diff options
Diffstat (limited to '')
38 files changed, 2806 insertions, 2186 deletions
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.rst b/Documentation/RCU/Design/Data-Structures/Data-Structures.rst index 4a48e20a46f2..b34990c7c377 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.rst +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.rst @@ -963,7 +963,7 @@ exit and perhaps also vice versa. Therefore, whenever the ``->dynticks_nesting`` field is incremented up from zero, the ``->dynticks_nmi_nesting`` field is set to a large positive number, and whenever the ``->dynticks_nesting`` field is decremented down to zero, -the the ``->dynticks_nmi_nesting`` field is set to zero. Assuming that +the ``->dynticks_nmi_nesting`` field is set to zero. Assuming that the number of misnested interrupts is not sufficient to overflow the counter, this approach corrects the ``->dynticks_nmi_nesting`` field every time the corresponding CPU enters the idle loop from process @@ -973,7 +973,7 @@ The ``->dynticks`` field counts the corresponding CPU's transitions to and from either dyntick-idle or user mode, so that this counter has an even value when the CPU is in dyntick-idle mode or user mode and an odd value otherwise. The transitions to/from user mode need to be counted -for user mode adaptive-ticks support (see timers/NO_HZ.txt). +for user mode adaptive-ticks support (see Documentation/timers/no_hz.rst). The ``->rcu_need_heavy_qs`` field is used to record the fact that the RCU core code would really like to see a quiescent state from the diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst index 72f0f6fbd53c..c9c957c85bac 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst @@ -38,7 +38,7 @@ sections. RCU-preempt Expedited Grace Periods =================================== -``CONFIG_PREEMPT=y`` kernels implement RCU-preempt. +``CONFIG_PREEMPTION=y`` kernels implement RCU-preempt. The overall flow of the handling of a given CPU by an RCU-preempt expedited grace period is shown in the following diagram: @@ -112,7 +112,7 @@ things. RCU-sched Expedited Grace Periods --------------------------------- -``CONFIG_PREEMPT=n`` kernels implement RCU-sched. The overall flow of +``CONFIG_PREEMPTION=n`` kernels implement RCU-sched. The overall flow of the handling of a given CPU by an RCU-sched expedited grace period is shown in the following diagram: @@ -406,7 +406,7 @@ In earlier implementations, the task requesting the expedited grace period also drove it to completion. This straightforward approach had the disadvantage of needing to account for POSIX signals sent to user tasks, so more recent implemementations use the Linux kernel's -`workqueues <https://www.kernel.org/doc/Documentation/core-api/workqueue.rst>`__. +workqueues (see Documentation/core-api/workqueue.rst). The requesting task still does counter snapshotting and funnel-lock processing, but the task reaching the top of the funnel lock does a diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg index 98af66557908..16b1ff0ad38c 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - 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style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst index 1a8b129cfc04..7fdf151a8680 100644 --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst @@ -4,7 +4,7 @@ A Tour Through TREE_RCU's Grace-Period Memory Ordering August 8, 2017 -This article was contributed by Paul E. McKenney +This article was contributed by Paul E. McKenney Introduction ============ @@ -21,7 +21,7 @@ Any code that happens after the end of a given RCU grace period is guaranteed to see the effects of all accesses prior to the beginning of that grace period that are within RCU read-side critical sections. Similarly, any code that happens before the beginning of a given RCU grace -period is guaranteed to see the effects of all accesses following the end +period is guaranteed to not see the effects of all accesses following the end of that grace period that are within RCU read-side critical sections. Note well that RCU-sched read-side critical sections include any region @@ -48,7 +48,7 @@ Tree RCU Grace Period Memory Ordering Building Blocks The workhorse for RCU's grace-period memory ordering is the critical section for the ``rcu_node`` structure's -``->lock``. These critical sections use helper functions for lock +``->lock``. These critical sections use helper functions for lock acquisition, including ``raw_spin_lock_rcu_node()``, ``raw_spin_lock_irq_rcu_node()``, and ``raw_spin_lock_irqsave_rcu_node()``. Their lock-release counterparts are ``raw_spin_unlock_rcu_node()``, @@ -102,9 +102,9 @@ lock-acquisition and lock-release functions:: 23 r3 = READ_ONCE(x); 24 } 25 - 26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0); + 26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0); -The ``WARN_ON()`` is evaluated at “the end of time”, +The ``WARN_ON()`` is evaluated at "the end of time", after all changes have propagated throughout the system. Without the ``smp_mb__after_unlock_lock()`` provided by the acquisition functions, this ``WARN_ON()`` could trigger, for example @@ -112,6 +112,35 @@ on PowerPC. The ``smp_mb__after_unlock_lock()`` invocations prevent this ``WARN_ON()`` from triggering. ++-----------------------------------------------------------------------+ +| **Quick Quiz**: | ++-----------------------------------------------------------------------+ +| But the chain of rcu_node-structure lock acquisitions guarantees | +| that new readers will see all of the updater's pre-grace-period | +| accesses and also guarantees that the updater's post-grace-period | +| accesses will see all of the old reader's accesses. So why do we | +| need all of those calls to smp_mb__after_unlock_lock()? | ++-----------------------------------------------------------------------+ +| **Answer**: | ++-----------------------------------------------------------------------+ +| Because we must provide ordering for RCU's polling grace-period | +| primitives, for example, get_state_synchronize_rcu() and | +| poll_state_synchronize_rcu(). Consider this code:: | +| | +| CPU 0 CPU 1 | +| ---- ---- | +| WRITE_ONCE(X, 1) WRITE_ONCE(Y, 1) | +| g = get_state_synchronize_rcu() smp_mb() | +| while (!poll_state_synchronize_rcu(g)) r1 = READ_ONCE(X) | +| continue; | +| r0 = READ_ONCE(Y) | +| | +| RCU guarantees that the outcome r0 == 0 && r1 == 0 will not | +| happen, even if CPU 1 is in an RCU extended quiescent state | +| (idle or offline) and thus won't interact directly with the RCU | +| core processing at all. | ++-----------------------------------------------------------------------+ + This approach must be extended to include idle CPUs, which need RCU's grace-period memory ordering guarantee to extend to any RCU read-side critical sections preceding and following the current @@ -173,49 +202,44 @@ newly arrived RCU callbacks against future grace periods: 1 static void rcu_prepare_for_idle(void) 2 { 3 bool needwake; - 4 struct rcu_data *rdp; - 5 struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); - 6 struct rcu_node *rnp; - 7 struct rcu_state *rsp; - 8 int tne; - 9 - 10 if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) || - 11 rcu_is_nocb_cpu(smp_processor_id())) - 12 return; + 4 struct rcu_data *rdp = this_cpu_ptr(&rcu_data); + 5 struct rcu_node *rnp; + 6 int tne; + 7 + 8 lockdep_assert_irqs_disabled(); + 9 if (rcu_rdp_is_offloaded(rdp)) + 10 return; + 11 + 12 /* Handle nohz enablement switches conservatively. */ 13 tne = READ_ONCE(tick_nohz_active); - 14 if (tne != rdtp->tick_nohz_enabled_snap) { - 15 if (rcu_cpu_has_callbacks(NULL)) - 16 invoke_rcu_core(); - 17 rdtp->tick_nohz_enabled_snap = tne; + 14 if (tne != rdp->tick_nohz_enabled_snap) { + 15 if (!rcu_segcblist_empty(&rdp->cblist)) + 16 invoke_rcu_core(); /* force nohz to see update. */ + 17 rdp->tick_nohz_enabled_snap = tne; 18 return; - 19 } + 19 } 20 if (!tne) 21 return; - 22 if (rdtp->all_lazy && - 23 rdtp->nonlazy_posted != rdtp->nonlazy_posted_snap) { - 24 rdtp->all_lazy = false; - 25 rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted; - 26 invoke_rcu_core(); - 27 return; - 28 } - 29 if (rdtp->last_accelerate == jiffies) - 30 return; - 31 rdtp->last_accelerate = jiffies; - 32 for_each_rcu_flavor(rsp) { - 33 rdp = this_cpu_ptr(rsp->rda); - 34 if (rcu_segcblist_pend_cbs(&rdp->cblist)) - 35 continue; - 36 rnp = rdp->mynode; - 37 raw_spin_lock_rcu_node(rnp); - 38 needwake = rcu_accelerate_cbs(rsp, rnp, rdp); - 39 raw_spin_unlock_rcu_node(rnp); - 40 if (needwake) - 41 rcu_gp_kthread_wake(rsp); - 42 } - 43 } + 22 + 23 /* + 24 * If we have not yet accelerated this jiffy, accelerate all + 25 * callbacks on this CPU. + 26 */ + 27 if (rdp->last_accelerate == jiffies) + 28 return; + 29 rdp->last_accelerate = jiffies; + 30 if (rcu_segcblist_pend_cbs(&rdp->cblist)) { + 31 rnp = rdp->mynode; + 32 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */ + 33 needwake = rcu_accelerate_cbs(rnp, rdp); + 34 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */ + 35 if (needwake) + 36 rcu_gp_kthread_wake(); + 37 } + 38 } But the only part of ``rcu_prepare_for_idle()`` that really matters for -this discussion are lines 37–39. We will therefore abbreviate this +this discussion are lines 32–34. We will therefore abbreviate this function as follows: .. kernel-figure:: rcu_node-lock.svg @@ -339,14 +363,14 @@ The diagram below shows the path of ordering if the leftmost leftmost ``rcu_node`` structure offlines its last CPU and if the next ``rcu_node`` structure has no online CPUs). -.. kernel-figure:: TreeRCU-gp-init-1.svg +.. kernel-figure:: TreeRCU-gp-init-2.svg The final ``rcu_gp_init()`` pass through the ``rcu_node`` tree traverses breadth-first, setting each ``rcu_node`` structure's ``->gp_seq`` field to the newly advanced value from the ``rcu_state`` structure, as shown in the following diagram. -.. kernel-figure:: TreeRCU-gp-init-1.svg +.. kernel-figure:: TreeRCU-gp-init-3.svg This change will also cause each CPU's next call to ``__note_gp_changes()`` to notice that a new grace period has started, @@ -473,7 +497,7 @@ read-side critical sections that follow the idle period (the oval near the bottom of the diagram above). Plumbing this into the full grace-period execution is described -`below <#Forcing%20Quiescent%20States>`__. +`below <Forcing Quiescent States_>`__. CPU-Hotplug Interface ^^^^^^^^^^^^^^^^^^^^^ @@ -494,7 +518,7 @@ mask to detect CPUs having gone offline since the beginning of this grace period. Plumbing this into the full grace-period execution is described -`below <#Forcing%20Quiescent%20States>`__. +`below <Forcing Quiescent States_>`__. Forcing Quiescent States ^^^^^^^^^^^^^^^^^^^^^^^^ @@ -532,7 +556,7 @@ from other CPUs. | RCU. But this diagram is complex enough as it is, so simplicity | | overrode accuracy. You can think of it as poetic license, or you can | | think of it as misdirection that is resolved in the | -| `stitched-together diagram <#Putting%20It%20All%20Together>`__. | +| `stitched-together diagram <Putting It All Together_>`__. | +-----------------------------------------------------------------------+ Grace-Period Cleanup @@ -596,7 +620,7 @@ maintain ordering. For example, if the callback function wakes up a task that runs on some other CPU, proper ordering must in place in both the callback function and the task being awakened. To see why this is important, consider the top half of the `grace-period -cleanup <#Grace-Period%20Cleanup>`__ diagram. The callback might be +cleanup`_ diagram. The callback might be running on a CPU corresponding to the leftmost leaf ``rcu_node`` structure, and awaken a task that is to run on a CPU corresponding to the rightmost leaf ``rcu_node`` structure, and the grace-period kernel diff --git a/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg index 4b4014fda770..87851a8fac1e 100644 --- a/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg +++ b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg @@ -88,7 +88,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -103,7 +103,7 @@ id="text2993" y="-261.66608" x="412.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -135,7 +135,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="268.18076" id="text4429" @@ -146,7 +146,7 @@ y="268.18076">WRITE_ONCE(a, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="439.13766" id="text4441" @@ -157,7 +157,7 @@ y="439.13766">WRITE_ONCE(b, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.60869" y="309.29346" id="text4445" @@ -168,7 +168,7 @@ y="309.29346">r1 = READ_ONCE(a);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.14423" y="520.61786" id="text4449" @@ -179,7 +179,7 @@ y="520.61786">WRITE_ONCE(c, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="384.71124" id="text4453" @@ -190,7 +190,7 @@ y="384.71124">r2 = READ_ONCE(b);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="582.13617" id="text4457" @@ -201,7 +201,7 @@ y="582.13617">r3 = READ_ONCE(c);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.08231" y="213.91006" id="text4461" @@ -212,7 +212,7 @@ y="213.91006">thread0()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="252.34512" y="213.91006" id="text4461-6" @@ -223,7 +223,7 @@ y="213.91006">thread1()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.42557" y="213.91006" id="text4461-2" @@ -251,7 +251,7 @@ inkscape:connector-curvature="0" /> <text xml:space="preserve" - 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style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="597.40289" id="text4429-8-9-3" @@ -284,7 +284,7 @@ y="597.40289">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="453.15311" id="text4429-8-9-3-1" @@ -300,7 +300,7 @@ inkscape:connector-curvature="0" /> <text xml:space="preserve" - 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style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="254.85066" y="348.96619" id="text4648-4-3" diff --git a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg index 48cd1623d4d4..e2a8af592bab 100644 --- a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg +++ b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -131,7 +131,7 @@ id="text2993" y="-261.66608" x="436.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -163,7 +163,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="268.18076" id="text4429" @@ -174,7 +174,7 @@ y="268.18076">WRITE_ONCE(a, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="487.13766" id="text4441" @@ -185,7 +185,7 @@ y="487.13766">WRITE_ONCE(b, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.60869" y="297.29346" id="text4445" @@ -196,7 +196,7 @@ y="297.29346">r1 = READ_ONCE(a);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.14423" y="554.61786" id="text4449" @@ -207,7 +207,7 @@ y="554.61786">WRITE_ONCE(c, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="370.71124" id="text4453" @@ -218,7 +218,7 @@ y="370.71124">WRITE_ONCE(d, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="572.13617" id="text4457" @@ -229,7 +229,7 @@ y="572.13617">r2 = READ_ONCE(c);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.08231" y="213.91006" id="text4461" @@ -240,7 +240,7 @@ y="213.91006">thread0()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="252.34512" y="213.91006" id="text4461-6" @@ -251,7 +251,7 @@ y="213.91006">thread1()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.42557" y="213.91006" id="text4461-2" @@ -281,7 +281,7 @@ sodipodi:nodetypes="cc" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="251.53981" id="text4429-8" @@ -292,7 +292,7 @@ y="251.53981">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="353.91556" id="text4429-8-9" @@ -303,7 +303,7 @@ y="353.91556">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="587.40289" id="text4429-8-9-3" @@ -314,7 +314,7 @@ y="587.40289">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="501.15311" id="text4429-8-9-3-1" @@ -331,7 +331,7 @@ sodipodi:nodetypes="cc" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="394.94427" y="331.66351" id="text4648" @@ -355,7 +355,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.11968" y="523.77856" id="text4648-4" @@ -392,7 +392,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="254.85066" y="336.96619" id="text4648-4-3" @@ -421,7 +421,7 @@ id="text2993-7" y="-261.66608" x="440.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -453,7 +453,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="541.70508" y="387.6217" id="text4445-0" @@ -464,7 +464,7 @@ y="387.6217">r3 = READ_ONCE(d);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="541.2406" y="646.94611" id="text4449-6" @@ -488,7 +488,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="540.94702" y="427.29443" id="text4648-4-3-1" @@ -499,7 +499,7 @@ y="427.29443">QS</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="461.83929" id="text4453-7" @@ -510,7 +510,7 @@ y="461.83929">r4 = READ_ONCE(b);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="669.26422" id="text4457-9" @@ -521,7 +521,7 @@ y="669.26422">r5 = READ_ONCE(e);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="445.04358" id="text4429-8-9-33" @@ -532,7 +532,7 @@ y="445.04358">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="684.53094" id="text4429-8-9-3-8" @@ -543,7 +543,7 @@ y="684.53094">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="685.11914" y="422.79153" id="text4648-9" @@ -567,7 +567,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="397.85934" y="609.59003" id="text4648-5" @@ -591,7 +591,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="256.75986" y="586.99133" id="text4648-5-2" @@ -615,7 +615,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="546.22791" y="213.91006" id="text4461-2-5" @@ -626,7 +626,7 @@ y="213.91006">thread3()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="684.00067" y="213.91006" id="text4461-2-1" diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index fd5e2cbc4935..a0f8164c8513 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -45,7 +45,7 @@ requirements: #. `Other RCU Flavors`_ #. `Possible Future Changes`_ -This is followed by a `summary <#Summary>`__, however, the answers to +This is followed by a summary_, however, the answers to each quick quiz immediately follows the quiz. Select the big white space with your mouse to see the answer. @@ -72,13 +72,13 @@ understanding of this guarantee. RCU's grace-period guarantee allows updaters to wait for the completion of all pre-existing RCU read-side critical sections. An RCU read-side -critical section begins with the marker ``rcu_read_lock()`` and ends -with the marker ``rcu_read_unlock()``. These markers may be nested, and +critical section begins with the marker rcu_read_lock() and ends +with the marker rcu_read_unlock(). These markers may be nested, and RCU treats a nested set as one big RCU read-side critical section. -Production-quality implementations of ``rcu_read_lock()`` and -``rcu_read_unlock()`` are extremely lightweight, and in fact have +Production-quality implementations of rcu_read_lock() and +rcu_read_unlock() are extremely lightweight, and in fact have exactly zero overhead in Linux kernels built for production use with -``CONFIG_PREEMPT=n``. +``CONFIG_PREEMPTION=n``. This guarantee allows ordering to be enforced with extremely low overhead to readers, for example: @@ -102,12 +102,12 @@ overhead to readers, for example: 15 WRITE_ONCE(y, 1); 16 } -Because the ``synchronize_rcu()`` on line 14 waits for all pre-existing -readers, any instance of ``thread0()`` that loads a value of zero from -``x`` must complete before ``thread1()`` stores to ``y``, so that +Because the synchronize_rcu() on line 14 waits for all pre-existing +readers, any instance of thread0() that loads a value of zero from +``x`` must complete before thread1() stores to ``y``, so that instance must also load a value of zero from ``y``. Similarly, any -instance of ``thread0()`` that loads a value of one from ``y`` must have -started after the ``synchronize_rcu()`` started, and must therefore also +instance of thread0() that loads a value of one from ``y`` must have +started after the synchronize_rcu() started, and must therefore also load a value of one from ``x``. Therefore, the outcome: :: @@ -121,14 +121,14 @@ cannot happen. +-----------------------------------------------------------------------+ | Wait a minute! You said that updaters can make useful forward | | progress concurrently with readers, but pre-existing readers will | -| block ``synchronize_rcu()``!!! | +| block synchronize_rcu()!!! | | Just who are you trying to fool??? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | First, if updaters do not wish to be blocked by readers, they can use | -| ``call_rcu()`` or ``kfree_rcu()``, which will be discussed later. | -| Second, even when using ``synchronize_rcu()``, the other update-side | +| call_rcu() or kfree_rcu(), which will be discussed later. | +| Second, even when using synchronize_rcu(), the other update-side | | code does run concurrently with readers, whether pre-existing or not. | +-----------------------------------------------------------------------+ @@ -170,34 +170,34 @@ recovery from node failure, more or less as follows: 29 WRITE_ONCE(state, STATE_NORMAL); 30 } -The RCU read-side critical section in ``do_something_dlm()`` works with -the ``synchronize_rcu()`` in ``start_recovery()`` to guarantee that -``do_something()`` never runs concurrently with ``recovery()``, but with -little or no synchronization overhead in ``do_something_dlm()``. +The RCU read-side critical section in do_something_dlm() works with +the synchronize_rcu() in start_recovery() to guarantee that +do_something() never runs concurrently with recovery(), but with +little or no synchronization overhead in do_something_dlm(). +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Why is the ``synchronize_rcu()`` on line 28 needed? | +| Why is the synchronize_rcu() on line 28 needed? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Without that extra grace period, memory reordering could result in | -| ``do_something_dlm()`` executing ``do_something()`` concurrently with | -| the last bits of ``recovery()``. | +| do_something_dlm() executing do_something() concurrently with | +| the last bits of recovery(). | +-----------------------------------------------------------------------+ In order to avoid fatal problems such as deadlocks, an RCU read-side -critical section must not contain calls to ``synchronize_rcu()``. +critical section must not contain calls to synchronize_rcu(). Similarly, an RCU read-side critical section must not contain anything that waits, directly or indirectly, on completion of an invocation of -``synchronize_rcu()``. +synchronize_rcu(). Although RCU's grace-period guarantee is useful in and of itself, with `quite a few use cases <https://lwn.net/Articles/573497/>`__, it would be good to be able to use RCU to coordinate read-side access to linked data structures. For this, the grace-period guarantee is not sufficient, -as can be seen in function ``add_gp_buggy()`` below. We will look at the +as can be seen in function add_gp_buggy() below. We will look at the reader's code later, but in the meantime, just think of the reader as locklessly picking up the ``gp`` pointer, and, if the value loaded is non-\ ``NULL``, locklessly accessing the ``->a`` and ``->b`` fields. @@ -256,8 +256,8 @@ Publish/Subscribe Guarantee RCU's publish-subscribe guarantee allows data to be inserted into a linked data structure without disrupting RCU readers. The updater uses -``rcu_assign_pointer()`` to insert the new data, and readers use -``rcu_dereference()`` to access data, whether new or old. The following +rcu_assign_pointer() to insert the new data, and readers use +rcu_dereference() to access data, whether new or old. The following shows an example of insertion: :: @@ -279,7 +279,7 @@ shows an example of insertion: 15 return true; 16 } -The ``rcu_assign_pointer()`` on line 13 is conceptually equivalent to a +The rcu_assign_pointer() on line 13 is conceptually equivalent to a simple assignment statement, but also guarantees that its assignment will happen after the two assignments in lines 11 and 12, similar to the C11 ``memory_order_release`` store operation. It also prevents any @@ -289,7 +289,7 @@ number of “interesting” compiler optimizations, for example, the use of +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| But ``rcu_assign_pointer()`` does nothing to prevent the two | +| But rcu_assign_pointer() does nothing to prevent the two | | assignments to ``p->a`` and ``p->b`` from being reordered. Can't that | | also cause problems? | +-----------------------------------------------------------------------+ @@ -303,7 +303,7 @@ number of “interesting” compiler optimizations, for example, the use of It is tempting to assume that the reader need not do anything special to control its accesses to the RCU-protected data, as shown in -``do_something_gp_buggy()`` below: +do_something_gp_buggy() below: :: @@ -321,11 +321,10 @@ control its accesses to the RCU-protected data, as shown in 12 } However, this temptation must be resisted because there are a -surprisingly large number of ways that the compiler (to say nothing of -`DEC Alpha CPUs <https://h71000.www7.hp.com/wizard/wiz_2637.html>`__) -can trip this code up. For but one example, if the compiler were short -of registers, it might choose to refetch from ``gp`` rather than keeping -a separate copy in ``p`` as follows: +surprisingly large number of ways that the compiler (or weak ordering +CPUs like the DEC Alpha) can trip this code up. For but one example, if +the compiler were short of registers, it might choose to refetch from +``gp`` rather than keeping a separate copy in ``p`` as follows: :: @@ -345,7 +344,7 @@ If this function ran concurrently with a series of updates that replaced the current structure with a new one, the fetches of ``gp->a`` and ``gp->b`` might well come from two different structures, which could cause serious confusion. To prevent this (and much else besides), -``do_something_gp()`` uses ``rcu_dereference()`` to fetch from ``gp``: +do_something_gp() uses rcu_dereference() to fetch from ``gp``: :: @@ -362,21 +361,23 @@ cause serious confusion. To prevent this (and much else besides), 11 return false; 12 } -The ``rcu_dereference()`` uses volatile casts and (for DEC Alpha) memory -barriers in the Linux kernel. Should a `high-quality implementation of -C11 ``memory_order_consume`` -[PDF] <http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf>`__ -ever appear, then ``rcu_dereference()`` could be implemented as a +The rcu_dereference() uses volatile casts and (for DEC Alpha) memory +barriers in the Linux kernel. Should a |high-quality implementation of +C11 memory_order_consume [PDF]|_ +ever appear, then rcu_dereference() could be implemented as a ``memory_order_consume`` load. Regardless of the exact implementation, a -pointer fetched by ``rcu_dereference()`` may not be used outside of the +pointer fetched by rcu_dereference() may not be used outside of the outermost RCU read-side critical section containing that -``rcu_dereference()``, unless protection of the corresponding data +rcu_dereference(), unless protection of the corresponding data element has been passed from RCU to some other synchronization -mechanism, most commonly locking or `reference -counting <https://www.kernel.org/doc/Documentation/RCU/rcuref.txt>`__. +mechanism, most commonly locking or reference counting +(see ../../rcuref.rst). -In short, updaters use ``rcu_assign_pointer()`` and readers use -``rcu_dereference()``, and these two RCU API elements work together to +.. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF] +.. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf + +In short, updaters use rcu_assign_pointer() and readers use +rcu_dereference(), and these two RCU API elements work together to ensure that readers have a consistent view of newly added data elements. Of course, it is also necessary to remove elements from RCU-protected @@ -388,9 +389,9 @@ data structures, for example, using the following process: the newly removed data element). #. At this point, only the updater has a reference to the newly removed data element, so it can safely reclaim the data element, for example, - by passing it to ``kfree()``. + by passing it to kfree(). -This process is implemented by ``remove_gp_synchronous()``: +This process is implemented by remove_gp_synchronous(): :: @@ -413,16 +414,16 @@ This process is implemented by ``remove_gp_synchronous()``: This function is straightforward, with line 13 waiting for a grace period before line 14 frees the old data element. This waiting ensures -that readers will reach line 7 of ``do_something_gp()`` before the data -element referenced by ``p`` is freed. The ``rcu_access_pointer()`` on -line 6 is similar to ``rcu_dereference()``, except that: +that readers will reach line 7 of do_something_gp() before the data +element referenced by ``p`` is freed. The rcu_access_pointer() on +line 6 is similar to rcu_dereference(), except that: -#. The value returned by ``rcu_access_pointer()`` cannot be +#. The value returned by rcu_access_pointer() cannot be dereferenced. If you want to access the value pointed to as well as - the pointer itself, use ``rcu_dereference()`` instead of - ``rcu_access_pointer()``. -#. The call to ``rcu_access_pointer()`` need not be protected. In - contrast, ``rcu_dereference()`` must either be within an RCU + the pointer itself, use rcu_dereference() instead of + rcu_access_pointer(). +#. The call to rcu_access_pointer() need not be protected. In + contrast, rcu_dereference() must either be within an RCU read-side critical section or in a code segment where the pointer cannot change, for example, in code protected by the corresponding update-side lock. @@ -430,13 +431,13 @@ line 6 is similar to ``rcu_dereference()``, except that: +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Without the ``rcu_dereference()`` or the ``rcu_access_pointer()``, | +| Without the rcu_dereference() or the rcu_access_pointer(), | | what destructive optimizations might the compiler make use of? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| Let's start with what happens to ``do_something_gp()`` if it fails to | -| use ``rcu_dereference()``. It could reuse a value formerly fetched | +| Let's start with what happens to do_something_gp() if it fails to | +| use rcu_dereference(). It could reuse a value formerly fetched | | from this same pointer. It could also fetch the pointer from ``gp`` | | in a byte-at-a-time manner, resulting in *load tearing*, in turn | | resulting a bytewise mash-up of two distinct pointer values. It might | @@ -445,15 +446,15 @@ line 6 is similar to ``rcu_dereference()``, except that: | update has changed the pointer to match the wrong guess. Too bad | | about any dereferences that returned pre-initialization garbage in | | the meantime! | -| For ``remove_gp_synchronous()``, as long as all modifications to | +| For remove_gp_synchronous(), as long as all modifications to | | ``gp`` are carried out while holding ``gp_lock``, the above | | optimizations are harmless. However, ``sparse`` will complain if you | | define ``gp`` with ``__rcu`` and then access it without using either | -| ``rcu_access_pointer()`` or ``rcu_dereference()``. | +| rcu_access_pointer() or rcu_dereference(). | +-----------------------------------------------------------------------+ In short, RCU's publish-subscribe guarantee is provided by the -combination of ``rcu_assign_pointer()`` and ``rcu_dereference()``. This +combination of rcu_assign_pointer() and rcu_dereference(). This guarantee allows data elements to be safely added to RCU-protected linked data structures without disrupting RCU readers. This guarantee can be used in combination with the grace-period guarantee to also allow @@ -462,9 +463,9 @@ again without disrupting RCU readers. This guarantee was only partially premeditated. DYNIX/ptx used an explicit memory barrier for publication, but had nothing resembling -``rcu_dereference()`` for subscription, nor did it have anything -resembling the ``smp_read_barrier_depends()`` that was later subsumed -into ``rcu_dereference()`` and later still into ``READ_ONCE()``. The +rcu_dereference() for subscription, nor did it have anything +resembling the dependency-ordering barrier that was later subsumed +into rcu_dereference() and later still into READ_ONCE(). The need for these operations made itself known quite suddenly at a late-1990s meeting with the DEC Alpha architects, back in the days when DEC was still a free-standing company. It took the Alpha architects a @@ -474,7 +475,7 @@ documentation did not make this point clear. More recent work with the C and C++ standards committees have provided much education on tricks and traps from the compiler. In short, compilers were much less tricky in the early 1990s, but in 2015, don't even think about omitting -``rcu_dereference()``! +rcu_dereference()! Memory-Barrier Guarantees ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -484,31 +485,31 @@ demonstrates the need for RCU's stringent memory-ordering guarantees on systems with more than one CPU: #. Each CPU that has an RCU read-side critical section that begins - before ``synchronize_rcu()`` starts is guaranteed to execute a full + before synchronize_rcu() starts is guaranteed to execute a full memory barrier between the time that the RCU read-side critical - section ends and the time that ``synchronize_rcu()`` returns. Without + section ends and the time that synchronize_rcu() returns. Without this guarantee, a pre-existing RCU read-side critical section might hold a reference to the newly removed ``struct foo`` after the - ``kfree()`` on line 14 of ``remove_gp_synchronous()``. + kfree() on line 14 of remove_gp_synchronous(). #. Each CPU that has an RCU read-side critical section that ends after - ``synchronize_rcu()`` returns is guaranteed to execute a full memory - barrier between the time that ``synchronize_rcu()`` begins and the + synchronize_rcu() returns is guaranteed to execute a full memory + barrier between the time that synchronize_rcu() begins and the time that the RCU read-side critical section begins. Without this guarantee, a later RCU read-side critical section running after the - ``kfree()`` on line 14 of ``remove_gp_synchronous()`` might later run - ``do_something_gp()`` and find the newly deleted ``struct foo``. -#. If the task invoking ``synchronize_rcu()`` remains on a given CPU, + kfree() on line 14 of remove_gp_synchronous() might later run + do_something_gp() and find the newly deleted ``struct foo``. +#. If the task invoking synchronize_rcu() remains on a given CPU, then that CPU is guaranteed to execute a full memory barrier sometime - during the execution of ``synchronize_rcu()``. This guarantee ensures - that the ``kfree()`` on line 14 of ``remove_gp_synchronous()`` really + during the execution of synchronize_rcu(). This guarantee ensures + that the kfree() on line 14 of remove_gp_synchronous() really does execute after the removal on line 11. -#. If the task invoking ``synchronize_rcu()`` migrates among a group of +#. If the task invoking synchronize_rcu() migrates among a group of CPUs during that invocation, then each of the CPUs in that group is guaranteed to execute a full memory barrier sometime during the - execution of ``synchronize_rcu()``. This guarantee also ensures that - the ``kfree()`` on line 14 of ``remove_gp_synchronous()`` really does + execution of synchronize_rcu(). This guarantee also ensures that + the kfree() on line 14 of remove_gp_synchronous() really does execute after the removal on line 11, but also in the case where the - thread executing the ``synchronize_rcu()`` migrates in the meantime. + thread executing the synchronize_rcu() migrates in the meantime. +-----------------------------------------------------------------------+ | **Quick Quiz**: | @@ -516,19 +517,19 @@ systems with more than one CPU: | Given that multiple CPUs can start RCU read-side critical sections at | | any time without any ordering whatsoever, how can RCU possibly tell | | whether or not a given RCU read-side critical section starts before a | -| given instance of ``synchronize_rcu()``? | +| given instance of synchronize_rcu()? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | If RCU cannot tell whether or not a given RCU read-side critical | -| section starts before a given instance of ``synchronize_rcu()``, then | +| section starts before a given instance of synchronize_rcu(), then | | it must assume that the RCU read-side critical section started first. | -| In other words, a given instance of ``synchronize_rcu()`` can avoid | +| In other words, a given instance of synchronize_rcu() can avoid | | waiting on a given RCU read-side critical section only if it can | -| prove that ``synchronize_rcu()`` started first. | -| A related question is “When ``rcu_read_lock()`` doesn't generate any | +| prove that synchronize_rcu() started first. | +| A related question is “When rcu_read_lock() doesn't generate any | | code, why does it matter how it relates to a grace period?” The | -| answer is that it is not the relationship of ``rcu_read_lock()`` | +| answer is that it is not the relationship of rcu_read_lock() | | itself that is important, but rather the relationship of the code | | within the enclosed RCU read-side critical section to the code | | preceding and following the grace period. If we take this viewpoint, | @@ -556,14 +557,14 @@ systems with more than one CPU: | Yes, they really are required. To see why the first guarantee is | | required, consider the following sequence of events: | | | -| #. CPU 1: ``rcu_read_lock()`` | +| #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp); /* Very likely to return p. */`` | | #. CPU 0: ``list_del_rcu(p);`` | -| #. CPU 0: ``synchronize_rcu()`` starts. | +| #. CPU 0: synchronize_rcu() starts. | | #. CPU 1: ``do_something_with(q->a);`` | | ``/* No smp_mb(), so might happen after kfree(). */`` | -| #. CPU 1: ``rcu_read_unlock()`` | -| #. CPU 0: ``synchronize_rcu()`` returns. | +| #. CPU 1: rcu_read_unlock() | +| #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | | | Therefore, there absolutely must be a full memory barrier between the | @@ -574,14 +575,14 @@ systems with more than one CPU: | is roughly similar: | | | | #. CPU 0: ``list_del_rcu(p);`` | -| #. CPU 0: ``synchronize_rcu()`` starts. | -| #. CPU 1: ``rcu_read_lock()`` | +| #. CPU 0: synchronize_rcu() starts. | +| #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp);`` | | ``/* Might return p if no memory barrier. */`` | -| #. CPU 0: ``synchronize_rcu()`` returns. | +| #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | #. CPU 1: ``do_something_with(q->a); /* Boom!!! */`` | -| #. CPU 1: ``rcu_read_unlock()`` | +| #. CPU 1: rcu_read_unlock() | | | | And similarly, without a memory barrier between the beginning of the | | grace period and the beginning of the RCU read-side critical section, | @@ -597,7 +598,7 @@ systems with more than one CPU: +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| You claim that ``rcu_read_lock()`` and ``rcu_read_unlock()`` generate | +| You claim that rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code in some kernel builds. This means that the | | compiler might arbitrarily rearrange consecutive RCU read-side | | critical sections. Given such rearrangement, if a given RCU read-side | @@ -607,11 +608,11 @@ systems with more than one CPU: +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| In cases where ``rcu_read_lock()`` and ``rcu_read_unlock()`` generate | +| In cases where rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code, RCU infers quiescent states only at special | | locations, for example, within the scheduler. Because calls to | -| ``schedule()`` had better prevent calling-code accesses to shared | -| variables from being rearranged across the call to ``schedule()``, if | +| schedule() had better prevent calling-code accesses to shared | +| variables from being rearranged across the call to schedule(), if | | RCU detects the end of a given RCU read-side critical section, it | | will necessarily detect the end of all prior RCU read-side critical | | sections, no matter how aggressively the compiler scrambles the code. | @@ -655,8 +656,8 @@ read-side critical section might search for a given data element, and then might acquire the update-side spinlock in order to update that element, all while remaining in that RCU read-side critical section. Of course, it is necessary to exit the RCU read-side critical section -before invoking ``synchronize_rcu()``, however, this inconvenience can -be avoided through use of the ``call_rcu()`` and ``kfree_rcu()`` API +before invoking synchronize_rcu(), however, this inconvenience can +be avoided through use of the call_rcu() and kfree_rcu() API members described later in this document. +-----------------------------------------------------------------------+ @@ -694,10 +695,10 @@ these non-guarantees were premeditated. Readers Impose Minimal Ordering ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Reader-side markers such as ``rcu_read_lock()`` and -``rcu_read_unlock()`` provide absolutely no ordering guarantees except +Reader-side markers such as rcu_read_lock() and +rcu_read_unlock() provide absolutely no ordering guarantees except through their interaction with the grace-period APIs such as -``synchronize_rcu()``. To see this, consider the following pair of +synchronize_rcu(). To see this, consider the following pair of threads: :: @@ -722,7 +723,7 @@ threads: 18 rcu_read_unlock(); 19 } -After ``thread0()`` and ``thread1()`` execute concurrently, it is quite +After thread0() and thread1() execute concurrently, it is quite possible to have :: @@ -730,7 +731,7 @@ possible to have (r1 == 1 && r2 == 0) (that is, ``y`` appears to have been assigned before ``x``), which would -not be possible if ``rcu_read_lock()`` and ``rcu_read_unlock()`` had +not be possible if rcu_read_lock() and rcu_read_unlock() had much in the way of ordering properties. But they do not, so the CPU is within its rights to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. @@ -742,14 +743,14 @@ significant ordering constraints would slow down these fast-path APIs. +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| No, the volatile casts in ``READ_ONCE()`` and ``WRITE_ONCE()`` | +| No, the volatile casts in READ_ONCE() and WRITE_ONCE() | | prevent the compiler from reordering in this particular case. | +-----------------------------------------------------------------------+ Readers Do Not Exclude Updaters ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Neither ``rcu_read_lock()`` nor ``rcu_read_unlock()`` exclude updates. +Neither rcu_read_lock() nor rcu_read_unlock() exclude updates. All they do is to prevent grace periods from ending. The following example illustrates this: @@ -775,19 +776,19 @@ example illustrates this: 18 spin_unlock(&my_lock); 19 } -If the ``thread0()`` function's ``rcu_read_lock()`` excluded the -``thread1()`` function's update, the ``WARN_ON()`` could never fire. But -the fact is that ``rcu_read_lock()`` does not exclude much of anything -aside from subsequent grace periods, of which ``thread1()`` has none, so -the ``WARN_ON()`` can and does fire. +If the thread0() function's rcu_read_lock() excluded the +thread1() function's update, the WARN_ON() could never fire. But +the fact is that rcu_read_lock() does not exclude much of anything +aside from subsequent grace periods, of which thread1() has none, so +the WARN_ON() can and does fire. Updaters Only Wait For Old Readers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -It might be tempting to assume that after ``synchronize_rcu()`` +It might be tempting to assume that after synchronize_rcu() completes, there are no readers executing. This temptation must be avoided because new readers can start immediately after -``synchronize_rcu()`` starts, and ``synchronize_rcu()`` is under no +synchronize_rcu() starts, and synchronize_rcu() is under no obligation to wait for these new readers. +-----------------------------------------------------------------------+ @@ -799,10 +800,10 @@ obligation to wait for these new readers. +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| For no time at all. Even if ``synchronize_rcu()`` were to wait until | +| For no time at all. Even if synchronize_rcu() were to wait until | | all readers had completed, a new reader might start immediately after | -| ``synchronize_rcu()`` completed. Therefore, the code following | -| ``synchronize_rcu()`` can *never* rely on there being no readers. | +| synchronize_rcu() completed. Therefore, the code following | +| synchronize_rcu() can *never* rely on there being no readers. | +-----------------------------------------------------------------------+ Grace Periods Don't Partition Read-Side Critical Sections @@ -892,12 +893,12 @@ period is known to end before the second grace period starts: 28 rcu_read_unlock(); 29 } -Here, if ``(r1 == 1)``, then ``thread0()``'s write to ``b`` must happen -before the end of ``thread1()``'s grace period. If in addition -``(r4 == 1)``, then ``thread3()``'s read from ``b`` must happen after -the beginning of ``thread2()``'s grace period. If it is also the case -that ``(r2 == 1)``, then the end of ``thread1()``'s grace period must -precede the beginning of ``thread2()``'s grace period. This mean that +Here, if ``(r1 == 1)``, then thread0()'s write to ``b`` must happen +before the end of thread1()'s grace period. If in addition +``(r4 == 1)``, then thread3()'s read from ``b`` must happen after +the beginning of thread2()'s grace period. If it is also the case +that ``(r2 == 1)``, then the end of thread1()'s grace period must +precede the beginning of thread2()'s grace period. This mean that the two RCU read-side critical sections cannot overlap, guaranteeing that ``(r3 == 1)``. As a result, the outcome: @@ -1076,8 +1077,8 @@ is captured by the following list of situations: b. Wait-free read-side primitives for real-time use. This focus on read-mostly situations means that RCU must interoperate -with other synchronization primitives. For example, the ``add_gp()`` and -``remove_gp_synchronous()`` examples discussed earlier use RCU to +with other synchronization primitives. For example, the add_gp() and +remove_gp_synchronous() examples discussed earlier use RCU to protect readers and locking to coordinate updaters. However, the need extends much farther, requiring that a variety of synchronization primitives be legal within RCU read-side critical sections, including @@ -1096,7 +1097,7 @@ memory barriers. | case, voluntary context switch) within an RCU read-side critical | | section. However, sleeping locks may be used within userspace RCU | | read-side critical sections, and also within Linux-kernel sleepable | -| RCU `(SRCU) <#Sleepable%20RCU>`__ read-side critical sections. In | +| RCU `(SRCU) <Sleepable RCU_>`__ read-side critical sections. In | | addition, the -rt patchset turns spinlocks into a sleeping locks so | | that the corresponding critical sections can be preempted, which also | | means that these sleeplockified spinlocks (but not other sleeping | @@ -1104,11 +1105,11 @@ memory barriers. | sections. | | Note that it *is* legal for a normal RCU read-side critical section | | to conditionally acquire a sleeping locks (as in | -| ``mutex_trylock()``), but only as long as it does not loop | +| mutex_trylock()), but only as long as it does not loop | | indefinitely attempting to conditionally acquire that sleeping locks. | -| The key point is that things like ``mutex_trylock()`` either return | +| The key point is that things like mutex_trylock() either return | | with the mutex held, or return an error indication if the mutex was | -| not immediately available. Either way, ``mutex_trylock()`` returns | +| not immediately available. Either way, mutex_trylock() returns | | immediately without sleeping. | +-----------------------------------------------------------------------+ @@ -1182,66 +1183,66 @@ and has become decreasingly so as memory sizes have expanded and memory costs have plummeted. However, as I learned from Matt Mackall's `bloatwatch <http://elinux.org/Linux_Tiny-FAQ>`__ efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (``CONFIG_PREEMPT=n``) kernels, and thus `tiny -RCU <https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com>`__ +non-preemptible (``CONFIG_PREEMPTION=n``) kernels, and thus `tiny +RCU <https://lore.kernel.org/r/20090113221724.GA15307@linux.vnet.ibm.com>`__ was born. Josh Triplett has since taken over the small-memory banner with his `Linux kernel tinification <https://tiny.wiki.kernel.org/>`__ -project, which resulted in `SRCU <#Sleepable%20RCU>`__ becoming optional +project, which resulted in `SRCU <Sleepable RCU_>`__ becoming optional for those kernels not needing it. The remaining performance requirements are, for the most part, unsurprising. For example, in keeping with RCU's read-side -specialization, ``rcu_dereference()`` should have negligible overhead +specialization, rcu_dereference() should have negligible overhead (for example, suppression of a few minor compiler optimizations). -Similarly, in non-preemptible environments, ``rcu_read_lock()`` and -``rcu_read_unlock()`` should have exactly zero overhead. +Similarly, in non-preemptible environments, rcu_read_lock() and +rcu_read_unlock() should have exactly zero overhead. In preemptible environments, in the case where the RCU read-side critical section was not preempted (as will be the case for the -highest-priority real-time process), ``rcu_read_lock()`` and -``rcu_read_unlock()`` should have minimal overhead. In particular, they +highest-priority real-time process), rcu_read_lock() and +rcu_read_unlock() should have minimal overhead. In particular, they should not contain atomic read-modify-write operations, memory-barrier instructions, preemption disabling, interrupt disabling, or backwards branches. However, in the case where the RCU read-side critical section -was preempted, ``rcu_read_unlock()`` may acquire spinlocks and disable +was preempted, rcu_read_unlock() may acquire spinlocks and disable interrupts. This is why it is better to nest an RCU read-side critical section within a preempt-disable region than vice versa, at least in cases where that critical section is short enough to avoid unduly degrading real-time latencies. -The ``synchronize_rcu()`` grace-period-wait primitive is optimized for +The synchronize_rcu() grace-period-wait primitive is optimized for throughput. It may therefore incur several milliseconds of latency in addition to the duration of the longest RCU read-side critical section. On the other hand, multiple concurrent invocations of -``synchronize_rcu()`` are required to use batching optimizations so that +synchronize_rcu() are required to use batching optimizations so that they can be satisfied by a single underlying grace-period-wait operation. For example, in the Linux kernel, it is not unusual for a single grace-period-wait operation to serve more than `1,000 separate invocations <https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response>`__ -of ``synchronize_rcu()``, thus amortizing the per-invocation overhead +of synchronize_rcu(), thus amortizing the per-invocation overhead down to nearly zero. However, the grace-period optimization is also required to avoid measurable degradation of real-time scheduling and interrupt latencies. -In some cases, the multi-millisecond ``synchronize_rcu()`` latencies are -unacceptable. In these cases, ``synchronize_rcu_expedited()`` may be +In some cases, the multi-millisecond synchronize_rcu() latencies are +unacceptable. In these cases, synchronize_rcu_expedited() may be used instead, reducing the grace-period latency down to a few tens of microseconds on small systems, at least in cases where the RCU read-side critical sections are short. There are currently no special latency -requirements for ``synchronize_rcu_expedited()`` on large systems, but, +requirements for synchronize_rcu_expedited() on large systems, but, consistent with the empirical nature of the RCU specification, that is subject to change. However, there most definitely are scalability -requirements: A storm of ``synchronize_rcu_expedited()`` invocations on +requirements: A storm of synchronize_rcu_expedited() invocations on 4096 CPUs should at least make reasonable forward progress. In return -for its shorter latencies, ``synchronize_rcu_expedited()`` is permitted +for its shorter latencies, synchronize_rcu_expedited() is permitted to impose modest degradation of real-time latency on non-idle online CPUs. Here, “modest” means roughly the same latency degradation as a scheduling-clock interrupt. There are a number of situations where even -``synchronize_rcu_expedited()``'s reduced grace-period latency is -unacceptable. In these situations, the asynchronous ``call_rcu()`` can -be used in place of ``synchronize_rcu()`` as follows: +synchronize_rcu_expedited()'s reduced grace-period latency is +unacceptable. In these situations, the asynchronous call_rcu() can +be used in place of synchronize_rcu() as follows: :: @@ -1275,19 +1276,19 @@ be used in place of ``synchronize_rcu()`` as follows: 28 } A definition of ``struct foo`` is finally needed, and appears on -lines 1-5. The function ``remove_gp_cb()`` is passed to ``call_rcu()`` +lines 1-5. The function remove_gp_cb() is passed to call_rcu() on line 25, and will be invoked after the end of a subsequent grace -period. This gets the same effect as ``remove_gp_synchronous()``, but +period. This gets the same effect as remove_gp_synchronous(), but without forcing the updater to wait for a grace period to elapse. The -``call_rcu()`` function may be used in a number of situations where -neither ``synchronize_rcu()`` nor ``synchronize_rcu_expedited()`` would -be legal, including within preempt-disable code, ``local_bh_disable()`` +call_rcu() function may be used in a number of situations where +neither synchronize_rcu() nor synchronize_rcu_expedited() would +be legal, including within preempt-disable code, local_bh_disable() code, interrupt-disable code, and interrupt handlers. However, even -``call_rcu()`` is illegal within NMI handlers and from idle and offline -CPUs. The callback function (``remove_gp_cb()`` in this case) will be +call_rcu() is illegal within NMI handlers and from idle and offline +CPUs. The callback function (remove_gp_cb() in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, either within a real softirq handler or under the -protection of ``local_bh_disable()``. In both the Linux kernel and in +protection of local_bh_disable(). In both the Linux kernel and in userspace, it is bad practice to write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. @@ -1295,23 +1296,23 @@ threads or (in the Linux kernel) workqueues. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Why does line 19 use ``rcu_access_pointer()``? After all, | -| ``call_rcu()`` on line 25 stores into the structure, which would | +| Why does line 19 use rcu_access_pointer()? After all, | +| call_rcu() on line 25 stores into the structure, which would | | interact badly with concurrent insertions. Doesn't this mean that | -| ``rcu_dereference()`` is required? | +| rcu_dereference() is required? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Presumably the ``->gp_lock`` acquired on line 18 excludes any | -| changes, including any insertions that ``rcu_dereference()`` would | +| changes, including any insertions that rcu_dereference() would | | protect against. Therefore, any insertions will be delayed until | | after ``->gp_lock`` is released on line 25, which in turn means that | -| ``rcu_access_pointer()`` suffices. | +| rcu_access_pointer() suffices. | +-----------------------------------------------------------------------+ -However, all that ``remove_gp_cb()`` is doing is invoking ``kfree()`` on +However, all that remove_gp_cb() is doing is invoking kfree() on the data element. This is a common idiom, and is supported by -``kfree_rcu()``, which allows “fire and forget” operation as shown +kfree_rcu(), which allows “fire and forget” operation as shown below: :: @@ -1338,20 +1339,20 @@ below: 20 return true; 21 } -Note that ``remove_gp_faf()`` simply invokes ``kfree_rcu()`` and +Note that remove_gp_faf() simply invokes kfree_rcu() and proceeds, without any need to pay any further attention to the -subsequent grace period and ``kfree()``. It is permissible to invoke -``kfree_rcu()`` from the same environments as for ``call_rcu()``. -Interestingly enough, DYNIX/ptx had the equivalents of ``call_rcu()`` -and ``kfree_rcu()``, but not ``synchronize_rcu()``. This was due to the +subsequent grace period and kfree(). It is permissible to invoke +kfree_rcu() from the same environments as for call_rcu(). +Interestingly enough, DYNIX/ptx had the equivalents of call_rcu() +and kfree_rcu(), but not synchronize_rcu(). This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few -places that needed something like ``synchronize_rcu()`` simply +places that needed something like synchronize_rcu() simply open-coded it. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Earlier it was claimed that ``call_rcu()`` and ``kfree_rcu()`` | +| Earlier it was claimed that call_rcu() and kfree_rcu() | | allowed updaters to avoid being blocked by readers. But how can that | | be correct, given that the invocation of the callback and the freeing | | of the memory (respectively) must still wait for a grace period to | @@ -1363,16 +1364,16 @@ open-coded it. | definition would say that updates in garbage-collected languages | | cannot complete until the next time the garbage collector runs, which | | does not seem at all reasonable. The key point is that in most cases, | -| an updater using either ``call_rcu()`` or ``kfree_rcu()`` can proceed | -| to the next update as soon as it has invoked ``call_rcu()`` or | -| ``kfree_rcu()``, without having to wait for a subsequent grace | +| an updater using either call_rcu() or kfree_rcu() can proceed | +| to the next update as soon as it has invoked call_rcu() or | +| kfree_rcu(), without having to wait for a subsequent grace | | period. | +-----------------------------------------------------------------------+ But what if the updater must wait for the completion of code to be executed after the end of the grace period, but has other tasks that can be carried out in the meantime? The polling-style -``get_state_synchronize_rcu()`` and ``cond_synchronize_rcu()`` functions +get_state_synchronize_rcu() and cond_synchronize_rcu() functions may be used for this purpose, as shown below: :: @@ -1397,11 +1398,11 @@ may be used for this purpose, as shown below: 18 return true; 19 } -On line 14, ``get_state_synchronize_rcu()`` obtains a “cookie” from RCU, +On line 14, get_state_synchronize_rcu() obtains a “cookie” from RCU, then line 15 carries out other tasks, and finally, line 16 returns immediately if a grace period has elapsed in the meantime, but otherwise waits as required. The need for ``get_state_synchronize_rcu`` and -``cond_synchronize_rcu()`` has appeared quite recently, so it is too +cond_synchronize_rcu() has appeared quite recently, so it is too early to tell whether they will stand the test of time. RCU thus provides a range of tools to allow updaters to strike the @@ -1421,8 +1422,8 @@ example, an infinite loop in an RCU read-side critical section must by definition prevent later grace periods from ever completing. For a more involved example, consider a 64-CPU system built with ``CONFIG_RCU_NOCB_CPU=y`` and booted with ``rcu_nocbs=1-63``, where -CPUs 1 through 63 spin in tight loops that invoke ``call_rcu()``. Even -if these tight loops also contain calls to ``cond_resched()`` (thus +CPUs 1 through 63 spin in tight loops that invoke call_rcu(). Even +if these tight loops also contain calls to cond_resched() (thus allowing grace periods to complete), CPU 0 simply will not be able to invoke callbacks as fast as the other 63 CPUs can register them, at least not until the system runs out of memory. In both of these @@ -1435,21 +1436,21 @@ RCU takes the following steps to encourage timely completion of grace periods: #. If a grace period fails to complete within 100 milliseconds, RCU - causes future invocations of ``cond_resched()`` on the holdout CPUs + causes future invocations of cond_resched() on the holdout CPUs to provide an RCU quiescent state. RCU also causes those CPUs' - ``need_resched()`` invocations to return ``true``, but only after the + need_resched() invocations to return ``true``, but only after the corresponding CPU's next scheduling-clock. #. CPUs mentioned in the ``nohz_full`` kernel boot parameter can run indefinitely in the kernel without scheduling-clock interrupts, which - defeats the above ``need_resched()`` strategem. RCU will therefore - invoke ``resched_cpu()`` on any ``nohz_full`` CPUs still holding out + defeats the above need_resched() strategem. RCU will therefore + invoke resched_cpu() on any ``nohz_full`` CPUs still holding out after 109 milliseconds. #. In kernels built with ``CONFIG_RCU_BOOST=y``, if a given task that has been preempted within an RCU read-side critical section is holding out for more than 500 milliseconds, RCU will resort to priority boosting. #. If a CPU is still holding out 10 seconds into the grace period, RCU - will invoke ``resched_cpu()`` on it regardless of its ``nohz_full`` + will invoke resched_cpu() on it regardless of its ``nohz_full`` state. The above values are defaults for systems running with ``HZ=1000``. They @@ -1457,10 +1458,10 @@ will vary as the value of ``HZ`` varies, and can also be changed using the relevant Kconfig options and kernel boot parameters. RCU currently does not do much sanity checking of these parameters, so please use caution when changing them. Note that these forward-progress measures -are provided only for RCU, not for `SRCU <#Sleepable%20RCU>`__ or `Tasks -RCU <#Tasks%20RCU>`__. +are provided only for RCU, not for `SRCU <Sleepable RCU_>`__ or `Tasks +RCU`_. -RCU takes the following steps in ``call_rcu()`` to encourage timely +RCU takes the following steps in call_rcu() to encourage timely invocation of callbacks when any given non-\ ``rcu_nocbs`` CPU has 10,000 callbacks, or has 10,000 more callbacks than it had the last time encouragement was provided: @@ -1477,12 +1478,12 @@ encouragement was provided: Again, these are default values when running at ``HZ=1000``, and can be overridden. Again, these forward-progress measures are provided only for -RCU, not for `SRCU <#Sleepable%20RCU>`__ or `Tasks -RCU <#Tasks%20RCU>`__. Even for RCU, callback-invocation forward +RCU, not for `SRCU <Sleepable RCU_>`__ or `Tasks +RCU`_. Even for RCU, callback-invocation forward progress for ``rcu_nocbs`` CPUs is much less well-developed, in part because workloads benefiting from ``rcu_nocbs`` CPUs tend to invoke -``call_rcu()`` relatively infrequently. If workloads emerge that need -both ``rcu_nocbs`` CPUs and high ``call_rcu()`` invocation rates, then +call_rcu() relatively infrequently. If workloads emerge that need +both ``rcu_nocbs`` CPUs and high call_rcu() invocation rates, then additional forward-progress work will be required. Composability @@ -1496,11 +1497,11 @@ in fact may be nested arbitrarily deeply. In practice, as with all real-world implementations of composable constructs, there are limitations. -Implementations of RCU for which ``rcu_read_lock()`` and -``rcu_read_unlock()`` generate no code, such as Linux-kernel RCU when -``CONFIG_PREEMPT=n``, can be nested arbitrarily deeply. After all, there +Implementations of RCU for which rcu_read_lock() and +rcu_read_unlock() generate no code, such as Linux-kernel RCU when +``CONFIG_PREEMPTION=n``, can be nested arbitrarily deeply. After all, there is no overhead. Except that if all these instances of -``rcu_read_lock()`` and ``rcu_read_unlock()`` are visible to the +rcu_read_lock() and rcu_read_unlock() are visible to the compiler, compilation will eventually fail due to exhausting memory, mass storage, or user patience, whichever comes first. If the nesting is not visible to the compiler, as is the case with mutually recursive @@ -1558,11 +1559,11 @@ argue that such workloads should instead use something other than RCU, the fact remains that RCU must handle such workloads gracefully. This requirement is another factor driving batching of grace periods, but it is also the driving force behind the checks for large numbers of queued -RCU callbacks in the ``call_rcu()`` code path. Finally, high update +RCU callbacks in the call_rcu() code path. Finally, high update rates should not delay RCU read-side critical sections, although some small read-side delays can occur when using -``synchronize_rcu_expedited()``, courtesy of this function's use of -``smp_call_function_single()``. +synchronize_rcu_expedited(), courtesy of this function's use of +smp_call_function_single(). Although all three of these corner cases were understood in the early 1990s, a simple user-level test consisting of ``close(open(path))`` in a @@ -1583,48 +1584,48 @@ Software-Engineering Requirements Between Murphy's Law and “To err is human”, it is necessary to guard against mishaps and misuse: -#. It is all too easy to forget to use ``rcu_read_lock()`` everywhere +#. It is all too easy to forget to use rcu_read_lock() everywhere that it is needed, so kernels built with ``CONFIG_PROVE_RCU=y`` will - splat if ``rcu_dereference()`` is used outside of an RCU read-side + splat if rcu_dereference() is used outside of an RCU read-side critical section. Update-side code can use - ``rcu_dereference_protected()``, which takes a `lockdep + rcu_dereference_protected(), which takes a `lockdep expression <https://lwn.net/Articles/371986/>`__ to indicate what is providing the protection. If the indicated protection is not provided, a lockdep splat is emitted. Code shared between readers and updaters can use - ``rcu_dereference_check()``, which also takes a lockdep expression, - and emits a lockdep splat if neither ``rcu_read_lock()`` nor the + rcu_dereference_check(), which also takes a lockdep expression, + and emits a lockdep splat if neither rcu_read_lock() nor the indicated protection is in place. In addition, - ``rcu_dereference_raw()`` is used in those (hopefully rare) cases + rcu_dereference_raw() is used in those (hopefully rare) cases where the required protection cannot be easily described. Finally, - ``rcu_read_lock_held()`` is provided to allow a function to verify + rcu_read_lock_held() is provided to allow a function to verify that it has been invoked within an RCU read-side critical section. I was made aware of this set of requirements shortly after Thomas Gleixner audited a number of RCU uses. #. A given function might wish to check for RCU-related preconditions upon entry, before using any other RCU API. The - ``rcu_lockdep_assert()`` does this job, asserting the expression in + rcu_lockdep_assert() does this job, asserting the expression in kernels having lockdep enabled and doing nothing otherwise. -#. It is also easy to forget to use ``rcu_assign_pointer()`` and - ``rcu_dereference()``, perhaps (incorrectly) substituting a simple +#. It is also easy to forget to use rcu_assign_pointer() and + rcu_dereference(), perhaps (incorrectly) substituting a simple assignment. To catch this sort of error, a given RCU-protected pointer may be tagged with ``__rcu``, after which sparse will complain about simple-assignment accesses to that pointer. Arnd Bergmann made me aware of this requirement, and also supplied the needed `patch series <https://lwn.net/Articles/376011/>`__. #. Kernels built with ``CONFIG_DEBUG_OBJECTS_RCU_HEAD=y`` will splat if - a data element is passed to ``call_rcu()`` twice in a row, without a + a data element is passed to call_rcu() twice in a row, without a grace period in between. (This error is similar to a double free.) The corresponding ``rcu_head`` structures that are dynamically allocated are automatically tracked, but ``rcu_head`` structures allocated on the stack must be initialized with - ``init_rcu_head_on_stack()`` and cleaned up with - ``destroy_rcu_head_on_stack()``. Similarly, statically allocated + init_rcu_head_on_stack() and cleaned up with + destroy_rcu_head_on_stack(). Similarly, statically allocated non-stack ``rcu_head`` structures must be initialized with - ``init_rcu_head()`` and cleaned up with ``destroy_rcu_head()``. + init_rcu_head() and cleaned up with destroy_rcu_head(). Mathieu Desnoyers made me aware of this requirement, and also supplied the needed - `patch <https://lkml.kernel.org/g/20100319013024.GA28456@Krystal>`__. + `patch <https://lore.kernel.org/r/20100319013024.GA28456@Krystal>`__. #. An infinite loop in an RCU read-side critical section will eventually trigger an RCU CPU stall warning splat, with the duration of “eventually” being controlled by the ``RCU_CPU_STALL_TIMEOUT`` @@ -1638,9 +1639,9 @@ against mishaps and misuse: ``rcupdate.rcu_cpu_stall_suppress`` to suppress the splats. This kernel parameter may also be set via ``sysfs``. Furthermore, RCU CPU stall warnings are counter-productive during sysrq dumps and during - panics. RCU therefore supplies the ``rcu_sysrq_start()`` and - ``rcu_sysrq_end()`` API members to be called before and after long - sysrq dumps. RCU also supplies the ``rcu_panic()`` notifier that is + panics. RCU therefore supplies the rcu_sysrq_start() and + rcu_sysrq_end() API members to be called before and after long + sysrq dumps. RCU also supplies the rcu_panic() notifier that is automatically invoked at the beginning of a panic to suppress further RCU CPU stall warnings. @@ -1656,7 +1657,7 @@ against mishaps and misuse: synchronization mechanism, for example, reference counting. #. In kernels built with ``CONFIG_RCU_TRACE=y``, RCU-related information is provided via event tracing. -#. Open-coded use of ``rcu_assign_pointer()`` and ``rcu_dereference()`` +#. Open-coded use of rcu_assign_pointer() and rcu_dereference() to create typical linked data structures can be surprisingly error-prone. Therefore, RCU-protected `linked lists <https://lwn.net/Articles/609973/#RCU%20List%20APIs>`__ and, @@ -1665,12 +1666,11 @@ against mishaps and misuse: other special-purpose RCU-protected data structures are available in the Linux kernel and the userspace RCU library. #. Some linked structures are created at compile time, but still require - ``__rcu`` checking. The ``RCU_POINTER_INITIALIZER()`` macro serves + ``__rcu`` checking. The RCU_POINTER_INITIALIZER() macro serves this purpose. -#. It is not necessary to use ``rcu_assign_pointer()`` when creating +#. It is not necessary to use rcu_assign_pointer() when creating linked structures that are to be published via a single external - pointer. The ``RCU_INIT_POINTER()`` macro is provided for this task - and also for assigning ``NULL`` pointers at runtime. + pointer. The RCU_INIT_POINTER() macro is provided for this task. This not a hard-and-fast list: RCU's diagnostic capabilities will continue to be guided by the number and type of usage bugs found in @@ -1716,7 +1716,7 @@ requires almost all of them be hidden behind a ``CONFIG_RCU_EXPERT`` This all should be quite obvious, but the fact remains that Linus Torvalds recently had to -`remind <https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com>`__ +`remind <https://lore.kernel.org/r/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com>`__ me of this requirement. Firmware Interface @@ -1743,17 +1743,17 @@ Early Boot ~~~~~~~~~~ The Linux kernel's boot sequence is an interesting process, and RCU is -used early, even before ``rcu_init()`` is invoked. In fact, a number of +used early, even before rcu_init() is invoked. In fact, a number of RCU's primitives can be used as soon as the initial task's ``task_struct`` is available and the boot CPU's per-CPU variables are -set up. The read-side primitives (``rcu_read_lock()``, -``rcu_read_unlock()``, ``rcu_dereference()``, and -``rcu_access_pointer()``) will operate normally very early on, as will -``rcu_assign_pointer()``. +set up. The read-side primitives (rcu_read_lock(), +rcu_read_unlock(), rcu_dereference(), and +rcu_access_pointer()) will operate normally very early on, as will +rcu_assign_pointer(). -Although ``call_rcu()`` may be invoked at any time during boot, +Although call_rcu() may be invoked at any time during boot, callbacks are not guaranteed to be invoked until after all of RCU's -kthreads have been spawned, which occurs at ``early_initcall()`` time. +kthreads have been spawned, which occurs at early_initcall() time. This delay in callback invocation is due to the fact that RCU does not invoke callbacks until it is fully initialized, and this full initialization cannot occur until after the scheduler has initialized @@ -1762,22 +1762,22 @@ it would be possible to invoke callbacks earlier, however, this is not a panacea because there would be severe restrictions on what operations those callbacks could invoke. -Perhaps surprisingly, ``synchronize_rcu()`` and -``synchronize_rcu_expedited()``, will operate normally during very early +Perhaps surprisingly, synchronize_rcu() and +synchronize_rcu_expedited(), will operate normally during very early boot, the reason being that there is only one CPU and preemption is -disabled. This means that the call ``synchronize_rcu()`` (or friends) +disabled. This means that the call synchronize_rcu() (or friends) itself is a quiescent state and thus a grace period, so the early-boot implementation can be a no-op. However, once the scheduler has spawned its first kthread, this early -boot trick fails for ``synchronize_rcu()`` (as well as for -``synchronize_rcu_expedited()``) in ``CONFIG_PREEMPT=y`` kernels. The +boot trick fails for synchronize_rcu() (as well as for +synchronize_rcu_expedited()) in ``CONFIG_PREEMPTION=y`` kernels. The reason is that an RCU read-side critical section might be preempted, -which means that a subsequent ``synchronize_rcu()`` really does have to +which means that a subsequent synchronize_rcu() really does have to wait for something, as opposed to simply returning immediately. -Unfortunately, ``synchronize_rcu()`` can't do this until all of its +Unfortunately, synchronize_rcu() can't do this until all of its kthreads are spawned, which doesn't happen until some time during -``early_initcalls()`` time. But this is no excuse: RCU is nevertheless +early_initcalls() time. But this is no excuse: RCU is nevertheless required to correctly handle synchronous grace periods during this time period. Once all of its kthreads are up and running, RCU starts running normally. @@ -1820,7 +1820,7 @@ Interrupts and NMIs The Linux kernel has interrupts, and RCU read-side critical sections are legal within interrupt handlers and within interrupt-disabled regions of -code, as are invocations of ``call_rcu()``. +code, as are invocations of call_rcu(). Some Linux-kernel architectures can enter an interrupt handler from non-idle process context, and then just never leave it, instead @@ -1832,22 +1832,22 @@ way during a rewrite of RCU's dyntick-idle code. The Linux kernel has non-maskable interrupts (NMIs), and RCU read-side critical sections are legal within NMI handlers. Thankfully, RCU -update-side primitives, including ``call_rcu()``, are prohibited within +update-side primitives, including call_rcu(), are prohibited within NMI handlers. The name notwithstanding, some Linux-kernel architectures can have nested NMIs, which RCU must handle correctly. Andy Lutomirski `surprised -me <https://lkml.kernel.org/r/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com>`__ +me <https://lore.kernel.org/r/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com>`__ with this requirement; he also kindly surprised me with `an -algorithm <https://lkml.kernel.org/r/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com>`__ +algorithm <https://lore.kernel.org/r/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com>`__ that meets this requirement. Furthermore, NMI handlers can be interrupted by what appear to RCU to be normal interrupts. One way that this can happen is for code that -directly invokes ``rcu_irq_enter()`` and ``rcu_irq_exit()`` to be called +directly invokes ct_irq_enter() and ct_irq_exit() to be called from an NMI handler. This astonishing fact of life prompted the current -code structure, which has ``rcu_irq_enter()`` invoking -``rcu_nmi_enter()`` and ``rcu_irq_exit()`` invoking ``rcu_nmi_exit()``. +code structure, which has ct_irq_enter() invoking +ct_nmi_enter() and ct_irq_exit() invoking ct_nmi_exit(). And yes, I also learned of this requirement the hard way. Loadable Modules @@ -1857,45 +1857,45 @@ The Linux kernel has loadable modules, and these modules can also be unloaded. After a given module has been unloaded, any attempt to call one of its functions results in a segmentation fault. The module-unload functions must therefore cancel any delayed calls to loadable-module -functions, for example, any outstanding ``mod_timer()`` must be dealt -with via ``del_timer_sync()`` or similar. +functions, for example, any outstanding mod_timer() must be dealt +with via del_timer_sync() or similar. Unfortunately, there is no way to cancel an RCU callback; once you -invoke ``call_rcu()``, the callback function is eventually going to be +invoke call_rcu(), the callback function is eventually going to be invoked, unless the system goes down first. Because it is normally considered socially irresponsible to crash the system in response to a module unload request, we need some other way to deal with in-flight RCU callbacks. -RCU therefore provides ``rcu_barrier()``, which waits until all +RCU therefore provides rcu_barrier(), which waits until all in-flight RCU callbacks have been invoked. If a module uses -``call_rcu()``, its exit function should therefore prevent any future -invocation of ``call_rcu()``, then invoke ``rcu_barrier()``. In theory, -the underlying module-unload code could invoke ``rcu_barrier()`` +call_rcu(), its exit function should therefore prevent any future +invocation of call_rcu(), then invoke rcu_barrier(). In theory, +the underlying module-unload code could invoke rcu_barrier() unconditionally, but in practice this would incur unacceptable latencies. Nikita Danilov noted this requirement for an analogous filesystem-unmount situation, and Dipankar Sarma incorporated -``rcu_barrier()`` into RCU. The need for ``rcu_barrier()`` for module +rcu_barrier() into RCU. The need for rcu_barrier() for module unloading became apparent later. .. important:: - The ``rcu_barrier()`` function is not, repeat, + The rcu_barrier() function is not, repeat, *not*, obligated to wait for a grace period. It is instead only required to wait for RCU callbacks that have already been posted. Therefore, if there are no RCU callbacks posted anywhere in the system, - ``rcu_barrier()`` is within its rights to return immediately. Even if - there are callbacks posted, ``rcu_barrier()`` does not necessarily need + rcu_barrier() is within its rights to return immediately. Even if + there are callbacks posted, rcu_barrier() does not necessarily need to wait for a grace period. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Wait a minute! Each RCU callbacks must wait for a grace period to | -| complete, and ``rcu_barrier()`` must wait for each pre-existing | -| callback to be invoked. Doesn't ``rcu_barrier()`` therefore need to | +| complete, and rcu_barrier() must wait for each pre-existing | +| callback to be invoked. Doesn't rcu_barrier() therefore need to | | wait for a full grace period if there is even one callback posted | | anywhere in the system? | +-----------------------------------------------------------------------+ @@ -1904,14 +1904,14 @@ unloading became apparent later. | Absolutely not!!! | | Yes, each RCU callbacks must wait for a grace period to complete, but | | it might well be partly (or even completely) finished waiting by the | -| time ``rcu_barrier()`` is invoked. In that case, ``rcu_barrier()`` | +| time rcu_barrier() is invoked. In that case, rcu_barrier() | | need only wait for the remaining portion of the grace period to | | elapse. So even if there are quite a few callbacks posted, | -| ``rcu_barrier()`` might well return quite quickly. | +| rcu_barrier() might well return quite quickly. | | | | So if you need to wait for a grace period as well as for all | | pre-existing callbacks, you will need to invoke both | -| ``synchronize_rcu()`` and ``rcu_barrier()``. If latency is a concern, | +| synchronize_rcu() and rcu_barrier(). If latency is a concern, | | you can always use workqueues to invoke them concurrently. | +-----------------------------------------------------------------------+ @@ -1920,7 +1920,7 @@ Hotplug CPU The Linux kernel supports CPU hotplug, which means that CPUs can come and go. It is of course illegal to use any RCU API member from an -offline CPU, with the exception of `SRCU <#Sleepable%20RCU>`__ read-side +offline CPU, with the exception of `SRCU <Sleepable RCU_>`__ read-side critical sections. This requirement was present from day one in DYNIX/ptx, but on the other hand, the Linux kernel's CPU-hotplug implementation is “interesting.” @@ -1929,76 +1929,77 @@ The Linux-kernel CPU-hotplug implementation has notifiers that are used to allow the various kernel subsystems (including RCU) to respond appropriately to a given CPU-hotplug operation. Most RCU operations may be invoked from CPU-hotplug notifiers, including even synchronous -grace-period operations such as ``synchronize_rcu()`` and -``synchronize_rcu_expedited()``. - -However, all-callback-wait operations such as ``rcu_barrier()`` are also -not supported, due to the fact that there are phases of CPU-hotplug -operations where the outgoing CPU's callbacks will not be invoked until -after the CPU-hotplug operation ends, which could also result in -deadlock. Furthermore, ``rcu_barrier()`` blocks CPU-hotplug operations -during its execution, which results in another type of deadlock when -invoked from a CPU-hotplug notifier. +grace-period operations such as (synchronize_rcu() and +synchronize_rcu_expedited()). However, these synchronous operations +do block and therefore cannot be invoked from notifiers that execute via +stop_machine(), specifically those between the ``CPUHP_AP_OFFLINE`` +and ``CPUHP_AP_ONLINE`` states. + +In addition, all-callback-wait operations such as rcu_barrier() may +not be invoked from any CPU-hotplug notifier. This restriction is due +to the fact that there are phases of CPU-hotplug operations where the +outgoing CPU's callbacks will not be invoked until after the CPU-hotplug +operation ends, which could also result in deadlock. Furthermore, +rcu_barrier() blocks CPU-hotplug operations during its execution, +which results in another type of deadlock when invoked from a CPU-hotplug +notifier. + +Finally, RCU must avoid deadlocks due to interaction between hotplug, +timers and grace period processing. It does so by maintaining its own set +of books that duplicate the centrally maintained ``cpu_online_mask``, +and also by reporting quiescent states explicitly when a CPU goes +offline. This explicit reporting of quiescent states avoids any need +for the force-quiescent-state loop (FQS) to report quiescent states for +offline CPUs. However, as a debugging measure, the FQS loop does splat +if offline CPUs block an RCU grace period for too long. + +An offline CPU's quiescent state will be reported either: + +1. As the CPU goes offline using RCU's hotplug notifier (rcu_report_dead()). +2. When grace period initialization (rcu_gp_init()) detects a + race either with CPU offlining or with a task unblocking on a leaf + ``rcu_node`` structure whose CPUs are all offline. + +The CPU-online path (rcu_cpu_starting()) should never need to report +a quiescent state for an offline CPU. However, as a debugging measure, +it does emit a warning if a quiescent state was not already reported +for that CPU. + +During the checking/modification of RCU's hotplug bookkeeping, the +corresponding CPU's leaf node lock is held. This avoids race conditions +between RCU's hotplug notifier hooks, the grace period initialization +code, and the FQS loop, all of which refer to or modify this bookkeeping. Scheduler and RCU ~~~~~~~~~~~~~~~~~ -RCU depends on the scheduler, and the scheduler uses RCU to protect some -of its data structures. The preemptible-RCU ``rcu_read_unlock()`` -implementation must therefore be written carefully to avoid deadlocks -involving the scheduler's runqueue and priority-inheritance locks. In -particular, ``rcu_read_unlock()`` must tolerate an interrupt where the -interrupt handler invokes both ``rcu_read_lock()`` and -``rcu_read_unlock()``. This possibility requires ``rcu_read_unlock()`` -to use negative nesting levels to avoid destructive recursion via -interrupt handler's use of RCU. - -This scheduler-RCU requirement came as a `complete -surprise <https://lwn.net/Articles/453002/>`__. - -As noted above, RCU makes use of kthreads, and it is necessary to avoid -excessive CPU-time accumulation by these kthreads. This requirement was -no surprise, but RCU's violation of it when running context-switch-heavy -workloads when built with ``CONFIG_NO_HZ_FULL=y`` `did come as a -surprise +RCU makes use of kthreads, and it is necessary to avoid excessive CPU-time +accumulation by these kthreads. This requirement was no surprise, but +RCU's violation of it when running context-switch-heavy workloads when +built with ``CONFIG_NO_HZ_FULL=y`` `did come as a surprise [PDF] <http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf>`__. RCU has made good progress towards meeting this requirement, even for context-switch-heavy ``CONFIG_NO_HZ_FULL=y`` workloads, but there is room for further improvement. -It is forbidden to hold any of scheduler's runqueue or -priority-inheritance spinlocks across an ``rcu_read_unlock()`` unless -interrupts have been disabled across the entire RCU read-side critical -section, that is, up to and including the matching ``rcu_read_lock()``. -Violating this restriction can result in deadlocks involving these -scheduler spinlocks. There was hope that this restriction might be -lifted when interrupt-disabled calls to ``rcu_read_unlock()`` started -deferring the reporting of the resulting RCU-preempt quiescent state -until the end of the corresponding interrupts-disabled region. -Unfortunately, timely reporting of the corresponding quiescent state to -expedited grace periods requires a call to ``raise_softirq()``, which -can acquire these scheduler spinlocks. In addition, real-time systems -using RCU priority boosting need this restriction to remain in effect -because deferred quiescent-state reporting would also defer deboosting, -which in turn would degrade real-time latencies. - -In theory, if a given RCU read-side critical section could be guaranteed -to be less than one second in duration, holding a scheduler spinlock -across that critical section's ``rcu_read_unlock()`` would require only -that preemption be disabled across the entire RCU read-side critical -section, not interrupts. Unfortunately, given the possibility of vCPU -preemption, long-running interrupts, and so on, it is not possible in -practice to guarantee that a given RCU read-side critical section will -complete in less than one second. Therefore, as noted above, if -scheduler spinlocks are held across a given call to -``rcu_read_unlock()``, interrupts must be disabled across the entire RCU -read-side critical section. +There is no longer any prohibition against holding any of +scheduler's runqueue or priority-inheritance spinlocks across an +rcu_read_unlock(), even if interrupts and preemption were enabled +somewhere within the corresponding RCU read-side critical section. +Therefore, it is now perfectly legal to execute rcu_read_lock() +with preemption enabled, acquire one of the scheduler locks, and hold +that lock across the matching rcu_read_unlock(). + +Similarly, the RCU flavor consolidation has removed the need for negative +nesting. The fact that interrupt-disabled regions of code act as RCU +read-side critical sections implicitly avoids earlier issues that used +to result in destructive recursion via interrupt handler's use of RCU. Tracing and RCU ~~~~~~~~~~~~~~~ It is possible to use tracing on RCU code, but tracing itself uses RCU. -For this reason, ``rcu_dereference_raw_check()`` is provided for use +For this reason, rcu_dereference_raw_check() is provided for use by tracing, which avoids the destructive recursion that could otherwise ensue. This API is also used by virtualization in some architectures, where RCU readers execute in environments in which tracing cannot be @@ -2009,12 +2010,12 @@ Accesses to User Memory and RCU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The kernel needs to access user-space memory, for example, to access data -referenced by system-call parameters. The ``get_user()`` macro does this job. +referenced by system-call parameters. The get_user() macro does this job. However, user-space memory might well be paged out, which means that -``get_user()`` might well page-fault and thus block while waiting for the +get_user() might well page-fault and thus block while waiting for the resulting I/O to complete. It would be a very bad thing for the compiler to -reorder a ``get_user()`` invocation into an RCU read-side critical section. +reorder a get_user() invocation into an RCU read-side critical section. For example, suppose that the source code looked like this: @@ -2039,23 +2040,23 @@ the following: 5 rcu_read_unlock(); 6 do_something_with(v, user_v); -If the compiler did make this transformation in a ``CONFIG_PREEMPT=n`` kernel -build, and if ``get_user()`` did page fault, the result would be a quiescent +If the compiler did make this transformation in a ``CONFIG_PREEMPTION=n`` kernel +build, and if get_user() did page fault, the result would be a quiescent state in the middle of an RCU read-side critical section. This misplaced quiescent state could result in line 4 being a use-after-free access, which could be bad for your kernel's actuarial statistics. Similar examples -can be constructed with the call to ``get_user()`` preceding the -``rcu_read_lock()``. +can be constructed with the call to get_user() preceding the +rcu_read_lock(). -Unfortunately, ``get_user()`` doesn't have any particular ordering properties, +Unfortunately, get_user() doesn't have any particular ordering properties, and in some architectures the underlying ``asm`` isn't even marked ``volatile``. And even if it was marked ``volatile``, the above access to ``p->value`` is not volatile, so the compiler would not have any reason to keep those two accesses in order. -Therefore, the Linux-kernel definitions of ``rcu_read_lock()`` and -``rcu_read_unlock()`` must act as compiler barriers, at least for outermost -instances of ``rcu_read_lock()`` and ``rcu_read_unlock()`` within a nested set +Therefore, the Linux-kernel definitions of rcu_read_lock() and +rcu_read_unlock() must act as compiler barriers, at least for outermost +instances of rcu_read_lock() and rcu_read_unlock() within a nested set of RCU read-side critical sections. Energy Efficiency @@ -2070,26 +2071,26 @@ call. Because RCU avoids interrupting idle CPUs, it is illegal to execute an RCU read-side critical section on an idle CPU. (Kernels built with -``CONFIG_PROVE_RCU=y`` will splat if you try it.) The ``RCU_NONIDLE()`` +``CONFIG_PROVE_RCU=y`` will splat if you try it.) The RCU_NONIDLE() macro and ``_rcuidle`` event tracing is provided to work around this -restriction. In addition, ``rcu_is_watching()`` may be used to test +restriction. In addition, rcu_is_watching() may be used to test whether or not it is currently legal to run RCU read-side critical sections on this CPU. I learned of the need for diagnostics on the one -hand and ``RCU_NONIDLE()`` on the other while inspecting idle-loop code. +hand and RCU_NONIDLE() on the other while inspecting idle-loop code. Steven Rostedt supplied ``_rcuidle`` event tracing, which is used quite heavily in the idle loop. However, there are some restrictions on the -code placed within ``RCU_NONIDLE()``: +code placed within RCU_NONIDLE(): #. Blocking is prohibited. In practice, this is not a serious restriction given that idle tasks are prohibited from blocking to begin with. -#. Although nesting ``RCU_NONIDLE()`` is permitted, they cannot nest +#. Although nesting RCU_NONIDLE() is permitted, they cannot nest indefinitely deeply. However, given that they can be nested on the order of a million deep, even on 32-bit systems, this should not be a serious restriction. This nesting limit would probably be reached long after the compiler OOMed or the stack overflowed. -#. Any code path that enters ``RCU_NONIDLE()`` must sequence out of that - same ``RCU_NONIDLE()``. For example, the following is grossly +#. Any code path that enters RCU_NONIDLE() must sequence out of that + same RCU_NONIDLE(). For example, the following is grossly illegal: :: @@ -2102,7 +2103,7 @@ code placed within ``RCU_NONIDLE()``: It is just as illegal to transfer control into the middle of - ``RCU_NONIDLE()``'s argument. Yes, in theory, you could transfer in + RCU_NONIDLE()'s argument. Yes, in theory, you could transfer in as long as you also transferred out, but in practice you could also expect to get sharply worded review comments. @@ -2176,7 +2177,7 @@ handles these states differently: However, RCU must be reliably informed as to whether any given CPU is currently in the idle loop, and, for ``NO_HZ_FULL``, also whether that CPU is executing in usermode, as discussed -`earlier <#Energy%20Efficiency>`__. It also requires that the +`earlier <Energy Efficiency_>`__. It also requires that the scheduling-clock interrupt be enabled when RCU needs it to be: #. If a CPU is either idle or executing in usermode, and RCU believes it @@ -2191,12 +2192,12 @@ scheduling-clock interrupt be enabled when RCU needs it to be: this sort of thing. #. If a CPU is in a portion of the kernel that is absolutely positively no-joking guaranteed to never execute any RCU read-side critical - sections, and RCU believes this CPU to to be idle, no problem. This + sections, and RCU believes this CPU to be idle, no problem. This sort of thing is used by some architectures for light-weight exception handlers, which can then avoid the overhead of - ``rcu_irq_enter()`` and ``rcu_irq_exit()`` at exception entry and + ct_irq_enter() and ct_irq_exit() at exception entry and exit, respectively. Some go further and avoid the entireties of - ``irq_enter()`` and ``irq_exit()``. + irq_enter() and irq_exit(). Just make very sure you are running some of your tests with ``CONFIG_PROVE_RCU=y``, just in case one of your code paths was in fact joking about not doing RCU read-side critical sections. @@ -2220,12 +2221,12 @@ scheduling-clock interrupt be enabled when RCU needs it to be: | **Quick Quiz**: | +-----------------------------------------------------------------------+ | But what if my driver has a hardware interrupt handler that can run | -| for many seconds? I cannot invoke ``schedule()`` from an hardware | +| for many seconds? I cannot invoke schedule() from an hardware | | interrupt handler, after all! | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| One approach is to do ``rcu_irq_exit();rcu_irq_enter();`` every so | +| One approach is to do ``ct_irq_exit();ct_irq_enter();`` every so | | often. But given that long-running interrupt handlers can cause other | | problems, not least for response time, shouldn't you work to keep | | your interrupt handler's runtime within reasonable bounds? | @@ -2242,8 +2243,8 @@ Memory Efficiency Although small-memory non-realtime systems can simply use Tiny RCU, code size is only one aspect of memory efficiency. Another aspect is the size -of the ``rcu_head`` structure used by ``call_rcu()`` and -``kfree_rcu()``. Although this structure contains nothing more than a +of the ``rcu_head`` structure used by call_rcu() and +kfree_rcu(). Although this structure contains nothing more than a pair of pointers, it does appear in many RCU-protected data structures, including some that are size critical. The ``page`` structure is a case in point, as evidenced by the many occurrences of the ``union`` keyword @@ -2253,7 +2254,7 @@ This need for memory efficiency is one reason that RCU uses hand-crafted singly linked lists to track the ``rcu_head`` structures that are waiting for a grace period to elapse. It is also the reason why ``rcu_head`` structures do not contain debug information, such as fields -tracking the file and line of the ``call_rcu()`` or ``kfree_rcu()`` that +tracking the file and line of the call_rcu() or kfree_rcu() that posted them. Although this information might appear in debug-only kernel builds at some point, in the meantime, the ``->func`` field will often provide the needed debug information. @@ -2263,18 +2264,18 @@ more extreme measures. Returning to the ``page`` structure, the ``rcu_head`` field shares storage with a great many other structures that are used at various points in the corresponding page's lifetime. In order to correctly resolve certain `race -conditions <https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com>`__, +conditions <https://lore.kernel.org/r/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com>`__, the Linux kernel's memory-management subsystem needs a particular bit to remain zero during all phases of grace-period processing, and that bit happens to map to the bottom bit of the ``rcu_head`` structure's -``->next`` field. RCU makes this guarantee as long as ``call_rcu()`` is -used to post the callback, as opposed to ``kfree_rcu()`` or some future -“lazy” variant of ``call_rcu()`` that might one day be created for +``->next`` field. RCU makes this guarantee as long as call_rcu() is +used to post the callback, as opposed to kfree_rcu() or some future +“lazy” variant of call_rcu() that might one day be created for energy-efficiency purposes. That said, there are limits. RCU requires that the ``rcu_head`` structure be aligned to a two-byte boundary, and passing a misaligned -``rcu_head`` structure to one of the ``call_rcu()`` family of functions +``rcu_head`` structure to one of the call_rcu() family of functions will result in a splat. It is therefore necessary to exercise caution when packing structures containing fields of type ``rcu_head``. Why not a four-byte or even eight-byte alignment requirement? Because the m68k @@ -2293,12 +2294,12 @@ Performance, Scalability, Response Time, and Reliability ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Expanding on the `earlier -discussion <#Performance%20and%20Scalability>`__, RCU is used heavily by +discussion <Performance and Scalability_>`__, RCU is used heavily by hot code paths in performance-critical portions of the Linux kernel's networking, security, virtualization, and scheduling code paths. RCU must therefore use efficient implementations, especially in its read-side primitives. To that end, it would be good if preemptible RCU's -implementation of ``rcu_read_lock()`` could be inlined, however, doing +implementation of rcu_read_lock() could be inlined, however, doing this requires resolving ``#include`` issues with the ``task_struct`` structure. @@ -2311,23 +2312,23 @@ on the ``rcu_node`` structure. RCU is required to tolerate all CPUs continuously invoking any combination of RCU's runtime primitives with minimal per-operation overhead. In fact, in many cases, increasing load must *decrease* the per-operation overhead, witness the batching -optimizations for ``synchronize_rcu()``, ``call_rcu()``, -``synchronize_rcu_expedited()``, and ``rcu_barrier()``. As a general +optimizations for synchronize_rcu(), call_rcu(), +synchronize_rcu_expedited(), and rcu_barrier(). As a general rule, RCU must cheerfully accept whatever the rest of the Linux kernel decides to throw at it. The Linux kernel is used for real-time workloads, especially in conjunction with the `-rt -patchset <https://rt.wiki.kernel.org/index.php/Main_Page>`__. The +patchset <https://wiki.linuxfoundation.org/realtime/>`__. The real-time-latency response requirements are such that the traditional approach of disabling preemption across RCU read-side critical sections -is inappropriate. Kernels built with ``CONFIG_PREEMPT=y`` therefore use +is inappropriate. Kernels built with ``CONFIG_PREEMPTION=y`` therefore use an RCU implementation that allows RCU read-side critical sections to be preempted. This requirement made its presence known after users made it clear that an earlier `real-time patch <https://lwn.net/Articles/107930/>`__ did not meet their needs, in conjunction with some `RCU -issues <https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com>`__ +issues <https://lore.kernel.org/r/20050318002026.GA2693@us.ibm.com>`__ encountered by a very early version of the -rt patchset. In addition, RCU must make do with a sub-100-microsecond real-time @@ -2345,7 +2346,7 @@ number of race conditions. RCU must avoid degrading real-time response for CPU-bound threads, whether executing in usermode (which is one use case for ``CONFIG_NO_HZ_FULL=y``) or in the kernel. That said, CPU-bound loops in -the kernel must execute ``cond_resched()`` at least once per few tens of +the kernel must execute cond_resched() at least once per few tens of milliseconds in order to avoid receiving an IPI from RCU. Finally, RCU's status as a synchronization primitive means that any RCU @@ -2411,7 +2412,7 @@ grace periods from ever ending. The result was an out-of-memory condition and a system hang. The solution was the creation of RCU-bh, which does -``local_bh_disable()`` across its read-side critical sections, and which +local_bh_disable() across its read-side critical sections, and which uses the transition from one type of softirq processing to another as a quiescent state in addition to context switch, idle, user mode, and offline. This means that RCU-bh grace periods can complete even when @@ -2419,31 +2420,31 @@ some of the CPUs execute in softirq indefinitely, thus allowing algorithms based on RCU-bh to withstand network-based denial-of-service attacks. -Because ``rcu_read_lock_bh()`` and ``rcu_read_unlock_bh()`` disable and +Because rcu_read_lock_bh() and rcu_read_unlock_bh() disable and re-enable softirq handlers, any attempt to start a softirq handlers during the RCU-bh read-side critical section will be deferred. In this -case, ``rcu_read_unlock_bh()`` will invoke softirq processing, which can +case, rcu_read_unlock_bh() will invoke softirq processing, which can take considerable time. One can of course argue that this softirq overhead should be associated with the code following the RCU-bh -read-side critical section rather than ``rcu_read_unlock_bh()``, but the +read-side critical section rather than rcu_read_unlock_bh(), but the fact is that most profiling tools cannot be expected to make this sort of fine distinction. For example, suppose that a three-millisecond-long RCU-bh read-side critical section executes during a time of heavy networking load. There will very likely be an attempt to invoke at least one softirq handler during that three milliseconds, but any such invocation will be delayed until the time of the -``rcu_read_unlock_bh()``. This can of course make it appear at first -glance as if ``rcu_read_unlock_bh()`` was executing very slowly. +rcu_read_unlock_bh(). This can of course make it appear at first +glance as if rcu_read_unlock_bh() was executing very slowly. The `RCU-bh API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``rcu_read_lock_bh()``, ``rcu_read_unlock_bh()``, -``rcu_dereference_bh()``, ``rcu_dereference_bh_check()``, -``synchronize_rcu_bh()``, ``synchronize_rcu_bh_expedited()``, -``call_rcu_bh()``, ``rcu_barrier_bh()``, and -``rcu_read_lock_bh_held()``. However, the update-side APIs are now -simple wrappers for other RCU flavors, namely RCU-sched in -CONFIG_PREEMPT=n kernels and RCU-preempt otherwise. +includes rcu_read_lock_bh(), rcu_read_unlock_bh(), rcu_dereference_bh(), +rcu_dereference_bh_check(), and rcu_read_lock_bh_held(). However, the +old RCU-bh update-side APIs are now gone, replaced by synchronize_rcu(), +synchronize_rcu_expedited(), call_rcu(), and rcu_barrier(). In addition, +anything that disables bottom halves also marks an RCU-bh read-side +critical section, including local_bh_disable() and local_bh_enable(), +local_irq_save() and local_irq_restore(), and so on. Sched Flavor (Historical) ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -2460,33 +2461,33 @@ However, there are legitimate preemptible-RCU implementations that do not have this property, given that any point in the code outside of an RCU read-side critical section can be a quiescent state. Therefore, *RCU-sched* was created, which follows “classic” RCU in that an -RCU-sched grace period waits for for pre-existing interrupt and NMI -handlers. In kernels built with ``CONFIG_PREEMPT=n``, the RCU and +RCU-sched grace period waits for pre-existing interrupt and NMI +handlers. In kernels built with ``CONFIG_PREEMPTION=n``, the RCU and RCU-sched APIs have identical implementations, while kernels built with -``CONFIG_PREEMPT=y`` provide a separate implementation for each. +``CONFIG_PREEMPTION=y`` provide a separate implementation for each. -Note well that in ``CONFIG_PREEMPT=y`` kernels, -``rcu_read_lock_sched()`` and ``rcu_read_unlock_sched()`` disable and +Note well that in ``CONFIG_PREEMPTION=y`` kernels, +rcu_read_lock_sched() and rcu_read_unlock_sched() disable and re-enable preemption, respectively. This means that if there was a preemption attempt during the RCU-sched read-side critical section, -``rcu_read_unlock_sched()`` will enter the scheduler, with all the -latency and overhead entailed. Just as with ``rcu_read_unlock_bh()``, -this can make it look as if ``rcu_read_unlock_sched()`` was executing +rcu_read_unlock_sched() will enter the scheduler, with all the +latency and overhead entailed. Just as with rcu_read_unlock_bh(), +this can make it look as if rcu_read_unlock_sched() was executing very slowly. However, the highest-priority task won't be preempted, so -that task will enjoy low-overhead ``rcu_read_unlock_sched()`` +that task will enjoy low-overhead rcu_read_unlock_sched() invocations. The `RCU-sched API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``rcu_read_lock_sched()``, ``rcu_read_unlock_sched()``, -``rcu_read_lock_sched_notrace()``, ``rcu_read_unlock_sched_notrace()``, -``rcu_dereference_sched()``, ``rcu_dereference_sched_check()``, -``synchronize_sched()``, ``synchronize_rcu_sched_expedited()``, -``call_rcu_sched()``, ``rcu_barrier_sched()``, and -``rcu_read_lock_sched_held()``. However, anything that disables -preemption also marks an RCU-sched read-side critical section, including -``preempt_disable()`` and ``preempt_enable()``, ``local_irq_save()`` and -``local_irq_restore()``, and so on. +includes rcu_read_lock_sched(), rcu_read_unlock_sched(), +rcu_read_lock_sched_notrace(), rcu_read_unlock_sched_notrace(), +rcu_dereference_sched(), rcu_dereference_sched_check(), and +rcu_read_lock_sched_held(). However, the old RCU-sched update-side APIs +are now gone, replaced by synchronize_rcu(), synchronize_rcu_expedited(), +call_rcu(), and rcu_barrier(). In addition, anything that disables +preemption also marks an RCU-sched read-side critical section, +including preempt_disable() and preempt_enable(), local_irq_save() +and local_irq_restore(), and so on. Sleepable RCU ~~~~~~~~~~~~~ @@ -2508,7 +2509,7 @@ this structure must be passed in to each SRCU function, for example, structure. The key benefit of these domains is that a slow SRCU reader in one domain does not delay an SRCU grace period in some other domain. That said, one consequence of these domains is that read-side code must -pass a “cookie” from ``srcu_read_lock()`` to ``srcu_read_unlock()``, for +pass a “cookie” from srcu_read_lock() to srcu_read_unlock(), for example, as follows: :: @@ -2538,24 +2539,24 @@ period to elapse. For example, this results in a self-deadlock: 6 srcu_read_unlock(&ss, idx); However, if line 5 acquired a mutex that was held across a -``synchronize_srcu()`` for domain ``ss``, deadlock would still be +synchronize_srcu() for domain ``ss``, deadlock would still be possible. Furthermore, if line 5 acquired a mutex that was held across a -``synchronize_srcu()`` for some other domain ``ss1``, and if an +synchronize_srcu() for some other domain ``ss1``, and if an ``ss1``-domain SRCU read-side critical section acquired another mutex -that was held across as ``ss``-domain ``synchronize_srcu()``, deadlock +that was held across as ``ss``-domain synchronize_srcu(), deadlock would again be possible. Such a deadlock cycle could extend across an arbitrarily large number of different SRCU domains. Again, with great power comes great responsibility. Unlike the other RCU flavors, SRCU read-side critical sections can run on idle and even offline CPUs. This ability requires that -``srcu_read_lock()`` and ``srcu_read_unlock()`` contain memory barriers, +srcu_read_lock() and srcu_read_unlock() contain memory barriers, which means that SRCU readers will run a bit slower than would RCU -readers. It also motivates the ``smp_mb__after_srcu_read_unlock()`` API, -which, in combination with ``srcu_read_unlock()``, guarantees a full +readers. It also motivates the smp_mb__after_srcu_read_unlock() API, +which, in combination with srcu_read_unlock(), guarantees a full memory barrier. -Also unlike other RCU flavors, ``synchronize_srcu()`` may **not** be +Also unlike other RCU flavors, synchronize_srcu() may **not** be invoked from CPU-hotplug notifiers, due to the fact that SRCU grace periods make use of timers and the possibility of timers being temporarily “stranded” on the outgoing CPU. This stranding of timers @@ -2564,7 +2565,7 @@ the CPU-hotplug process. The problem is that if a notifier is waiting on an SRCU grace period, that grace period is waiting on a timer, and that timer is stranded on the outgoing CPU, then the notifier will never be awakened, in other words, deadlock has occurred. This same situation of -course also prohibits ``srcu_barrier()`` from being invoked from +course also prohibits srcu_barrier() from being invoked from CPU-hotplug notifiers. SRCU also differs from other RCU flavors in that SRCU's expedited and @@ -2575,12 +2576,12 @@ have not yet completed. (But please note that this is a property of the current implementation, not necessarily of future implementations.) In addition, if SRCU has been idle for longer than the interval specified by the ``srcutree.exp_holdoff`` kernel boot parameter (25 microseconds -by default), and if a ``synchronize_srcu()`` invocation ends this idle +by default), and if a synchronize_srcu() invocation ends this idle period, that invocation will be automatically expedited. As of v4.12, SRCU's callbacks are maintained per-CPU, eliminating a locking bottleneck present in prior kernel versions. Although this will -allow users to put much heavier stress on ``call_srcu()``, it is +allow users to put much heavier stress on call_srcu(), it is important to note that SRCU does not yet take any special steps to deal with callback flooding. So if you are posting (say) 10,000 SRCU callbacks per second per CPU, you are probably totally OK, but if you @@ -2591,14 +2592,32 @@ of your CPUs and the size of your memory. The `SRCU API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``srcu_read_lock()``, ``srcu_read_unlock()``, -``srcu_dereference()``, ``srcu_dereference_check()``, -``synchronize_srcu()``, ``synchronize_srcu_expedited()``, -``call_srcu()``, ``srcu_barrier()``, and ``srcu_read_lock_held()``. It -also includes ``DEFINE_SRCU()``, ``DEFINE_STATIC_SRCU()``, and -``init_srcu_struct()`` APIs for defining and initializing +includes srcu_read_lock(), srcu_read_unlock(), +srcu_dereference(), srcu_dereference_check(), +synchronize_srcu(), synchronize_srcu_expedited(), +call_srcu(), srcu_barrier(), and srcu_read_lock_held(). It +also includes DEFINE_SRCU(), DEFINE_STATIC_SRCU(), and +init_srcu_struct() APIs for defining and initializing ``srcu_struct`` structures. +More recently, the SRCU API has added polling interfaces: + +#. start_poll_synchronize_srcu() returns a cookie identifying + the completion of a future SRCU grace period and ensures + that this grace period will be started. +#. poll_state_synchronize_srcu() returns ``true`` iff the + specified cookie corresponds to an already-completed + SRCU grace period. +#. get_state_synchronize_srcu() returns a cookie just like + start_poll_synchronize_srcu() does, but differs in that + it does nothing to ensure that any future SRCU grace period + will be started. + +These functions are used to avoid unnecessary SRCU grace periods in +certain types of buffer-cache algorithms having multi-stage age-out +mechanisms. The idea is that by the time the block has aged completely +from the cache, an SRCU grace period will be very likely to have elapsed. + Tasks RCU ~~~~~~~~~ @@ -2607,29 +2626,66 @@ required to install different types of probes. It would be good to be able to free old trampolines, which sounds like a job for some form of RCU. However, because it is necessary to be able to install a trace anywhere in the code, it is not possible to use read-side markers such -as ``rcu_read_lock()`` and ``rcu_read_unlock()``. In addition, it does +as rcu_read_lock() and rcu_read_unlock(). In addition, it does not work to have these markers in the trampoline itself, because there -would need to be instructions following ``rcu_read_unlock()``. Although -``synchronize_rcu()`` would guarantee that execution reached the -``rcu_read_unlock()``, it would not be able to guarantee that execution -had completely left the trampoline. +would need to be instructions following rcu_read_unlock(). Although +synchronize_rcu() would guarantee that execution reached the +rcu_read_unlock(), it would not be able to guarantee that execution +had completely left the trampoline. Worse yet, in some situations +the trampoline's protection must extend a few instructions *prior* to +execution reaching the trampoline. For example, these few instructions +might calculate the address of the trampoline, so that entering the +trampoline would be pre-ordained a surprisingly long time before execution +actually reached the trampoline itself. The solution, in the form of `Tasks RCU <https://lwn.net/Articles/607117/>`__, is to have implicit read-side critical sections that are delimited by voluntary context switches, that -is, calls to ``schedule()``, ``cond_resched()``, and -``synchronize_rcu_tasks()``. In addition, transitions to and from +is, calls to schedule(), cond_resched(), and +synchronize_rcu_tasks(). In addition, transitions to and from userspace execution also delimit tasks-RCU read-side critical sections. The tasks-RCU API is quite compact, consisting only of -``call_rcu_tasks()``, ``synchronize_rcu_tasks()``, and -``rcu_barrier_tasks()``. In ``CONFIG_PREEMPT=n`` kernels, trampolines -cannot be preempted, so these APIs map to ``call_rcu()``, -``synchronize_rcu()``, and ``rcu_barrier()``, respectively. In -``CONFIG_PREEMPT=y`` kernels, trampolines can be preempted, and these +call_rcu_tasks(), synchronize_rcu_tasks(), and +rcu_barrier_tasks(). In ``CONFIG_PREEMPTION=n`` kernels, trampolines +cannot be preempted, so these APIs map to call_rcu(), +synchronize_rcu(), and rcu_barrier(), respectively. In +``CONFIG_PREEMPTION=y`` kernels, trampolines can be preempted, and these three APIs are therefore implemented by separate functions that check for voluntary context switches. +Tasks Rude RCU +~~~~~~~~~~~~~~ + +Some forms of tracing need to wait for all preemption-disabled regions +of code running on any online CPU, including those executed when RCU is +not watching. This means that synchronize_rcu() is insufficient, and +Tasks Rude RCU must be used instead. This flavor of RCU does its work by +forcing a workqueue to be scheduled on each online CPU, hence the "Rude" +moniker. And this operation is considered to be quite rude by real-time +workloads that don't want their ``nohz_full`` CPUs receiving IPIs and +by battery-powered systems that don't want their idle CPUs to be awakened. + +The tasks-rude-RCU API is also reader-marking-free and thus quite compact, +consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(), +and rcu_barrier_tasks_rude(). + +Tasks Trace RCU +~~~~~~~~~~~~~~~ + +Some forms of tracing need to sleep in readers, but cannot tolerate +SRCU's read-side overhead, which includes a full memory barrier in both +srcu_read_lock() and srcu_read_unlock(). This need is handled by a +Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with +readers. Real-time systems that cannot tolerate IPIs may build their +kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at +the expense of adding full memory barriers to the read-side primitives. + +The tasks-trace-RCU API is also reasonably compact, +consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(), +rcu_read_lock_trace_held(), call_rcu_tasks_trace(), +synchronize_rcu_tasks_trace(), and rcu_barrier_tasks_trace(). + Possible Future Changes ----------------------- @@ -2640,8 +2696,8 @@ grace-period state machine so as to avoid the need for the additional latency. RCU disables CPU hotplug in a few places, perhaps most notably in the -``rcu_barrier()`` operations. If there is a strong reason to use -``rcu_barrier()`` in CPU-hotplug notifiers, it will be necessary to +rcu_barrier() operations. If there is a strong reason to use +rcu_barrier() in CPU-hotplug notifiers, it will be necessary to avoid disabling CPU hotplug. This would introduce some complexity, so there had better be a *very* good reason. @@ -2658,7 +2714,7 @@ However, this combining tree does not spread its memory across NUMA nodes nor does it align the CPU groups with hardware features such as sockets or cores. Such spreading and alignment is currently believed to be unnecessary because the hotpath read-side primitives do not access -the combining tree, nor does ``call_rcu()`` in the common case. If you +the combining tree, nor does call_rcu() in the common case. If you believe that your architecture needs such spreading and alignment, then your architecture should also benefit from the ``rcutree.rcu_fanout_leaf`` boot parameter, which can be set to the @@ -2679,7 +2735,7 @@ likely that adjustments will be required to more gracefully handle extreme loads. It might also be necessary to be able to relate CPU utilization by RCU's kthreads and softirq handlers to the code that instigated this CPU utilization. For example, RCU callback overhead -might be charged back to the originating ``call_rcu()`` instance, though +might be charged back to the originating call_rcu() instance, though probably not in production kernels. Additional work may be required to provide reasonable forward-progress diff --git a/Documentation/RCU/NMI-RCU.rst b/Documentation/RCU/NMI-RCU.rst index 180958388ff9..2a92bc685ef1 100644 --- a/Documentation/RCU/NMI-RCU.rst +++ b/Documentation/RCU/NMI-RCU.rst @@ -8,8 +8,7 @@ Although RCU is usually used to protect read-mostly data structures, it is possible to use RCU to provide dynamic non-maskable interrupt handlers, as well as dynamic irq handlers. This document describes how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer -work in "arch/x86/oprofile/nmi_timer_int.c" and in -"arch/x86/kernel/traps.c". +work in "arch/x86/kernel/traps.c". The relevant pieces of code are listed below, each followed by a brief explanation:: diff --git a/Documentation/RCU/RTFP.txt b/Documentation/RCU/RTFP.txt index 9bccf16736f7..588d97366a46 100644 --- a/Documentation/RCU/RTFP.txt +++ b/Documentation/RCU/RTFP.txt @@ -683,7 +683,7 @@ Orran Krieger and Rusty Russell and Dipankar Sarma and Maneesh Soni" ,month="October" ,year="2001" ,note="Available: -\url{http://lkml.org/lkml/2001/10/13/105} +\url{https://lore.kernel.org/r/Pine.LNX.4.33.0110131015410.8707-100000@penguin.transmeta.com} [Viewed August 21, 2004]" ,annotation={ } @@ -826,7 +826,7 @@ Symposium on Distributed Computing} ,month="October" ,year="2002" ,note="Available: -\url{https://lkml.org/lkml/2002/10/24/262} +\url{https://lore.kernel.org/r/3DB86B05.447E7410@us.ibm.com} [Viewed February 15, 2014]" ,annotation={ Mingming Cao's patch to introduce RCU to SysV IPC. @@ -839,7 +839,7 @@ Symposium on Distributed Computing} ,month="March" ,year="2003" ,note="Available: -\url{http://lkml.org/lkml/2003/3/9/205} +\url{https://lore.kernel.org/r/Pine.LNX.4.44.0303091831560.2129-100000@home.transmeta.com} [Viewed March 13, 2006]" ,annotation={ Linus suggests replacing brlock with RCU and/or seqlocks: @@ -847,7 +847,7 @@ Symposium on Distributed Computing} 'It's entirely possible that the current user could be replaced by RCU and/or seqlocks, and we could get rid of brlocks entirely.' . - Steve Hemminger responds by replacing them with RCU. + Stephen Hemminger responds by replacing them with RCU. } } @@ -1036,15 +1036,15 @@ Add per-cpu batch counter" ,annotation={ RCU runs reasonably on a 512-CPU SGI using Manfred Spraul's patches, which may be found at: - https://lkml.org/lkml/2004/5/20/49 (split vars into cachelines) - https://lkml.org/lkml/2004/5/22/114 (cpu_quiet() patch) - https://lkml.org/lkml/2004/5/25/24 (0/5) - https://lkml.org/lkml/2004/5/25/23 (1/5) - https://lkml.org/lkml/2004/5/25/265 (works for Jack) - https://lkml.org/lkml/2004/5/25/20 (2/5) - https://lkml.org/lkml/2004/5/25/22 (3/5) - https://lkml.org/lkml/2004/5/25/19 (4/5) - https://lkml.org/lkml/2004/5/25/21 (5/5) + https://lore.kernel.org/r/40AC9823.6020709@colorfullife.com (split vars into cachelines) + https://lore.kernel.org/r/Pine.LNX.4.44.0405222141260.11106-100000@dbl.q-ag.de (cpu_quiet() patch) + https://lore.kernel.org/r/200405250535.i4P5ZJo8017583@dbl.q-ag.de (0/5) + https://lore.kernel.org/r/200405250535.i4P5ZKAQ017591@dbl.q-ag.de (1/5) + https://lore.kernel.org/r/20040525203215.GB5127@sgi.com (works for Jack) + https://lore.kernel.org/r/200405250535.i4P5ZLiR017599@dbl.q-ag.de (2/5) + https://lore.kernel.org/r/200405250535.i4P5ZMFt017607@dbl.q-ag.de (3/5) + https://lore.kernel.org/r/200405250535.i4P5ZN6g017615@dbl.q-ag.de (4/5) + https://lore.kernel.org/r/200405250535.i4P5ZO7I017623@dbl.q-ag.de (5/5) } } @@ -1106,7 +1106,7 @@ Oregon Health and Sciences University" ,month="August" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/8/6/237} +\url{https://lore.kernel.org/r/20040807192424.GF3936@in.ibm.com} [Viewed June 8, 2010]" ,annotation={ Introduce rcu_dereference(). @@ -1119,7 +1119,7 @@ Oregon Health and Sciences University" ,month="August" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/8/30/87} +\url{https://lore.kernel.org/r/1093873222.984.12.camel@new.localdomain} [Viewed February 17, 2005]" ,annotation={ Uses active code in rcu_read_lock() and rcu_read_unlock() to @@ -1186,7 +1186,7 @@ Oregon Health and Sciences University" ,month="October" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/10/23/241} +\url{https://lore.kernel.org/r/20041023202723.GA1930@us.ibm.com} [Viewed June 8, 2010]" ,annotation={ Introduce rcu_assign_pointer(). @@ -1203,7 +1203,7 @@ Oregon Health and Sciences University" ,annotation={ James Morris posts Kaigai Kohei's patch to LKML. [Viewed December 10, 2004] - Kaigai's patch is at https://lkml.org/lkml/2004/9/27/52 + Kaigai's patch is at https://lore.kernel.org/r/200409271057.i8RAvcA1007873@mailsv.bs1.fc.nec.co.jp } } @@ -1241,7 +1241,7 @@ Oregon Health and Sciences University" ,year="2005" ,day="17" ,note="Available: -\url{http://lkml.org/lkml/2005/3/17/199} +\url{https://lore.kernel.org/r/20050318002026.GA2693@us.ibm.com} [Viewed September 5, 2005]" ,annotation={ First posting showing how RCU can be safely adapted for @@ -1256,7 +1256,7 @@ Oregon Health and Sciences University" ,year="2005" ,day="18" ,note="Available: -\url{http://lkml.org/lkml/2005/3/18/122} +\url{https://lore.kernel.org/r/Pine.OSF.4.05.10503181336310.2466-100000@da410.phys.au.dk} [Viewed March 30, 2006]" ,annotation={ Esben Neilsen suggests read-side suppression of grace-period @@ -1302,7 +1302,7 @@ Data Structures" ,month="May" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/5/9/185} +\url{https://lore.kernel.org/r/20050510012444.GA3011@us.ibm.com} [Viewed May 13, 2005]" ,annotation={ First publication of working lock-based deferred free patches @@ -1385,7 +1385,7 @@ Data Structures" ,day="1" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/8/1/155} +\url{https://lore.kernel.org/r/20050801171137.GA1754@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First operating counter-based realtime RCU patch posted to LKML. @@ -1399,7 +1399,7 @@ Data Structures" ,day="8" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/8/8/108} +\url{https://lore.kernel.org/r/20050808144216.GA1307@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First operating counter-based realtime RCU patch posted to LKML, @@ -1415,7 +1415,7 @@ Data Structures" ,day="1" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/10/1/70} +\url{https://lore.kernel.org/r/20051001182056.GA1613@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First rcutorture patch. @@ -1429,7 +1429,7 @@ Data Structures" ,day="6" ,year="2006" ,note="Available: -\url{https://lkml.org/lkml/2006/1/7/22} +\url{https://lore.kernel.org/r/20060106.231054.43576567.davem@davemloft.net} [Viewed February 29, 2012]" ,annotation={ David Miller's view on hashed arrays of locks: used to really @@ -1464,7 +1464,7 @@ Distributed Processing Symposium" ,day="20" ,year="2006" ,note="Available: -\url{http://lkml.org/lkml/2006/6/20/238} +\url{https://lore.kernel.org/r/20060408134707.22479.33814.sendpatchset@linux.site} [Viewed March 25, 2008]" ,annotation={ RCU-protected radix tree. @@ -1554,7 +1554,7 @@ Revised: ,day="28" ,year="2006" ,note="Available: -\url{http://lkml.org/lkml/2006/9/28/160} +\url{https://lore.kernel.org/r/20060928142616.GA20185@infradead.org} [Viewed March 27, 2008]" } @@ -1593,7 +1593,7 @@ Revised: ,year="2006" ,day=26 ,note="Available: -\url{http://lkml.org/lkml/2006/10/26/73} +\url{https://lore.kernel.org/r/20061026105731.GE11803@in.ibm.com} [Viewed January 26, 2009]" ,annotation={ RCU-based reader-writer lock that allows readers to proceed with @@ -1612,12 +1612,12 @@ Revised: ,year="2006" ,day=17 ,note="Available: -\url{http://lkml.org/lkml/2006/11/17/56} +\url{https://lore.kernel.org/r/20061117092925.GT7164@kernel.dk} [Viewed May 28, 2007]" ,annotation={ SRCU's grace periods are too slow for Jens, even after a factor-of-three speedup. - Sped-up version of SRCU at http://lkml.org/lkml/2006/11/17/359. + Sped-up version of SRCU at https://lore.kernel.org/r/20061118002845.GF2632@us.ibm.com. } } @@ -1629,7 +1629,7 @@ Revised: ,year="2006" ,day=19 ,note="Available: -\url{http://lkml.org/lkml/2006/11/19/69} +\url{https://lore.kernel.org/r/20061119190027.GA3676@oleg} [Viewed May 28, 2007]" ,annotation={ First cut of QRCU. Expanded/corrected versions followed. @@ -1644,7 +1644,7 @@ Revised: ,year="2006" ,day=30 ,note="Available: -\url{http://lkml.org/lkml/2006/11/29/330} +\url{https://lore.kernel.org/r/20061130015714.GC1350@oleg} [Viewed November 26, 2008]" ,annotation={ Expanded/corrected version of QRCU. @@ -1709,7 +1709,7 @@ Revised: ,year="2007" ,day=3 ,note="Available: -\url{http://lkml.org/lkml/2007/1/3/112} +\url{https://lore.kernel.org/r/20070103152738.GA16063@localdomain} [Viewed May 28, 2007]" ,annotation={ Patch for list_splice_rcu(). @@ -1737,7 +1737,7 @@ Revised: ,year="2007" ,day=28 ,note="Available: -\url{http://lkml.org/lkml/2007/1/28/34} +\url{https://lore.kernel.org/r/20070128120509.719287000@programming.kicks-ass.net} [Viewed March 27, 2008]" ,annotation={ RCU-like implementation for frequent updaters and rare readers(!). @@ -1767,7 +1767,7 @@ Revised: ,year="2007" ,day=24 ,note="Available: -\url{http://lkml.org/lkml/2007/2/25/18} +\url{https://lore.kernel.org/r/20070225062349.GA17468@linux.vnet.ibm.com} [Viewed March 27, 2008]" ,annotation={ Patch for QRCU supplying lock-free fast path. @@ -1846,7 +1846,7 @@ Revised: ,annotation={ LWN article describing Promela and spin, and also using Oleg Nesterov's QRCU as an example (with Paul McKenney's fastpath). - Merged patch at: http://lkml.org/lkml/2007/2/25/18 + Merged patch at: https://lore.kernel.org/r/20070225062349.GA17468@linux.vnet.ibm.com } } @@ -1885,7 +1885,7 @@ Revised: ,day="10" ,year="2007" ,note="Available: -\url{http://lkml.org/lkml/2007/9/10/213} +\url{https://lore.kernel.org/r/20070910183004.GA3299@linux.vnet.ibm.com} [Viewed October 25, 2007]" ,annotation={ Final patch for preemptable RCU to -rt. (Later patches were @@ -1933,7 +1933,7 @@ Revised: ,day="20" ,year="2007" ,note="Available: -\url{http://lkml.org/lkml/2007/12/20/244} +\url{https://lore.kernel.org/r/20071220142540.GB22523@Krystal} [Viewed March 27, 2008]" ,annotation={ Request for call_rcu_sched() and rcu_barrier_sched(). @@ -2013,7 +2013,7 @@ Revised: ,day="29" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/1/29/208} +\url{https://lore.kernel.org/r/Pine.LNX.4.58.0801291113350.20371@gandalf.stny.rr.com} [Viewed March 27, 2008]" ,annotation={ Patch that prevents preemptible RCU from unnecessarily waking @@ -2028,7 +2028,7 @@ Revised: ,day="1" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/2/2/255} +\url{https://lore.kernel.org/r/20080202214124.GA28612@linux.vnet.ibm.com} [Viewed October 18, 2008]" ,annotation={ Explanation of compilers violating dependency ordering. @@ -2088,7 +2088,7 @@ lot of {Linux} into your technology!!!" ,day="3" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/6/2/539} +\url{https://lore.kernel.org/r/4844BE83.5010401@cn.fujitsu.com} [Viewed December 10, 2008]" ,annotation={ Updated RCU classic algorithm. Introduced multi-tailed list @@ -2122,7 +2122,7 @@ lot of {Linux} into your technology!!!" ,day="21" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/8/21/336} +\url{https://lore.kernel.org/r/48AD8969.7060900@colorfullife.com} [Viewed December 8, 2008]" ,annotation={ State-based RCU. One key thing that this patch does is to @@ -2137,7 +2137,7 @@ lot of {Linux} into your technology!!!" ,day="6" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/9/6/86} +\url{https://lore.kernel.org/r/48C2B1D2.5070801@colorfullife.com} [Viewed December 8, 2008]" ,annotation={ Manfred notes a fix required to my attempt to separate irq @@ -2183,7 +2183,7 @@ lot of {Linux} into your technology!!!" ,day="14" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/1/14/449} +\url{https://lore.kernel.org/r/20090114202044.GJ6734@linux.vnet.ibm.com} [Viewed January 15, 2009]" ,annotation={ Small-footprint implementation of RCU for uniprocessor @@ -2218,7 +2218,7 @@ lot of {Linux} into your technology!!!" git://lttng.org/userspace-rcu.git http://lttng.org/cgi-bin/gitweb.cgi?p=userspace-rcu.git http://lttng.org/urcu - http://lkml.org/lkml/2009/2/5/572 + https://lore.kernel.org/r/20090206030543.GB8560@Krystal } } @@ -2258,7 +2258,7 @@ lot of {Linux} into your technology!!!" ,day="25" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/6/25/306} +\url{https://lore.kernel.org/r/20090625160706.GA9467@linux.vnet.ibm.com} [Viewed August 16, 2009]" ,annotation={ First posting of expedited RCU to be accepted into -tip. @@ -2272,7 +2272,7 @@ lot of {Linux} into your technology!!!" ,day="23" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/7/23/294} +\url{https://lore.kernel.org/r/20090724001429.GA17374@linux.vnet.ibm.com} [Viewed August 15, 2009]" ,annotation={ First posting of simple and fast preemptable RCU. @@ -2350,7 +2350,7 @@ lot of {Linux} into your technology!!!" ,month="December" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/10/18/129} +\url{https://lore.kernel.org/r/20091018232918.GA7385@Krystal} [Viewed December 29, 2009]" ,annotation={ Mathieu proposed defer_rcu() with fixed-size per-thread pool @@ -2518,7 +2518,7 @@ lot of {Linux} into your technology!!!" ,month="January" ,year="2011" ,note="Available: -\url{https://lkml.org/lkml/2011/1/18/322} +\url{https://lore.kernel.org/r/AANLkTimajU0x1v6y3rH2+jr-bZ=tNLs1S_agXdGGAa3S@mail.gmail.com} [Viewed March 4, 2011]" ,annotation={ "The RCU-based name lookup is at the other end of the spectrum - the diff --git a/Documentation/RCU/arrayRCU.rst b/Documentation/RCU/arrayRCU.rst index 4051ea3871ef..a5f2ff8fc54c 100644 --- a/Documentation/RCU/arrayRCU.rst +++ b/Documentation/RCU/arrayRCU.rst @@ -33,8 +33,8 @@ Situation 1: Hash Tables Hash tables are often implemented as an array, where each array entry has a linked-list hash chain. Each hash chain can be protected by RCU -as described in the listRCU.txt document. This approach also applies -to other array-of-list situations, such as radix trees. +as described in listRCU.rst. This approach also applies to other +array-of-list situations, such as radix trees. .. _static_arrays: diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst new file mode 100644 index 000000000000..048c5bc1f813 --- /dev/null +++ b/Documentation/RCU/checklist.rst @@ -0,0 +1,491 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================ +Review Checklist for RCU Patches +================================ + + +This document contains a checklist for producing and reviewing patches +that make use of RCU. Violating any of the rules listed below will +result in the same sorts of problems that leaving out a locking primitive +would cause. This list is based on experiences reviewing such patches +over a rather long period of time, but improvements are always welcome! + +0. Is RCU being applied to a read-mostly situation? If the data + structure is updated more than about 10% of the time, then you + should strongly consider some other approach, unless detailed + performance measurements show that RCU is nonetheless the right + tool for the job. Yes, RCU does reduce read-side overhead by + increasing write-side overhead, which is exactly why normal uses + of RCU will do much more reading than updating. + + Another exception is where performance is not an issue, and RCU + provides a simpler implementation. An example of this situation + is the dynamic NMI code in the Linux 2.6 kernel, at least on + architectures where NMIs are rare. + + Yet another exception is where the low real-time latency of RCU's + read-side primitives is critically important. + + One final exception is where RCU readers are used to prevent + the ABA problem (https://en.wikipedia.org/wiki/ABA_problem) + for lockless updates. This does result in the mildly + counter-intuitive situation where rcu_read_lock() and + rcu_read_unlock() are used to protect updates, however, this + approach provides the same potential simplifications that garbage + collectors do. + +1. Does the update code have proper mutual exclusion? + + RCU does allow *readers* to run (almost) naked, but *writers* must + still use some sort of mutual exclusion, such as: + + a. locking, + b. atomic operations, or + c. restricting updates to a single task. + + If you choose #b, be prepared to describe how you have handled + memory barriers on weakly ordered machines (pretty much all of + them -- even x86 allows later loads to be reordered to precede + earlier stores), and be prepared to explain why this added + complexity is worthwhile. If you choose #c, be prepared to + explain how this single task does not become a major bottleneck on + big multiprocessor machines (for example, if the task is updating + information relating to itself that other tasks can read, there + by definition can be no bottleneck). Note that the definition + of "large" has changed significantly: Eight CPUs was "large" + in the year 2000, but a hundred CPUs was unremarkable in 2017. + +2. Do the RCU read-side critical sections make proper use of + rcu_read_lock() and friends? These primitives are needed + to prevent grace periods from ending prematurely, which + could result in data being unceremoniously freed out from + under your read-side code, which can greatly increase the + actuarial risk of your kernel. + + As a rough rule of thumb, any dereference of an RCU-protected + pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(), + rcu_read_lock_sched(), or by the appropriate update-side lock. + Explicit disabling of preemption (preempt_disable(), for example) + can serve as rcu_read_lock_sched(), but is less readable and + prevents lockdep from detecting locking issues. + + Please not that you *cannot* rely on code known to be built + only in non-preemptible kernels. Such code can and will break, + especially in kernels built with CONFIG_PREEMPT_COUNT=y. + + Letting RCU-protected pointers "leak" out of an RCU read-side + critical section is every bit as bad as letting them leak out + from under a lock. Unless, of course, you have arranged some + other means of protection, such as a lock or a reference count + *before* letting them out of the RCU read-side critical section. + +3. Does the update code tolerate concurrent accesses? + + The whole point of RCU is to permit readers to run without + any locks or atomic operations. This means that readers will + be running while updates are in progress. There are a number + of ways to handle this concurrency, depending on the situation: + + a. Use the RCU variants of the list and hlist update + primitives to add, remove, and replace elements on + an RCU-protected list. Alternatively, use the other + RCU-protected data structures that have been added to + the Linux kernel. + + This is almost always the best approach. + + b. Proceed as in (a) above, but also maintain per-element + locks (that are acquired by both readers and writers) + that guard per-element state. Of course, fields that + the readers refrain from accessing can be guarded by + some other lock acquired only by updaters, if desired. + + This works quite well, also. + + c. Make updates appear atomic to readers. For example, + pointer updates to properly aligned fields will + appear atomic, as will individual atomic primitives. + Sequences of operations performed under a lock will *not* + appear to be atomic to RCU readers, nor will sequences + of multiple atomic primitives. + + This can work, but is starting to get a bit tricky. + + d. Carefully order the updates and the reads so that + readers see valid data at all phases of the update. + This is often more difficult than it sounds, especially + given modern CPUs' tendency to reorder memory references. + One must usually liberally sprinkle memory barriers + (smp_wmb(), smp_rmb(), smp_mb()) through the code, + making it difficult to understand and to test. + + It is usually better to group the changing data into + a separate structure, so that the change may be made + to appear atomic by updating a pointer to reference + a new structure containing updated values. + +4. Weakly ordered CPUs pose special challenges. Almost all CPUs + are weakly ordered -- even x86 CPUs allow later loads to be + reordered to precede earlier stores. RCU code must take all of + the following measures to prevent memory-corruption problems: + + a. Readers must maintain proper ordering of their memory + accesses. The rcu_dereference() primitive ensures that + the CPU picks up the pointer before it picks up the data + that the pointer points to. This really is necessary + on Alpha CPUs. + + The rcu_dereference() primitive is also an excellent + documentation aid, letting the person reading the + code know exactly which pointers are protected by RCU. + Please note that compilers can also reorder code, and + they are becoming increasingly aggressive about doing + just that. The rcu_dereference() primitive therefore also + prevents destructive compiler optimizations. However, + with a bit of devious creativity, it is possible to + mishandle the return value from rcu_dereference(). + Please see rcu_dereference.rst for more information. + + The rcu_dereference() primitive is used by the + various "_rcu()" list-traversal primitives, such + as the list_for_each_entry_rcu(). Note that it is + perfectly legal (if redundant) for update-side code to + use rcu_dereference() and the "_rcu()" list-traversal + primitives. This is particularly useful in code that + is common to readers and updaters. However, lockdep + will complain if you access rcu_dereference() outside + of an RCU read-side critical section. See lockdep.rst + to learn what to do about this. + + Of course, neither rcu_dereference() nor the "_rcu()" + list-traversal primitives can substitute for a good + concurrency design coordinating among multiple updaters. + + b. If the list macros are being used, the list_add_tail_rcu() + and list_add_rcu() primitives must be used in order + to prevent weakly ordered machines from misordering + structure initialization and pointer planting. + Similarly, if the hlist macros are being used, the + hlist_add_head_rcu() primitive is required. + + c. If the list macros are being used, the list_del_rcu() + primitive must be used to keep list_del()'s pointer + poisoning from inflicting toxic effects on concurrent + readers. Similarly, if the hlist macros are being used, + the hlist_del_rcu() primitive is required. + + The list_replace_rcu() and hlist_replace_rcu() primitives + may be used to replace an old structure with a new one + in their respective types of RCU-protected lists. + + d. Rules similar to (4b) and (4c) apply to the "hlist_nulls" + type of RCU-protected linked lists. + + e. Updates must ensure that initialization of a given + structure happens before pointers to that structure are + publicized. Use the rcu_assign_pointer() primitive + when publicizing a pointer to a structure that can + be traversed by an RCU read-side critical section. + +5. If call_rcu() or call_srcu() is used, the callback function will + be called from softirq context. In particular, it cannot block. + If you need the callback to block, run that code in a workqueue + handler scheduled from the callback. The queue_rcu_work() + function does this for you in the case of call_rcu(). + +6. Since synchronize_rcu() can block, it cannot be called + from any sort of irq context. The same rule applies + for synchronize_srcu(), synchronize_rcu_expedited(), and + synchronize_srcu_expedited(). + + The expedited forms of these primitives have the same semantics + as the non-expedited forms, but expediting is both expensive and + (with the exception of synchronize_srcu_expedited()) unfriendly + to real-time workloads. Use of the expedited primitives should + be restricted to rare configuration-change operations that would + not normally be undertaken while a real-time workload is running. + However, real-time workloads can use rcupdate.rcu_normal kernel + boot parameter to completely disable expedited grace periods, + though this might have performance implications. + + In particular, if you find yourself invoking one of the expedited + primitives repeatedly in a loop, please do everyone a favor: + Restructure your code so that it batches the updates, allowing + a single non-expedited primitive to cover the entire batch. + This will very likely be faster than the loop containing the + expedited primitive, and will be much much easier on the rest + of the system, especially to real-time workloads running on + the rest of the system. + +7. As of v4.20, a given kernel implements only one RCU flavor, which + is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. + If the updater uses call_rcu() or synchronize_rcu(), then + the corresponding readers may use: (1) rcu_read_lock() and + rcu_read_unlock(), (2) any pair of primitives that disables + and re-enables softirq, for example, rcu_read_lock_bh() and + rcu_read_unlock_bh(), or (3) any pair of primitives that disables + and re-enables preemption, for example, rcu_read_lock_sched() and + rcu_read_unlock_sched(). If the updater uses synchronize_srcu() + or call_srcu(), then the corresponding readers must use + srcu_read_lock() and srcu_read_unlock(), and with the same + srcu_struct. The rules for the expedited RCU grace-period-wait + primitives are the same as for their non-expedited counterparts. + + If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(), + then the readers must refrain from executing voluntary + context switches, that is, from blocking. If the updater uses + call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then + the corresponding readers must use rcu_read_lock_trace() and + rcu_read_unlock_trace(). If an updater uses call_rcu_tasks_rude() + or synchronize_rcu_tasks_rude(), then the corresponding readers + must use anything that disables interrupts. + + Mixing things up will result in confusion and broken kernels, and + has even resulted in an exploitable security issue. Therefore, + when using non-obvious pairs of primitives, commenting is + of course a must. One example of non-obvious pairing is + the XDP feature in networking, which calls BPF programs from + network-driver NAPI (softirq) context. BPF relies heavily on RCU + protection for its data structures, but because the BPF program + invocation happens entirely within a single local_bh_disable() + section in a NAPI poll cycle, this usage is safe. The reason + that this usage is safe is that readers can use anything that + disables BH when updaters use call_rcu() or synchronize_rcu(). + +8. Although synchronize_rcu() is slower than is call_rcu(), it + usually results in simpler code. So, unless update performance is + critically important, the updaters cannot block, or the latency of + synchronize_rcu() is visible from userspace, synchronize_rcu() + should be used in preference to call_rcu(). Furthermore, + kfree_rcu() usually results in even simpler code than does + synchronize_rcu() without synchronize_rcu()'s multi-millisecond + latency. So please take advantage of kfree_rcu()'s "fire and + forget" memory-freeing capabilities where it applies. + + An especially important property of the synchronize_rcu() + primitive is that it automatically self-limits: if grace periods + are delayed for whatever reason, then the synchronize_rcu() + primitive will correspondingly delay updates. In contrast, + code using call_rcu() should explicitly limit update rate in + cases where grace periods are delayed, as failing to do so can + result in excessive realtime latencies or even OOM conditions. + + Ways of gaining this self-limiting property when using call_rcu() + include: + + a. Keeping a count of the number of data-structure elements + used by the RCU-protected data structure, including + those waiting for a grace period to elapse. Enforce a + limit on this number, stalling updates as needed to allow + previously deferred frees to complete. Alternatively, + limit only the number awaiting deferred free rather than + the total number of elements. + + One way to stall the updates is to acquire the update-side + mutex. (Don't try this with a spinlock -- other CPUs + spinning on the lock could prevent the grace period + from ever ending.) Another way to stall the updates + is for the updates to use a wrapper function around + the memory allocator, so that this wrapper function + simulates OOM when there is too much memory awaiting an + RCU grace period. There are of course many other + variations on this theme. + + b. Limiting update rate. For example, if updates occur only + once per hour, then no explicit rate limiting is + required, unless your system is already badly broken. + Older versions of the dcache subsystem take this approach, + guarding updates with a global lock, limiting their rate. + + c. Trusted update -- if updates can only be done manually by + superuser or some other trusted user, then it might not + be necessary to automatically limit them. The theory + here is that superuser already has lots of ways to crash + the machine. + + d. Periodically invoke synchronize_rcu(), permitting a limited + number of updates per grace period. Better yet, periodically + invoke rcu_barrier() to wait for all outstanding callbacks. + + The same cautions apply to call_srcu() and kfree_rcu(). + + Note that although these primitives do take action to avoid memory + exhaustion when any given CPU has too many callbacks, a determined + user could still exhaust memory. This is especially the case + if a system with a large number of CPUs has been configured to + offload all of its RCU callbacks onto a single CPU, or if the + system has relatively little free memory. + +9. All RCU list-traversal primitives, which include + rcu_dereference(), list_for_each_entry_rcu(), and + list_for_each_safe_rcu(), must be either within an RCU read-side + critical section or must be protected by appropriate update-side + locks. RCU read-side critical sections are delimited by + rcu_read_lock() and rcu_read_unlock(), or by similar primitives + such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which + case the matching rcu_dereference() primitive must be used in + order to keep lockdep happy, in this case, rcu_dereference_bh(). + + The reason that it is permissible to use RCU list-traversal + primitives when the update-side lock is held is that doing so + can be quite helpful in reducing code bloat when common code is + shared between readers and updaters. Additional primitives + are provided for this case, as discussed in lockdep.rst. + + One exception to this rule is when data is only ever added to + the linked data structure, and is never removed during any + time that readers might be accessing that structure. In such + cases, READ_ONCE() may be used in place of rcu_dereference() + and the read-side markers (rcu_read_lock() and rcu_read_unlock(), + for example) may be omitted. + +10. Conversely, if you are in an RCU read-side critical section, + and you don't hold the appropriate update-side lock, you *must* + use the "_rcu()" variants of the list macros. Failing to do so + will break Alpha, cause aggressive compilers to generate bad code, + and confuse people trying to read your code. + +11. Any lock acquired by an RCU callback must be acquired elsewhere + with softirq disabled, e.g., via spin_lock_irqsave(), + spin_lock_bh(), etc. Failing to disable softirq on a given + acquisition of that lock will result in deadlock as soon as + the RCU softirq handler happens to run your RCU callback while + interrupting that acquisition's critical section. + +12. RCU callbacks can be and are executed in parallel. In many cases, + the callback code simply wrappers around kfree(), so that this + is not an issue (or, more accurately, to the extent that it is + an issue, the memory-allocator locking handles it). However, + if the callbacks do manipulate a shared data structure, they + must use whatever locking or other synchronization is required + to safely access and/or modify that data structure. + + Do not assume that RCU callbacks will be executed on the same + CPU that executed the corresponding call_rcu() or call_srcu(). + For example, if a given CPU goes offline while having an RCU + callback pending, then that RCU callback will execute on some + surviving CPU. (If this was not the case, a self-spawning RCU + callback would prevent the victim CPU from ever going offline.) + Furthermore, CPUs designated by rcu_nocbs= might well *always* + have their RCU callbacks executed on some other CPUs, in fact, + for some real-time workloads, this is the whole point of using + the rcu_nocbs= kernel boot parameter. + +13. Unlike other forms of RCU, it *is* permissible to block in an + SRCU read-side critical section (demarked by srcu_read_lock() + and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". + Please note that if you don't need to sleep in read-side critical + sections, you should be using RCU rather than SRCU, because RCU + is almost always faster and easier to use than is SRCU. + + Also unlike other forms of RCU, explicit initialization and + cleanup is required either at build time via DEFINE_SRCU() + or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() + and cleanup_srcu_struct(). These last two are passed a + "struct srcu_struct" that defines the scope of a given + SRCU domain. Once initialized, the srcu_struct is passed + to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), + synchronize_srcu_expedited(), and call_srcu(). A given + synchronize_srcu() waits only for SRCU read-side critical + sections governed by srcu_read_lock() and srcu_read_unlock() + calls that have been passed the same srcu_struct. This property + is what makes sleeping read-side critical sections tolerable -- + a given subsystem delays only its own updates, not those of other + subsystems using SRCU. Therefore, SRCU is less prone to OOM the + system than RCU would be if RCU's read-side critical sections + were permitted to sleep. + + The ability to sleep in read-side critical sections does not + come for free. First, corresponding srcu_read_lock() and + srcu_read_unlock() calls must be passed the same srcu_struct. + Second, grace-period-detection overhead is amortized only + over those updates sharing a given srcu_struct, rather than + being globally amortized as they are for other forms of RCU. + Therefore, SRCU should be used in preference to rw_semaphore + only in extremely read-intensive situations, or in situations + requiring SRCU's read-side deadlock immunity or low read-side + realtime latency. You should also consider percpu_rw_semaphore + when you need lightweight readers. + + SRCU's expedited primitive (synchronize_srcu_expedited()) + never sends IPIs to other CPUs, so it is easier on + real-time workloads than is synchronize_rcu_expedited(). + + Note that rcu_assign_pointer() relates to SRCU just as it does to + other forms of RCU, but instead of rcu_dereference() you should + use srcu_dereference() in order to avoid lockdep splats. + +14. The whole point of call_rcu(), synchronize_rcu(), and friends + is to wait until all pre-existing readers have finished before + carrying out some otherwise-destructive operation. It is + therefore critically important to *first* remove any path + that readers can follow that could be affected by the + destructive operation, and *only then* invoke call_rcu(), + synchronize_rcu(), or friends. + + Because these primitives only wait for pre-existing readers, it + is the caller's responsibility to guarantee that any subsequent + readers will execute safely. + +15. The various RCU read-side primitives do *not* necessarily contain + memory barriers. You should therefore plan for the CPU + and the compiler to freely reorder code into and out of RCU + read-side critical sections. It is the responsibility of the + RCU update-side primitives to deal with this. + + For SRCU readers, you can use smp_mb__after_srcu_read_unlock() + immediately after an srcu_read_unlock() to get a full barrier. + +16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the + __rcu sparse checks to validate your RCU code. These can help + find problems as follows: + + CONFIG_PROVE_LOCKING: + check that accesses to RCU-protected data + structures are carried out under the proper RCU + read-side critical section, while holding the right + combination of locks, or whatever other conditions + are appropriate. + + CONFIG_DEBUG_OBJECTS_RCU_HEAD: + check that you don't pass the + same object to call_rcu() (or friends) before an RCU + grace period has elapsed since the last time that you + passed that same object to call_rcu() (or friends). + + __rcu sparse checks: + tag the pointer to the RCU-protected data + structure with __rcu, and sparse will warn you if you + access that pointer without the services of one of the + variants of rcu_dereference(). + + These debugging aids can help you find problems that are + otherwise extremely difficult to spot. + +17. If you register a callback using call_rcu() or call_srcu(), and + pass in a function defined within a loadable module, then it in + necessary to wait for all pending callbacks to be invoked after + the last invocation and before unloading that module. Note that + it is absolutely *not* sufficient to wait for a grace period! + The current (say) synchronize_rcu() implementation is *not* + guaranteed to wait for callbacks registered on other CPUs. + Or even on the current CPU if that CPU recently went offline + and came back online. + + You instead need to use one of the barrier functions: + + - call_rcu() -> rcu_barrier() + - call_srcu() -> srcu_barrier() + + However, these barrier functions are absolutely *not* guaranteed + to wait for a grace period. In fact, if there are no call_rcu() + callbacks waiting anywhere in the system, rcu_barrier() is within + its rights to return immediately. + + So if you need to wait for both an RCU grace period and for + all pre-existing call_rcu() callbacks, you will need to execute + both rcu_barrier() and synchronize_rcu(), if necessary, using + something like workqueues to execute them concurrently. + + See rcubarrier.rst for more information. diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt deleted file mode 100644 index e98ff261a438..000000000000 --- a/Documentation/RCU/checklist.txt +++ /dev/null @@ -1,458 +0,0 @@ -Review Checklist for RCU Patches - - -This document contains a checklist for producing and reviewing patches -that make use of RCU. Violating any of the rules listed below will -result in the same sorts of problems that leaving out a locking primitive -would cause. This list is based on experiences reviewing such patches -over a rather long period of time, but improvements are always welcome! - -0. Is RCU being applied to a read-mostly situation? If the data - structure is updated more than about 10% of the time, then you - should strongly consider some other approach, unless detailed - performance measurements show that RCU is nonetheless the right - tool for the job. Yes, RCU does reduce read-side overhead by - increasing write-side overhead, which is exactly why normal uses - of RCU will do much more reading than updating. - - Another exception is where performance is not an issue, and RCU - provides a simpler implementation. An example of this situation - is the dynamic NMI code in the Linux 2.6 kernel, at least on - architectures where NMIs are rare. - - Yet another exception is where the low real-time latency of RCU's - read-side primitives is critically important. - - One final exception is where RCU readers are used to prevent - the ABA problem (https://en.wikipedia.org/wiki/ABA_problem) - for lockless updates. This does result in the mildly - counter-intuitive situation where rcu_read_lock() and - rcu_read_unlock() are used to protect updates, however, this - approach provides the same potential simplifications that garbage - collectors do. - -1. Does the update code have proper mutual exclusion? - - RCU does allow -readers- to run (almost) naked, but -writers- must - still use some sort of mutual exclusion, such as: - - a. locking, - b. atomic operations, or - c. restricting updates to a single task. - - If you choose #b, be prepared to describe how you have handled - memory barriers on weakly ordered machines (pretty much all of - them -- even x86 allows later loads to be reordered to precede - earlier stores), and be prepared to explain why this added - complexity is worthwhile. If you choose #c, be prepared to - explain how this single task does not become a major bottleneck on - big multiprocessor machines (for example, if the task is updating - information relating to itself that other tasks can read, there - by definition can be no bottleneck). Note that the definition - of "large" has changed significantly: Eight CPUs was "large" - in the year 2000, but a hundred CPUs was unremarkable in 2017. - -2. Do the RCU read-side critical sections make proper use of - rcu_read_lock() and friends? These primitives are needed - to prevent grace periods from ending prematurely, which - could result in data being unceremoniously freed out from - under your read-side code, which can greatly increase the - actuarial risk of your kernel. - - As a rough rule of thumb, any dereference of an RCU-protected - pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(), - rcu_read_lock_sched(), or by the appropriate update-side lock. - Disabling of preemption can serve as rcu_read_lock_sched(), but - is less readable and prevents lockdep from detecting locking issues. - - Letting RCU-protected pointers "leak" out of an RCU read-side - critical section is every bid as bad as letting them leak out - from under a lock. Unless, of course, you have arranged some - other means of protection, such as a lock or a reference count - -before- letting them out of the RCU read-side critical section. - -3. Does the update code tolerate concurrent accesses? - - The whole point of RCU is to permit readers to run without - any locks or atomic operations. This means that readers will - be running while updates are in progress. There are a number - of ways to handle this concurrency, depending on the situation: - - a. Use the RCU variants of the list and hlist update - primitives to add, remove, and replace elements on - an RCU-protected list. Alternatively, use the other - RCU-protected data structures that have been added to - the Linux kernel. - - This is almost always the best approach. - - b. Proceed as in (a) above, but also maintain per-element - locks (that are acquired by both readers and writers) - that guard per-element state. Of course, fields that - the readers refrain from accessing can be guarded by - some other lock acquired only by updaters, if desired. - - This works quite well, also. - - c. Make updates appear atomic to readers. For example, - pointer updates to properly aligned fields will - appear atomic, as will individual atomic primitives. - Sequences of operations performed under a lock will -not- - appear to be atomic to RCU readers, nor will sequences - of multiple atomic primitives. - - This can work, but is starting to get a bit tricky. - - d. Carefully order the updates and the reads so that - readers see valid data at all phases of the update. - This is often more difficult than it sounds, especially - given modern CPUs' tendency to reorder memory references. - One must usually liberally sprinkle memory barriers - (smp_wmb(), smp_rmb(), smp_mb()) through the code, - making it difficult to understand and to test. - - It is usually better to group the changing data into - a separate structure, so that the change may be made - to appear atomic by updating a pointer to reference - a new structure containing updated values. - -4. Weakly ordered CPUs pose special challenges. Almost all CPUs - are weakly ordered -- even x86 CPUs allow later loads to be - reordered to precede earlier stores. RCU code must take all of - the following measures to prevent memory-corruption problems: - - a. Readers must maintain proper ordering of their memory - accesses. The rcu_dereference() primitive ensures that - the CPU picks up the pointer before it picks up the data - that the pointer points to. This really is necessary - on Alpha CPUs. If you don't believe me, see: - - http://www.openvms.compaq.com/wizard/wiz_2637.html - - The rcu_dereference() primitive is also an excellent - documentation aid, letting the person reading the - code know exactly which pointers are protected by RCU. - Please note that compilers can also reorder code, and - they are becoming increasingly aggressive about doing - just that. The rcu_dereference() primitive therefore also - prevents destructive compiler optimizations. However, - with a bit of devious creativity, it is possible to - mishandle the return value from rcu_dereference(). - Please see rcu_dereference.txt in this directory for - more information. - - The rcu_dereference() primitive is used by the - various "_rcu()" list-traversal primitives, such - as the list_for_each_entry_rcu(). Note that it is - perfectly legal (if redundant) for update-side code to - use rcu_dereference() and the "_rcu()" list-traversal - primitives. This is particularly useful in code that - is common to readers and updaters. However, lockdep - will complain if you access rcu_dereference() outside - of an RCU read-side critical section. See lockdep.txt - to learn what to do about this. - - Of course, neither rcu_dereference() nor the "_rcu()" - list-traversal primitives can substitute for a good - concurrency design coordinating among multiple updaters. - - b. If the list macros are being used, the list_add_tail_rcu() - and list_add_rcu() primitives must be used in order - to prevent weakly ordered machines from misordering - structure initialization and pointer planting. - Similarly, if the hlist macros are being used, the - hlist_add_head_rcu() primitive is required. - - c. If the list macros are being used, the list_del_rcu() - primitive must be used to keep list_del()'s pointer - poisoning from inflicting toxic effects on concurrent - readers. Similarly, if the hlist macros are being used, - the hlist_del_rcu() primitive is required. - - The list_replace_rcu() and hlist_replace_rcu() primitives - may be used to replace an old structure with a new one - in their respective types of RCU-protected lists. - - d. Rules similar to (4b) and (4c) apply to the "hlist_nulls" - type of RCU-protected linked lists. - - e. Updates must ensure that initialization of a given - structure happens before pointers to that structure are - publicized. Use the rcu_assign_pointer() primitive - when publicizing a pointer to a structure that can - be traversed by an RCU read-side critical section. - -5. If call_rcu() or call_srcu() is used, the callback function will - be called from softirq context. In particular, it cannot block. - -6. Since synchronize_rcu() can block, it cannot be called - from any sort of irq context. The same rule applies - for synchronize_srcu(), synchronize_rcu_expedited(), and - synchronize_srcu_expedited(). - - The expedited forms of these primitives have the same semantics - as the non-expedited forms, but expediting is both expensive and - (with the exception of synchronize_srcu_expedited()) unfriendly - to real-time workloads. Use of the expedited primitives should - be restricted to rare configuration-change operations that would - not normally be undertaken while a real-time workload is running. - However, real-time workloads can use rcupdate.rcu_normal kernel - boot parameter to completely disable expedited grace periods, - though this might have performance implications. - - In particular, if you find yourself invoking one of the expedited - primitives repeatedly in a loop, please do everyone a favor: - Restructure your code so that it batches the updates, allowing - a single non-expedited primitive to cover the entire batch. - This will very likely be faster than the loop containing the - expedited primitive, and will be much much easier on the rest - of the system, especially to real-time workloads running on - the rest of the system. - -7. As of v4.20, a given kernel implements only one RCU flavor, - which is RCU-sched for PREEMPT=n and RCU-preempt for PREEMPT=y. - If the updater uses call_rcu() or synchronize_rcu(), - then the corresponding readers my use rcu_read_lock() and - rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(), - or any pair of primitives that disables and re-enables preemption, - for example, rcu_read_lock_sched() and rcu_read_unlock_sched(). - If the updater uses synchronize_srcu() or call_srcu(), - then the corresponding readers must use srcu_read_lock() and - srcu_read_unlock(), and with the same srcu_struct. The rules for - the expedited primitives are the same as for their non-expedited - counterparts. Mixing things up will result in confusion and - broken kernels, and has even resulted in an exploitable security - issue. - - One exception to this rule: rcu_read_lock() and rcu_read_unlock() - may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() - in cases where local bottom halves are already known to be - disabled, for example, in irq or softirq context. Commenting - such cases is a must, of course! And the jury is still out on - whether the increased speed is worth it. - -8. Although synchronize_rcu() is slower than is call_rcu(), it - usually results in simpler code. So, unless update performance is - critically important, the updaters cannot block, or the latency of - synchronize_rcu() is visible from userspace, synchronize_rcu() - should be used in preference to call_rcu(). Furthermore, - kfree_rcu() usually results in even simpler code than does - synchronize_rcu() without synchronize_rcu()'s multi-millisecond - latency. So please take advantage of kfree_rcu()'s "fire and - forget" memory-freeing capabilities where it applies. - - An especially important property of the synchronize_rcu() - primitive is that it automatically self-limits: if grace periods - are delayed for whatever reason, then the synchronize_rcu() - primitive will correspondingly delay updates. In contrast, - code using call_rcu() should explicitly limit update rate in - cases where grace periods are delayed, as failing to do so can - result in excessive realtime latencies or even OOM conditions. - - Ways of gaining this self-limiting property when using call_rcu() - include: - - a. Keeping a count of the number of data-structure elements - used by the RCU-protected data structure, including - those waiting for a grace period to elapse. Enforce a - limit on this number, stalling updates as needed to allow - previously deferred frees to complete. Alternatively, - limit only the number awaiting deferred free rather than - the total number of elements. - - One way to stall the updates is to acquire the update-side - mutex. (Don't try this with a spinlock -- other CPUs - spinning on the lock could prevent the grace period - from ever ending.) Another way to stall the updates - is for the updates to use a wrapper function around - the memory allocator, so that this wrapper function - simulates OOM when there is too much memory awaiting an - RCU grace period. There are of course many other - variations on this theme. - - b. Limiting update rate. For example, if updates occur only - once per hour, then no explicit rate limiting is - required, unless your system is already badly broken. - Older versions of the dcache subsystem take this approach, - guarding updates with a global lock, limiting their rate. - - c. Trusted update -- if updates can only be done manually by - superuser or some other trusted user, then it might not - be necessary to automatically limit them. The theory - here is that superuser already has lots of ways to crash - the machine. - - d. Periodically invoke synchronize_rcu(), permitting a limited - number of updates per grace period. - - The same cautions apply to call_srcu() and kfree_rcu(). - - Note that although these primitives do take action to avoid memory - exhaustion when any given CPU has too many callbacks, a determined - user could still exhaust memory. This is especially the case - if a system with a large number of CPUs has been configured to - offload all of its RCU callbacks onto a single CPU, or if the - system has relatively little free memory. - -9. All RCU list-traversal primitives, which include - rcu_dereference(), list_for_each_entry_rcu(), and - list_for_each_safe_rcu(), must be either within an RCU read-side - critical section or must be protected by appropriate update-side - locks. RCU read-side critical sections are delimited by - rcu_read_lock() and rcu_read_unlock(), or by similar primitives - such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which - case the matching rcu_dereference() primitive must be used in - order to keep lockdep happy, in this case, rcu_dereference_bh(). - - The reason that it is permissible to use RCU list-traversal - primitives when the update-side lock is held is that doing so - can be quite helpful in reducing code bloat when common code is - shared between readers and updaters. Additional primitives - are provided for this case, as discussed in lockdep.txt. - -10. Conversely, if you are in an RCU read-side critical section, - and you don't hold the appropriate update-side lock, you -must- - use the "_rcu()" variants of the list macros. Failing to do so - will break Alpha, cause aggressive compilers to generate bad code, - and confuse people trying to read your code. - -11. Any lock acquired by an RCU callback must be acquired elsewhere - with softirq disabled, e.g., via spin_lock_irqsave(), - spin_lock_bh(), etc. Failing to disable softirq on a given - acquisition of that lock will result in deadlock as soon as - the RCU softirq handler happens to run your RCU callback while - interrupting that acquisition's critical section. - -12. RCU callbacks can be and are executed in parallel. In many cases, - the callback code simply wrappers around kfree(), so that this - is not an issue (or, more accurately, to the extent that it is - an issue, the memory-allocator locking handles it). However, - if the callbacks do manipulate a shared data structure, they - must use whatever locking or other synchronization is required - to safely access and/or modify that data structure. - - Do not assume that RCU callbacks will be executed on the same - CPU that executed the corresponding call_rcu() or call_srcu(). - For example, if a given CPU goes offline while having an RCU - callback pending, then that RCU callback will execute on some - surviving CPU. (If this was not the case, a self-spawning RCU - callback would prevent the victim CPU from ever going offline.) - Furthermore, CPUs designated by rcu_nocbs= might well -always- - have their RCU callbacks executed on some other CPUs, in fact, - for some real-time workloads, this is the whole point of using - the rcu_nocbs= kernel boot parameter. - -13. Unlike other forms of RCU, it -is- permissible to block in an - SRCU read-side critical section (demarked by srcu_read_lock() - and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". - Please note that if you don't need to sleep in read-side critical - sections, you should be using RCU rather than SRCU, because RCU - is almost always faster and easier to use than is SRCU. - - Also unlike other forms of RCU, explicit initialization and - cleanup is required either at build time via DEFINE_SRCU() - or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct() - and cleanup_srcu_struct(). These last two are passed a - "struct srcu_struct" that defines the scope of a given - SRCU domain. Once initialized, the srcu_struct is passed - to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(), - synchronize_srcu_expedited(), and call_srcu(). A given - synchronize_srcu() waits only for SRCU read-side critical - sections governed by srcu_read_lock() and srcu_read_unlock() - calls that have been passed the same srcu_struct. This property - is what makes sleeping read-side critical sections tolerable -- - a given subsystem delays only its own updates, not those of other - subsystems using SRCU. Therefore, SRCU is less prone to OOM the - system than RCU would be if RCU's read-side critical sections - were permitted to sleep. - - The ability to sleep in read-side critical sections does not - come for free. First, corresponding srcu_read_lock() and - srcu_read_unlock() calls must be passed the same srcu_struct. - Second, grace-period-detection overhead is amortized only - over those updates sharing a given srcu_struct, rather than - being globally amortized as they are for other forms of RCU. - Therefore, SRCU should be used in preference to rw_semaphore - only in extremely read-intensive situations, or in situations - requiring SRCU's read-side deadlock immunity or low read-side - realtime latency. You should also consider percpu_rw_semaphore - when you need lightweight readers. - - SRCU's expedited primitive (synchronize_srcu_expedited()) - never sends IPIs to other CPUs, so it is easier on - real-time workloads than is synchronize_rcu_expedited(). - - Note that rcu_assign_pointer() relates to SRCU just as it does to - other forms of RCU, but instead of rcu_dereference() you should - use srcu_dereference() in order to avoid lockdep splats. - -14. The whole point of call_rcu(), synchronize_rcu(), and friends - is to wait until all pre-existing readers have finished before - carrying out some otherwise-destructive operation. It is - therefore critically important to -first- remove any path - that readers can follow that could be affected by the - destructive operation, and -only- -then- invoke call_rcu(), - synchronize_rcu(), or friends. - - Because these primitives only wait for pre-existing readers, it - is the caller's responsibility to guarantee that any subsequent - readers will execute safely. - -15. The various RCU read-side primitives do -not- necessarily contain - memory barriers. You should therefore plan for the CPU - and the compiler to freely reorder code into and out of RCU - read-side critical sections. It is the responsibility of the - RCU update-side primitives to deal with this. - - For SRCU readers, you can use smp_mb__after_srcu_read_unlock() - immediately after an srcu_read_unlock() to get a full barrier. - -16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the - __rcu sparse checks to validate your RCU code. These can help - find problems as follows: - - CONFIG_PROVE_LOCKING: check that accesses to RCU-protected data - structures are carried out under the proper RCU - read-side critical section, while holding the right - combination of locks, or whatever other conditions - are appropriate. - - CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the - same object to call_rcu() (or friends) before an RCU - grace period has elapsed since the last time that you - passed that same object to call_rcu() (or friends). - - __rcu sparse checks: tag the pointer to the RCU-protected data - structure with __rcu, and sparse will warn you if you - access that pointer without the services of one of the - variants of rcu_dereference(). - - These debugging aids can help you find problems that are - otherwise extremely difficult to spot. - -17. If you register a callback using call_rcu() or call_srcu(), and - pass in a function defined within a loadable module, then it in - necessary to wait for all pending callbacks to be invoked after - the last invocation and before unloading that module. Note that - it is absolutely -not- sufficient to wait for a grace period! - The current (say) synchronize_rcu() implementation is -not- - guaranteed to wait for callbacks registered on other CPUs. - Or even on the current CPU if that CPU recently went offline - and came back online. - - You instead need to use one of the barrier functions: - - o call_rcu() -> rcu_barrier() - o call_srcu() -> srcu_barrier() - - However, these barrier functions are absolutely -not- guaranteed - to wait for a grace period. In fact, if there are no call_rcu() - callbacks waiting anywhere in the system, rcu_barrier() is within - its rights to return immediately. - - So if you need to wait for both an RCU grace period and for - all pre-existing call_rcu() callbacks, you will need to execute - both rcu_barrier() and synchronize_rcu(), if necessary, using - something like workqueues to to execute them concurrently. - - See rcubarrier.txt for more information. diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst index 81a0a1e5f767..e703d3dbe60c 100644 --- a/Documentation/RCU/index.rst +++ b/Documentation/RCU/index.rst @@ -1,3 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + .. _rcu_concepts: ============ @@ -8,10 +10,17 @@ RCU concepts :maxdepth: 3 arrayRCU + checklist + lockdep + lockdep-splat rcubarrier rcu_dereference whatisRCU rcu + rculist_nulls + rcuref + torture + stallwarn listRCU NMI-RCU UP diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst index 7956ff33042b..2a643e293fb4 100644 --- a/Documentation/RCU/listRCU.rst +++ b/Documentation/RCU/listRCU.rst @@ -4,12 +4,61 @@ Using RCU to Protect Read-Mostly Linked Lists ============================================= One of the best applications of RCU is to protect read-mostly linked lists -("struct list_head" in list.h). One big advantage of this approach +(``struct list_head`` in list.h). One big advantage of this approach is that all of the required memory barriers are included for you in the list macros. This document describes several applications of RCU, with the best fits first. -Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates + +Example 1: Read-mostly list: Deferred Destruction +------------------------------------------------- + +A widely used usecase for RCU lists in the kernel is lockless iteration over +all processes in the system. ``task_struct::tasks`` represents the list node that +links all the processes. The list can be traversed in parallel to any list +additions or removals. + +The traversal of the list is done using ``for_each_process()`` which is defined +by the 2 macros:: + + #define next_task(p) \ + list_entry_rcu((p)->tasks.next, struct task_struct, tasks) + + #define for_each_process(p) \ + for (p = &init_task ; (p = next_task(p)) != &init_task ; ) + +The code traversing the list of all processes typically looks like:: + + rcu_read_lock(); + for_each_process(p) { + /* Do something with p */ + } + rcu_read_unlock(); + +The simplified code for removing a process from a task list is:: + + void release_task(struct task_struct *p) + { + write_lock(&tasklist_lock); + list_del_rcu(&p->tasks); + write_unlock(&tasklist_lock); + call_rcu(&p->rcu, delayed_put_task_struct); + } + +When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under +``tasklist_lock`` writer lock protection, to remove the task from the list of +all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals +from corrupting the list. Readers using ``for_each_process()`` are not protected +with the ``tasklist_lock``. To prevent readers from noticing changes in the list +pointers, the ``task_struct`` object is freed only after one or more grace +periods elapse (with the help of call_rcu()). This deferring of destruction +ensures that any readers traversing the list will see valid ``p->tasks.next`` +pointers and deletion/freeing can happen in parallel with traversal of the list. +This pattern is also called an **existence lock**, since RCU pins the object in +memory until all existing readers finish. + + +Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates ---------------------------------------------------------------------- The best applications are cases where, if reader-writer locking were @@ -26,7 +75,7 @@ added or deleted, rather than being modified in place. A straightforward example of this use of RCU may be found in the system-call auditing support. For example, a reader-writer locked -implementation of audit_filter_task() might be as follows:: +implementation of ``audit_filter_task()`` might be as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -34,7 +83,7 @@ implementation of audit_filter_task() might be as follows:: enum audit_state state; read_lock(&auditsc_lock); - /* Note: audit_netlink_sem held by caller. */ + /* Note: audit_filter_mutex held by caller. */ list_for_each_entry(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { read_unlock(&auditsc_lock); @@ -58,7 +107,7 @@ This means that RCU can be easily applied to the read side, as follows:: enum audit_state state; rcu_read_lock(); - /* Note: audit_netlink_sem held by caller. */ + /* Note: audit_filter_mutex held by caller. */ list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { rcu_read_unlock(); @@ -69,18 +118,18 @@ This means that RCU can be easily applied to the read side, as follows:: return AUDIT_BUILD_CONTEXT; } -The read_lock() and read_unlock() calls have become rcu_read_lock() +The ``read_lock()`` and ``read_unlock()`` calls have become rcu_read_lock() and rcu_read_unlock(), respectively, and the list_for_each_entry() has -become list_for_each_entry_rcu(). The _rcu() list-traversal primitives +become list_for_each_entry_rcu(). The **_rcu()** list-traversal primitives insert the read-side memory barriers that are required on DEC Alpha CPUs. -The changes to the update side are also straightforward. A reader-writer -lock might be used as follows for deletion and insertion:: +The changes to the update side are also straightforward. A reader-writer lock +might be used as follows for deletion and insertion:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { - struct audit_entry *e; + struct audit_entry *e; write_lock(&auditsc_lock); list_for_each_entry(e, list, list) { @@ -113,9 +162,9 @@ Following are the RCU equivalents for these two functions:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { - struct audit_entry *e; + struct audit_entry *e; - /* Do not use the _rcu iterator here, since this is the only + /* No need to use the _rcu iterator here, since this is the only * deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { @@ -139,45 +188,45 @@ Following are the RCU equivalents for these two functions:: return 0; } -Normally, the write_lock() and write_unlock() would be replaced by -a spin_lock() and a spin_unlock(), but in this case, all callers hold -audit_netlink_sem, so no additional locking is required. The auditsc_lock -can therefore be eliminated, since use of RCU eliminates the need for -writers to exclude readers. Normally, the write_lock() calls would -be converted into spin_lock() calls. +Normally, the ``write_lock()`` and ``write_unlock()`` would be replaced by a +spin_lock() and a spin_unlock(). But in this case, all callers hold +``audit_filter_mutex``, so no additional locking is required. The +``auditsc_lock`` can therefore be eliminated, since use of RCU eliminates the +need for writers to exclude readers. The list_del(), list_add(), and list_add_tail() primitives have been replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). -The _rcu() list-manipulation primitives add memory barriers that are -needed on weakly ordered CPUs (most of them!). The list_del_rcu() -primitive omits the pointer poisoning debug-assist code that would -otherwise cause concurrent readers to fail spectacularly. +The **_rcu()** list-manipulation primitives add memory barriers that are needed on +weakly ordered CPUs (most of them!). The list_del_rcu() primitive omits the +pointer poisoning debug-assist code that would otherwise cause concurrent +readers to fail spectacularly. -So, when readers can tolerate stale data and when entries are either added -or deleted, without in-place modification, it is very easy to use RCU! +So, when readers can tolerate stale data and when entries are either added or +deleted, without in-place modification, it is very easy to use RCU! -Example 2: Handling In-Place Updates + +Example 3: Handling In-Place Updates ------------------------------------ -The system-call auditing code does not update auditing rules in place. -However, if it did, reader-writer-locked code to do so might look as -follows (presumably, the field_count is only permitted to decrease, -otherwise, the added fields would need to be filled in):: +The system-call auditing code does not update auditing rules in place. However, +if it did, the reader-writer-locked code to do so might look as follows +(assuming only ``field_count`` is updated, otherwise, the added fields would +need to be filled in):: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { - struct audit_entry *e; - struct audit_newentry *ne; + struct audit_entry *e; + struct audit_entry *ne; write_lock(&auditsc_lock); - /* Note: audit_netlink_sem held by caller. */ + /* Note: audit_filter_mutex held by caller. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { e->rule.action = newaction; - e->rule.file_count = newfield_count; + e->rule.field_count = newfield_count; write_unlock(&auditsc_lock); return 0; } @@ -188,16 +237,16 @@ otherwise, the added fields would need to be filled in):: The RCU version creates a copy, updates the copy, then replaces the old entry with the newly updated entry. This sequence of actions, allowing -concurrent reads while doing a copy to perform an update, is what gives -RCU ("read-copy update") its name. The RCU code is as follows:: +concurrent reads while making a copy to perform an update, is what gives +RCU (*read-copy update*) its name. The RCU code is as follows:: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { - struct audit_entry *e; - struct audit_newentry *ne; + struct audit_entry *e; + struct audit_entry *ne; list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { @@ -206,7 +255,7 @@ RCU ("read-copy update") its name. The RCU code is as follows:: return -ENOMEM; audit_copy_rule(&ne->rule, &e->rule); ne->rule.action = newaction; - ne->rule.file_count = newfield_count; + ne->rule.field_count = newfield_count; list_replace_rcu(&e->list, &ne->list); call_rcu(&e->rcu, audit_free_rule); return 0; @@ -215,34 +264,45 @@ RCU ("read-copy update") its name. The RCU code is as follows:: return -EFAULT; /* No matching rule */ } -Again, this assumes that the caller holds audit_netlink_sem. Normally, -the reader-writer lock would become a spinlock in this sort of code. +Again, this assumes that the caller holds ``audit_filter_mutex``. Normally, the +writer lock would become a spinlock in this sort of code. -Example 3: Eliminating Stale Data +Another use of this pattern can be found in the openswitch driver's *connection +tracking table* code in ``ct_limit_set()``. The table holds connection tracking +entries and has a limit on the maximum entries. There is one such table +per-zone and hence one *limit* per zone. The zones are mapped to their limits +through a hashtable using an RCU-managed hlist for the hash chains. When a new +limit is set, a new limit object is allocated and ``ct_limit_set()`` is called +to replace the old limit object with the new one using list_replace_rcu(). +The old limit object is then freed after a grace period using kfree_rcu(). + + +Example 4: Eliminating Stale Data --------------------------------- -The auditing examples above tolerate stale data, as do most algorithms +The auditing example above tolerates stale data, as do most algorithms that are tracking external state. Because there is a delay from the time the external state changes before Linux becomes aware of the change, -additional RCU-induced staleness is normally not a problem. +additional RCU-induced staleness is generally not a problem. However, there are many examples where stale data cannot be tolerated. -One example in the Linux kernel is the System V IPC (see the ipc_lock() -function in ipc/util.c). This code checks a "deleted" flag under a -per-entry spinlock, and, if the "deleted" flag is set, pretends that the +One example in the Linux kernel is the System V IPC (see the shm_lock() +function in ipc/shm.c). This code checks a *deleted* flag under a +per-entry spinlock, and, if the *deleted* flag is set, pretends that the entry does not exist. For this to be helpful, the search function must -return holding the per-entry spinlock, as ipc_lock() does in fact do. +return holding the per-entry spinlock, as shm_lock() does in fact do. + +.. _quick_quiz: Quick Quiz: - Why does the search function need to return holding the per-entry lock for - this deleted-flag technique to be helpful? + For the deleted-flag technique to be helpful, why is it necessary + to hold the per-entry lock while returning from the search function? -:ref:`Answer to Quick Quiz <answer_quick_quiz_list>` +:ref:`Answer to Quick Quiz <quick_quiz_answer>` -If the system-call audit module were to ever need to reject stale data, -one way to accomplish this would be to add a "deleted" flag and a "lock" -spinlock to the audit_entry structure, and modify audit_filter_task() -as follows:: +If the system-call audit module were to ever need to reject stale data, one way +to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the +audit_entry structure, and modify ``audit_filter_task()`` as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -267,20 +327,20 @@ as follows:: } Note that this example assumes that entries are only added and deleted. -Additional mechanism is required to deal correctly with the -update-in-place performed by audit_upd_rule(). For one thing, -audit_upd_rule() would need additional memory barriers to ensure -that the list_add_rcu() was really executed before the list_del_rcu(). +Additional mechanism is required to deal correctly with the update-in-place +performed by ``audit_upd_rule()``. For one thing, ``audit_upd_rule()`` would +need additional memory barriers to ensure that the list_add_rcu() was really +executed before the list_del_rcu(). -The audit_del_rule() function would need to set the "deleted" -flag under the spinlock as follows:: +The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the +spinlock as follows:: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { - struct audit_entry *e; + struct audit_entry *e; - /* Do not need to use the _rcu iterator here, since this + /* No need to use the _rcu iterator here, since this * is the only deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { @@ -295,6 +355,91 @@ flag under the spinlock as follows:: return -EFAULT; /* No matching rule */ } +This too assumes that the caller holds ``audit_filter_mutex``. + + +Example 5: Skipping Stale Objects +--------------------------------- + +For some usecases, reader performance can be improved by skipping stale objects +during read-side list traversal if the object in concern is pending destruction +after one or more grace periods. One such example can be found in the timerfd +subsystem. When a ``CLOCK_REALTIME`` clock is reprogrammed - for example due to +setting of the system time, then all programmed timerfds that depend on this +clock get triggered and processes waiting on them to expire are woken up in +advance of their scheduled expiry. To facilitate this, all such timers are added +to an RCU-managed ``cancel_list`` when they are setup in +``timerfd_setup_cancel()``:: + + static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags) + { + spin_lock(&ctx->cancel_lock); + if ((ctx->clockid == CLOCK_REALTIME && + (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) { + if (!ctx->might_cancel) { + ctx->might_cancel = true; + spin_lock(&cancel_lock); + list_add_rcu(&ctx->clist, &cancel_list); + spin_unlock(&cancel_lock); + } + } + spin_unlock(&ctx->cancel_lock); + } + +When a timerfd is freed (fd is closed), then the ``might_cancel`` flag of the +timerfd object is cleared, the object removed from the ``cancel_list`` and +destroyed:: + + int timerfd_release(struct inode *inode, struct file *file) + { + struct timerfd_ctx *ctx = file->private_data; + + spin_lock(&ctx->cancel_lock); + if (ctx->might_cancel) { + ctx->might_cancel = false; + spin_lock(&cancel_lock); + list_del_rcu(&ctx->clist); + spin_unlock(&cancel_lock); + } + spin_unlock(&ctx->cancel_lock); + + hrtimer_cancel(&ctx->t.tmr); + kfree_rcu(ctx, rcu); + return 0; + } + +If the ``CLOCK_REALTIME`` clock is set, for example by a time server, the +hrtimer framework calls ``timerfd_clock_was_set()`` which walks the +``cancel_list`` and wakes up processes waiting on the timerfd. While iterating +the ``cancel_list``, the ``might_cancel`` flag is consulted to skip stale +objects:: + + void timerfd_clock_was_set(void) + { + struct timerfd_ctx *ctx; + unsigned long flags; + + rcu_read_lock(); + list_for_each_entry_rcu(ctx, &cancel_list, clist) { + if (!ctx->might_cancel) + continue; + spin_lock_irqsave(&ctx->wqh.lock, flags); + if (ctx->moffs != ktime_mono_to_real(0)) { + ctx->moffs = KTIME_MAX; + ctx->ticks++; + wake_up_locked_poll(&ctx->wqh, EPOLLIN); + } + spin_unlock_irqrestore(&ctx->wqh.lock, flags); + } + rcu_read_unlock(); + } + +The key point here is, because RCU-traversal of the ``cancel_list`` happens +while objects are being added and removed to the list, sometimes the traversal +can step on an object that has been removed from the list. In this example, it +is seen that it is better to skip such objects using a flag. + + Summary ------- @@ -303,19 +448,21 @@ the most amenable to use of RCU. The simplest case is where entries are either added or deleted from the data structure (or atomically modified in place), but non-atomic in-place modifications can be handled by making a copy, updating the copy, then replacing the original with the copy. -If stale data cannot be tolerated, then a "deleted" flag may be used +If stale data cannot be tolerated, then a *deleted* flag may be used in conjunction with a per-entry spinlock in order to allow the search function to reject newly deleted data. -.. _answer_quick_quiz_list: +.. _quick_quiz_answer: Answer to Quick Quiz: - Why does the search function need to return holding the per-entry - lock for this deleted-flag technique to be helpful? + For the deleted-flag technique to be helpful, why is it necessary + to hold the per-entry lock while returning from the search function? If the search function drops the per-entry lock before returning, then the caller will be processing stale data in any case. If it is really OK to be processing stale data, then you don't need a - "deleted" flag. If processing stale data really is a problem, + *deleted* flag. If processing stale data really is a problem, then you need to hold the per-entry lock across all of the code that uses the value that was returned. + +:ref:`Back to Quick Quiz <quick_quiz>` diff --git a/Documentation/RCU/lockdep-splat.rst b/Documentation/RCU/lockdep-splat.rst new file mode 100644 index 000000000000..2a5c79db57dc --- /dev/null +++ b/Documentation/RCU/lockdep-splat.rst @@ -0,0 +1,115 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================= +Lockdep-RCU Splat +================= + +Lockdep-RCU was added to the Linux kernel in early 2010 +(http://lwn.net/Articles/371986/). This facility checks for some common +misuses of the RCU API, most notably using one of the rcu_dereference() +family to access an RCU-protected pointer without the proper protection. +When such misuse is detected, an lockdep-RCU splat is emitted. + +The usual cause of a lockdep-RCU slat is someone accessing an +RCU-protected data structure without either (1) being in the right kind of +RCU read-side critical section or (2) holding the right update-side lock. +This problem can therefore be serious: it might result in random memory +overwriting or worse. There can of course be false positives, this +being the real world and all that. + +So let's look at an example RCU lockdep splat from 3.0-rc5, one that +has long since been fixed:: + + ============================= + WARNING: suspicious RCU usage + ----------------------------- + block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage! + +other info that might help us debug this:: + + rcu_scheduler_active = 1, debug_locks = 0 + 3 locks held by scsi_scan_6/1552: + #0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>] + scsi_scan_host_selected+0x5a/0x150 + #1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>] + elevator_exit+0x22/0x60 + #2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>] + cfq_exit_queue+0x43/0x190 + + stack backtrace: + Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17 + Call Trace: + [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0 + [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120 + [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190 + [<ffffffff812a5046>] elevator_exit+0x36/0x60 + [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60 + [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10 + [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0 + [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10 + [<ffffffff817da069>] ? error_exit+0x29/0xb0 + [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80 + [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680 + [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c + [<ffffffff817da069>] ? error_exit+0x29/0xb0 + [<ffffffff812bcc60>] ? kobject_del+0x40/0x40 + [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0 + [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150 + [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90 + [<ffffffff8145f170>] do_scan_async+0x20/0x160 + [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90 + [<ffffffff810975b6>] kthread+0xa6/0xb0 + [<ffffffff817db154>] kernel_thread_helper+0x4/0x10 + [<ffffffff81066430>] ? finish_task_switch+0x80/0x110 + [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe + [<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70 + [<ffffffff817db150>] ? gs_change+0xb/0xb + +Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:: + + if (rcu_dereference(ioc->ioc_data) == cic) { + +This form says that it must be in a plain vanilla RCU read-side critical +section, but the "other info" list above shows that this is not the +case. Instead, we hold three locks, one of which might be RCU related. +And maybe that lock really does protect this reference. If so, the fix +is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to +take the struct request_queue "q" from cfq_exit_queue() as an argument, +which would permit us to invoke rcu_dereference_protected as follows:: + + if (rcu_dereference_protected(ioc->ioc_data, + lockdep_is_held(&q->queue_lock)) == cic) { + +With this change, there would be no lockdep-RCU splat emitted if this +code was invoked either from within an RCU read-side critical section +or with the ->queue_lock held. In particular, this would have suppressed +the above lockdep-RCU splat because ->queue_lock is held (see #2 in the +list above). + +On the other hand, perhaps we really do need an RCU read-side critical +section. In this case, the critical section must span the use of the +return value from rcu_dereference(), or at least until there is some +reference count incremented or some such. One way to handle this is to +add rcu_read_lock() and rcu_read_unlock() as follows:: + + rcu_read_lock(); + if (rcu_dereference(ioc->ioc_data) == cic) { + spin_lock(&ioc->lock); + rcu_assign_pointer(ioc->ioc_data, NULL); + spin_unlock(&ioc->lock); + } + rcu_read_unlock(); + +With this change, the rcu_dereference() is always within an RCU +read-side critical section, which again would have suppressed the +above lockdep-RCU splat. + +But in this particular case, we don't actually dereference the pointer +returned from rcu_dereference(). Instead, that pointer is just compared +to the cic pointer, which means that the rcu_dereference() can be replaced +by rcu_access_pointer() as follows:: + + if (rcu_access_pointer(ioc->ioc_data) == cic) { + +Because it is legal to invoke rcu_access_pointer() without protection, +this change would also suppress the above lockdep-RCU splat. diff --git a/Documentation/RCU/lockdep-splat.txt b/Documentation/RCU/lockdep-splat.txt deleted file mode 100644 index b8096316fd11..000000000000 --- a/Documentation/RCU/lockdep-splat.txt +++ /dev/null @@ -1,110 +0,0 @@ -Lockdep-RCU was added to the Linux kernel in early 2010 -(http://lwn.net/Articles/371986/). This facility checks for some common -misuses of the RCU API, most notably using one of the rcu_dereference() -family to access an RCU-protected pointer without the proper protection. -When such misuse is detected, an lockdep-RCU splat is emitted. - -The usual cause of a lockdep-RCU slat is someone accessing an -RCU-protected data structure without either (1) being in the right kind of -RCU read-side critical section or (2) holding the right update-side lock. -This problem can therefore be serious: it might result in random memory -overwriting or worse. There can of course be false positives, this -being the real world and all that. - -So let's look at an example RCU lockdep splat from 3.0-rc5, one that -has long since been fixed: - -============================= -WARNING: suspicious RCU usage ------------------------------ -block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage! - -other info that might help us debug this: - - -rcu_scheduler_active = 1, debug_locks = 0 -3 locks held by scsi_scan_6/1552: - #0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>] -scsi_scan_host_selected+0x5a/0x150 - #1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>] -elevator_exit+0x22/0x60 - #2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>] -cfq_exit_queue+0x43/0x190 - -stack backtrace: -Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17 -Call Trace: - [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0 - [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120 - [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190 - [<ffffffff812a5046>] elevator_exit+0x36/0x60 - [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60 - [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10 - [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0 - [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10 - [<ffffffff817da069>] ? error_exit+0x29/0xb0 - [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80 - [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680 - [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c - [<ffffffff817da069>] ? error_exit+0x29/0xb0 - [<ffffffff812bcc60>] ? kobject_del+0x40/0x40 - [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0 - [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150 - [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90 - [<ffffffff8145f170>] do_scan_async+0x20/0x160 - [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90 - [<ffffffff810975b6>] kthread+0xa6/0xb0 - [<ffffffff817db154>] kernel_thread_helper+0x4/0x10 - [<ffffffff81066430>] ? finish_task_switch+0x80/0x110 - [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe - [<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70 - [<ffffffff817db150>] ? gs_change+0xb/0xb - -Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows: - - if (rcu_dereference(ioc->ioc_data) == cic) { - -This form says that it must be in a plain vanilla RCU read-side critical -section, but the "other info" list above shows that this is not the -case. Instead, we hold three locks, one of which might be RCU related. -And maybe that lock really does protect this reference. If so, the fix -is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to -take the struct request_queue "q" from cfq_exit_queue() as an argument, -which would permit us to invoke rcu_dereference_protected as follows: - - if (rcu_dereference_protected(ioc->ioc_data, - lockdep_is_held(&q->queue_lock)) == cic) { - -With this change, there would be no lockdep-RCU splat emitted if this -code was invoked either from within an RCU read-side critical section -or with the ->queue_lock held. In particular, this would have suppressed -the above lockdep-RCU splat because ->queue_lock is held (see #2 in the -list above). - -On the other hand, perhaps we really do need an RCU read-side critical -section. In this case, the critical section must span the use of the -return value from rcu_dereference(), or at least until there is some -reference count incremented or some such. One way to handle this is to -add rcu_read_lock() and rcu_read_unlock() as follows: - - rcu_read_lock(); - if (rcu_dereference(ioc->ioc_data) == cic) { - spin_lock(&ioc->lock); - rcu_assign_pointer(ioc->ioc_data, NULL); - spin_unlock(&ioc->lock); - } - rcu_read_unlock(); - -With this change, the rcu_dereference() is always within an RCU -read-side critical section, which again would have suppressed the -above lockdep-RCU splat. - -But in this particular case, we don't actually dereference the pointer -returned from rcu_dereference(). Instead, that pointer is just compared -to the cic pointer, which means that the rcu_dereference() can be replaced -by rcu_access_pointer() as follows: - - if (rcu_access_pointer(ioc->ioc_data) == cic) { - -Because it is legal to invoke rcu_access_pointer() without protection, -this change would also suppress the above lockdep-RCU splat. diff --git a/Documentation/RCU/lockdep.rst b/Documentation/RCU/lockdep.rst new file mode 100644 index 000000000000..a94f55991a71 --- /dev/null +++ b/Documentation/RCU/lockdep.rst @@ -0,0 +1,116 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== +RCU and lockdep checking +======================== + +All flavors of RCU have lockdep checking available, so that lockdep is +aware of when each task enters and leaves any flavor of RCU read-side +critical section. Each flavor of RCU is tracked separately (but note +that this is not the case in 2.6.32 and earlier). This allows lockdep's +tracking to include RCU state, which can sometimes help when debugging +deadlocks and the like. + +In addition, RCU provides the following primitives that check lockdep's +state:: + + rcu_read_lock_held() for normal RCU. + rcu_read_lock_bh_held() for RCU-bh. + rcu_read_lock_sched_held() for RCU-sched. + srcu_read_lock_held() for SRCU. + +These functions are conservative, and will therefore return 1 if they +aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). +This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false +positives when lockdep is disabled. + +In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables +checking of rcu_dereference() primitives: + + rcu_dereference(p): + Check for RCU read-side critical section. + rcu_dereference_bh(p): + Check for RCU-bh read-side critical section. + rcu_dereference_sched(p): + Check for RCU-sched read-side critical section. + srcu_dereference(p, sp): + Check for SRCU read-side critical section. + rcu_dereference_check(p, c): + Use explicit check expression "c" along with + rcu_read_lock_held(). This is useful in code that is + invoked by both RCU readers and updaters. + rcu_dereference_bh_check(p, c): + Use explicit check expression "c" along with + rcu_read_lock_bh_held(). This is useful in code that + is invoked by both RCU-bh readers and updaters. + rcu_dereference_sched_check(p, c): + Use explicit check expression "c" along with + rcu_read_lock_sched_held(). This is useful in code that + is invoked by both RCU-sched readers and updaters. + srcu_dereference_check(p, c): + Use explicit check expression "c" along with + srcu_read_lock_held(). This is useful in code that + is invoked by both SRCU readers and updaters. + rcu_dereference_raw(p): + Don't check. (Use sparingly, if at all.) + rcu_dereference_protected(p, c): + Use explicit check expression "c", and omit all barriers + and compiler constraints. This is useful when the data + structure cannot change, for example, in code that is + invoked only by updaters. + rcu_access_pointer(p): + Return the value of the pointer and omit all barriers, + but retain the compiler constraints that prevent duplicating + or coalescsing. This is useful when testing the + value of the pointer itself, for example, against NULL. + +The rcu_dereference_check() check expression can be any boolean +expression, but would normally include a lockdep expression. However, +any boolean expression can be used. For a moderately ornate example, +consider the following:: + + file = rcu_dereference_check(fdt->fd[fd], + lockdep_is_held(&files->file_lock) || + atomic_read(&files->count) == 1); + +This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner, +and, if CONFIG_PROVE_RCU is configured, verifies that this expression +is used in: + +1. An RCU read-side critical section (implicit), or +2. with files->file_lock held, or +3. on an unshared files_struct. + +In case (1), the pointer is picked up in an RCU-safe manner for vanilla +RCU read-side critical sections, in case (2) the ->file_lock prevents +any change from taking place, and finally, in case (3) the current task +is the only task accessing the file_struct, again preventing any change +from taking place. If the above statement was invoked only from updater +code, it could instead be written as follows:: + + file = rcu_dereference_protected(fdt->fd[fd], + lockdep_is_held(&files->file_lock) || + atomic_read(&files->count) == 1); + +This would verify cases #2 and #3 above, and furthermore lockdep would +complain if this was used in an RCU read-side critical section unless one +of these two cases held. Because rcu_dereference_protected() omits all +barriers and compiler constraints, it generates better code than do the +other flavors of rcu_dereference(). On the other hand, it is illegal +to use rcu_dereference_protected() if either the RCU-protected pointer +or the RCU-protected data that it points to can change concurrently. + +Like rcu_dereference(), when lockdep is enabled, RCU list and hlist +traversal primitives check for being called from within an RCU read-side +critical section. However, a lockdep expression can be passed to them +as a additional optional argument. With this lockdep expression, these +traversal primitives will complain only if the lockdep expression is +false and they are called from outside any RCU read-side critical section. + +For example, the workqueue for_each_pwq() macro is intended to be used +either within an RCU read-side critical section or with wq->mutex held. +It is thus implemented as follows:: + + #define for_each_pwq(pwq, wq) + list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node, + lock_is_held(&(wq->mutex).dep_map)) diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt deleted file mode 100644 index 89db949eeca0..000000000000 --- a/Documentation/RCU/lockdep.txt +++ /dev/null @@ -1,112 +0,0 @@ -RCU and lockdep checking - -All flavors of RCU have lockdep checking available, so that lockdep is -aware of when each task enters and leaves any flavor of RCU read-side -critical section. Each flavor of RCU is tracked separately (but note -that this is not the case in 2.6.32 and earlier). This allows lockdep's -tracking to include RCU state, which can sometimes help when debugging -deadlocks and the like. - -In addition, RCU provides the following primitives that check lockdep's -state: - - rcu_read_lock_held() for normal RCU. - rcu_read_lock_bh_held() for RCU-bh. - rcu_read_lock_sched_held() for RCU-sched. - srcu_read_lock_held() for SRCU. - -These functions are conservative, and will therefore return 1 if they -aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). -This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false -positives when lockdep is disabled. - -In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables -checking of rcu_dereference() primitives: - - rcu_dereference(p): - Check for RCU read-side critical section. - rcu_dereference_bh(p): - Check for RCU-bh read-side critical section. - rcu_dereference_sched(p): - Check for RCU-sched read-side critical section. - srcu_dereference(p, sp): - Check for SRCU read-side critical section. - rcu_dereference_check(p, c): - Use explicit check expression "c" along with - rcu_read_lock_held(). This is useful in code that is - invoked by both RCU readers and updaters. - rcu_dereference_bh_check(p, c): - Use explicit check expression "c" along with - rcu_read_lock_bh_held(). This is useful in code that - is invoked by both RCU-bh readers and updaters. - rcu_dereference_sched_check(p, c): - Use explicit check expression "c" along with - rcu_read_lock_sched_held(). This is useful in code that - is invoked by both RCU-sched readers and updaters. - srcu_dereference_check(p, c): - Use explicit check expression "c" along with - srcu_read_lock_held()(). This is useful in code that - is invoked by both SRCU readers and updaters. - rcu_dereference_raw(p): - Don't check. (Use sparingly, if at all.) - rcu_dereference_protected(p, c): - Use explicit check expression "c", and omit all barriers - and compiler constraints. This is useful when the data - structure cannot change, for example, in code that is - invoked only by updaters. - rcu_access_pointer(p): - Return the value of the pointer and omit all barriers, - but retain the compiler constraints that prevent duplicating - or coalescsing. This is useful when when testing the - value of the pointer itself, for example, against NULL. - -The rcu_dereference_check() check expression can be any boolean -expression, but would normally include a lockdep expression. However, -any boolean expression can be used. For a moderately ornate example, -consider the following: - - file = rcu_dereference_check(fdt->fd[fd], - lockdep_is_held(&files->file_lock) || - atomic_read(&files->count) == 1); - -This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner, -and, if CONFIG_PROVE_RCU is configured, verifies that this expression -is used in: - -1. An RCU read-side critical section (implicit), or -2. with files->file_lock held, or -3. on an unshared files_struct. - -In case (1), the pointer is picked up in an RCU-safe manner for vanilla -RCU read-side critical sections, in case (2) the ->file_lock prevents -any change from taking place, and finally, in case (3) the current task -is the only task accessing the file_struct, again preventing any change -from taking place. If the above statement was invoked only from updater -code, it could instead be written as follows: - - file = rcu_dereference_protected(fdt->fd[fd], - lockdep_is_held(&files->file_lock) || - atomic_read(&files->count) == 1); - -This would verify cases #2 and #3 above, and furthermore lockdep would -complain if this was used in an RCU read-side critical section unless one -of these two cases held. Because rcu_dereference_protected() omits all -barriers and compiler constraints, it generates better code than do the -other flavors of rcu_dereference(). On the other hand, it is illegal -to use rcu_dereference_protected() if either the RCU-protected pointer -or the RCU-protected data that it points to can change concurrently. - -Like rcu_dereference(), when lockdep is enabled, RCU list and hlist -traversal primitives check for being called from within an RCU read-side -critical section. However, a lockdep expression can be passed to them -as a additional optional argument. With this lockdep expression, these -traversal primitives will complain only if the lockdep expression is -false and they are called from outside any RCU read-side critical section. - -For example, the workqueue for_each_pwq() macro is intended to be used -either within an RCU read-side critical section or with wq->mutex held. -It is thus implemented as follows: - - #define for_each_pwq(pwq, wq) - list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node, - lock_is_held(&(wq->mutex).dep_map)) diff --git a/Documentation/RCU/rcu.rst b/Documentation/RCU/rcu.rst index 8dfb437dacc3..3cfe01ba9a49 100644 --- a/Documentation/RCU/rcu.rst +++ b/Documentation/RCU/rcu.rst @@ -10,9 +10,8 @@ A "grace period" must elapse between the two parts, and this grace period must be long enough that any readers accessing the item being deleted have since dropped their references. For example, an RCU-protected deletion from a linked list would first remove the item from the list, wait for -a grace period to elapse, then free the element. See the -Documentation/RCU/listRCU.rst file for more information on using RCU with -linked lists. +a grace period to elapse, then free the element. See listRCU.rst for more +information on using RCU with linked lists. Frequently Asked Questions -------------------------- @@ -50,7 +49,7 @@ Frequently Asked Questions - If I am running on a uniprocessor kernel, which can only do one thing at a time, why should I wait for a grace period? - See the Documentation/RCU/UP.rst file for more information. + See UP.rst for more information. - How can I see where RCU is currently used in the Linux kernel? @@ -64,22 +63,22 @@ Frequently Asked Questions - What guidelines should I follow when writing code that uses RCU? - See the checklist.txt file in this directory. + See checklist.rst. - Why the name "RCU"? - "RCU" stands for "read-copy update". The file Documentation/RCU/listRCU.rst - has more information on where this name came from, search for - "read-copy update" to find it. + "RCU" stands for "read-copy update". + listRCU.rst has more information on where this name came from, search + for "read-copy update" to find it. - I hear that RCU is patented? What is with that? Yes, it is. There are several known patents related to RCU, - search for the string "Patent" in RTFP.txt to find them. + search for the string "Patent" in Documentation/RCU/RTFP.txt to find them. Of these, one was allowed to lapse by the assignee, and the others have been contributed to the Linux kernel under GPL. There are now also LGPL implementations of user-level RCU - available (http://liburcu.org/). + available (https://liburcu.org/). - I hear that RCU needs work in order to support realtime kernels? @@ -88,5 +87,5 @@ Frequently Asked Questions - Where can I find more information on RCU? - See the RTFP.txt file in this directory. + See the Documentation/RCU/RTFP.txt file. Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/). diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst index c9667eb0d444..81e828c8313b 100644 --- a/Documentation/RCU/rcu_dereference.rst +++ b/Documentation/RCU/rcu_dereference.rst @@ -28,6 +28,12 @@ Follow these rules to keep your RCU code working properly: for an example where the compiler can in fact deduce the exact value of the pointer, and thus cause misordering. +- In the special case where data is added but is never removed + while readers are accessing the structure, READ_ONCE() may be used + instead of rcu_dereference(). In this case, use of READ_ONCE() + takes on the role of the lockless_dereference() primitive that + was removed in v4.15. + - You are only permitted to use rcu_dereference on pointer values. The compiler simply knows too much about integral values to trust it to carry dependencies through integer operations. @@ -37,7 +43,7 @@ Follow these rules to keep your RCU code working properly: - Set bits and clear bits down in the must-be-zero low-order bits of that pointer. This clearly means that the pointer must have alignment constraints, for example, this does - -not- work in general for char* pointers. + *not* work in general for char* pointers. - XOR bits to translate pointers, as is done in some classic buddy-allocator algorithms. @@ -122,10 +128,16 @@ Follow these rules to keep your RCU code working properly: This sort of comparison occurs frequently when scanning RCU-protected circular linked lists. - Note that if checks for being within an RCU read-side - critical section are not required and the pointer is never - dereferenced, rcu_access_pointer() should be used in place - of rcu_dereference(). + Note that if the pointer comparison is done outside + of an RCU read-side critical section, and the pointer + is never dereferenced, rcu_access_pointer() should be + used in place of rcu_dereference(). In most cases, + it is best to avoid accidental dereferences by testing + the rcu_access_pointer() return value directly, without + assigning it to a variable. + + Within an RCU read-side critical section, there is little + reason to use rcu_access_pointer(). - The comparison is against a pointer that references memory that was initialized "a long time ago." The reason @@ -168,7 +180,7 @@ Follow these rules to keep your RCU code working properly: Please see the "CONTROL DEPENDENCIES" section of Documentation/memory-barriers.txt for more details. - - The pointers are not equal -and- the compiler does + - The pointers are not equal *and* the compiler does not have enough information to deduce the value of the pointer. Note that the volatile cast in rcu_dereference() will normally prevent the compiler from knowing too much. @@ -354,7 +366,7 @@ in turn destroying the ordering between this load and the loads of the return values. This can result in "p->b" returning pre-initialization garbage values. -In short, rcu_dereference() is -not- optional when you are going to +In short, rcu_dereference() is *not* optional when you are going to dereference the resulting pointer. diff --git a/Documentation/RCU/rcubarrier.rst b/Documentation/RCU/rcubarrier.rst index f64f4413a47c..3b4a24877496 100644 --- a/Documentation/RCU/rcubarrier.rst +++ b/Documentation/RCU/rcubarrier.rst @@ -9,7 +9,7 @@ RCU (read-copy update) is a synchronization mechanism that can be thought of as a replacement for read-writer locking (among other things), but with very low-overhead readers that are immune to deadlock, priority inversion, and unbounded latency. RCU read-side critical sections are delimited -by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT +by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPTION kernels, generate no code whatsoever. This means that RCU writers are unaware of the presence of concurrent @@ -329,10 +329,10 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last to smp_call_function() and further to smp_call_function_on_cpu(), causing this latter to spin until the cross-CPU invocation of rcu_barrier_func() has completed. This by itself would prevent - a grace period from completing on non-CONFIG_PREEMPT kernels, + a grace period from completing on non-CONFIG_PREEMPTION kernels, since each CPU must undergo a context switch (or other quiescent state) before the grace period can complete. However, this is - of no use in CONFIG_PREEMPT kernels. + of no use in CONFIG_PREEMPTION kernels. Therefore, on_each_cpu() disables preemption across its call to smp_call_function() and also across the local call to diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst new file mode 100644 index 000000000000..ca4692775ad4 --- /dev/null +++ b/Documentation/RCU/rculist_nulls.rst @@ -0,0 +1,200 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================================= +Using RCU hlist_nulls to protect list and objects +================================================= + +This section describes how to use hlist_nulls to +protect read-mostly linked lists and +objects using SLAB_TYPESAFE_BY_RCU allocations. + +Please read the basics in listRCU.rst. + +Using 'nulls' +============= + +Using special makers (called 'nulls') is a convenient way +to solve following problem : + +A typical RCU linked list managing objects which are +allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can +use following algos : + +1) Lookup algo +-------------- + +:: + + rcu_read_lock() + begin: + obj = lockless_lookup(key); + if (obj) { + if (!try_get_ref(obj)) // might fail for free objects + goto begin; + /* + * Because a writer could delete object, and a writer could + * reuse these object before the RCU grace period, we + * must check key after getting the reference on object + */ + if (obj->key != key) { // not the object we expected + put_ref(obj); + goto begin; + } + } + rcu_read_unlock(); + +Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu() +but a version with an additional memory barrier (smp_rmb()) + +:: + + lockless_lookup(key) + { + struct hlist_node *node, *next; + for (pos = rcu_dereference((head)->first); + pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && + ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); + pos = rcu_dereference(next)) + if (obj->key == key) + return obj; + return NULL; + } + +And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb():: + + struct hlist_node *node; + for (pos = rcu_dereference((head)->first); + pos && ({ prefetch(pos->next); 1; }) && + ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); + pos = rcu_dereference(pos->next)) + if (obj->key == key) + return obj; + return NULL; + +Quoting Corey Minyard:: + + "If the object is moved from one list to another list in-between the + time the hash is calculated and the next field is accessed, and the + object has moved to the end of a new list, the traversal will not + complete properly on the list it should have, since the object will + be on the end of the new list and there's not a way to tell it's on a + new list and restart the list traversal. I think that this can be + solved by pre-fetching the "next" field (with proper barriers) before + checking the key." + +2) Insert algo +-------------- + +We need to make sure a reader cannot read the new 'obj->obj_next' value +and previous value of 'obj->key'. Or else, an item could be deleted +from a chain, and inserted into another chain. If new chain was empty +before the move, 'next' pointer is NULL, and lockless reader can +not detect it missed following items in original chain. + +:: + + /* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ + obj = kmem_cache_alloc(...); + lock_chain(); // typically a spin_lock() + obj->key = key; + /* + * we need to make sure obj->key is updated before obj->next + * or obj->refcnt + */ + smp_wmb(); + atomic_set(&obj->refcnt, 1); + hlist_add_head_rcu(&obj->obj_node, list); + unlock_chain(); // typically a spin_unlock() + + +3) Remove algo +-------------- +Nothing special here, we can use a standard RCU hlist deletion. +But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused +very very fast (before the end of RCU grace period) + +:: + + if (put_last_reference_on(obj) { + lock_chain(); // typically a spin_lock() + hlist_del_init_rcu(&obj->obj_node); + unlock_chain(); // typically a spin_unlock() + kmem_cache_free(cachep, obj); + } + + + +-------------------------------------------------------------------------- + +Avoiding extra smp_rmb() +======================== + +With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup() +and extra smp_wmb() in insert function. + +For example, if we choose to store the slot number as the 'nulls' +end-of-list marker for each slot of the hash table, we can detect +a race (some writer did a delete and/or a move of an object +to another chain) checking the final 'nulls' value if +the lookup met the end of chain. If final 'nulls' value +is not the slot number, then we must restart the lookup at +the beginning. If the object was moved to the same chain, +then the reader doesn't care : It might eventually +scan the list again without harm. + + +1) lookup algo +-------------- + +:: + + head = &table[slot]; + rcu_read_lock(); + begin: + hlist_nulls_for_each_entry_rcu(obj, node, head, member) { + if (obj->key == key) { + if (!try_get_ref(obj)) // might fail for free objects + goto begin; + if (obj->key != key) { // not the object we expected + put_ref(obj); + goto begin; + } + goto out; + } + /* + * if the nulls value we got at the end of this lookup is + * not the expected one, we must restart lookup. + * We probably met an item that was moved to another chain. + */ + if (get_nulls_value(node) != slot) + goto begin; + obj = NULL; + + out: + rcu_read_unlock(); + +2) Insert function +------------------ + +:: + + /* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ + obj = kmem_cache_alloc(cachep); + lock_chain(); // typically a spin_lock() + obj->key = key; + /* + * changes to obj->key must be visible before refcnt one + */ + smp_wmb(); + atomic_set(&obj->refcnt, 1); + /* + * insert obj in RCU way (readers might be traversing chain) + */ + hlist_nulls_add_head_rcu(&obj->obj_node, list); + unlock_chain(); // typically a spin_unlock() diff --git a/Documentation/RCU/rculist_nulls.txt b/Documentation/RCU/rculist_nulls.txt deleted file mode 100644 index 23f115dc87cf..000000000000 --- a/Documentation/RCU/rculist_nulls.txt +++ /dev/null @@ -1,172 +0,0 @@ -Using hlist_nulls to protect read-mostly linked lists and -objects using SLAB_TYPESAFE_BY_RCU allocations. - -Please read the basics in Documentation/RCU/listRCU.rst - -Using special makers (called 'nulls') is a convenient way -to solve following problem : - -A typical RCU linked list managing objects which are -allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can -use following algos : - -1) Lookup algo --------------- -rcu_read_lock() -begin: -obj = lockless_lookup(key); -if (obj) { - if (!try_get_ref(obj)) // might fail for free objects - goto begin; - /* - * Because a writer could delete object, and a writer could - * reuse these object before the RCU grace period, we - * must check key after getting the reference on object - */ - if (obj->key != key) { // not the object we expected - put_ref(obj); - goto begin; - } -} -rcu_read_unlock(); - -Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu() -but a version with an additional memory barrier (smp_rmb()) - -lockless_lookup(key) -{ - struct hlist_node *node, *next; - for (pos = rcu_dereference((head)->first); - pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && - ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); - pos = rcu_dereference(next)) - if (obj->key == key) - return obj; - return NULL; - -And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() : - - struct hlist_node *node; - for (pos = rcu_dereference((head)->first); - pos && ({ prefetch(pos->next); 1; }) && - ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); - pos = rcu_dereference(pos->next)) - if (obj->key == key) - return obj; - return NULL; -} - -Quoting Corey Minyard : - -"If the object is moved from one list to another list in-between the - time the hash is calculated and the next field is accessed, and the - object has moved to the end of a new list, the traversal will not - complete properly on the list it should have, since the object will - be on the end of the new list and there's not a way to tell it's on a - new list and restart the list traversal. I think that this can be - solved by pre-fetching the "next" field (with proper barriers) before - checking the key." - -2) Insert algo : ----------------- - -We need to make sure a reader cannot read the new 'obj->obj_next' value -and previous value of 'obj->key'. Or else, an item could be deleted -from a chain, and inserted into another chain. If new chain was empty -before the move, 'next' pointer is NULL, and lockless reader can -not detect it missed following items in original chain. - -/* - * Please note that new inserts are done at the head of list, - * not in the middle or end. - */ -obj = kmem_cache_alloc(...); -lock_chain(); // typically a spin_lock() -obj->key = key; -/* - * we need to make sure obj->key is updated before obj->next - * or obj->refcnt - */ -smp_wmb(); -atomic_set(&obj->refcnt, 1); -hlist_add_head_rcu(&obj->obj_node, list); -unlock_chain(); // typically a spin_unlock() - - -3) Remove algo --------------- -Nothing special here, we can use a standard RCU hlist deletion. -But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused -very very fast (before the end of RCU grace period) - -if (put_last_reference_on(obj) { - lock_chain(); // typically a spin_lock() - hlist_del_init_rcu(&obj->obj_node); - unlock_chain(); // typically a spin_unlock() - kmem_cache_free(cachep, obj); -} - - - --------------------------------------------------------------------------- -With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup() -and extra smp_wmb() in insert function. - -For example, if we choose to store the slot number as the 'nulls' -end-of-list marker for each slot of the hash table, we can detect -a race (some writer did a delete and/or a move of an object -to another chain) checking the final 'nulls' value if -the lookup met the end of chain. If final 'nulls' value -is not the slot number, then we must restart the lookup at -the beginning. If the object was moved to the same chain, -then the reader doesn't care : It might eventually -scan the list again without harm. - - -1) lookup algo - - head = &table[slot]; - rcu_read_lock(); -begin: - hlist_nulls_for_each_entry_rcu(obj, node, head, member) { - if (obj->key == key) { - if (!try_get_ref(obj)) // might fail for free objects - goto begin; - if (obj->key != key) { // not the object we expected - put_ref(obj); - goto begin; - } - goto out; - } -/* - * if the nulls value we got at the end of this lookup is - * not the expected one, we must restart lookup. - * We probably met an item that was moved to another chain. - */ - if (get_nulls_value(node) != slot) - goto begin; - obj = NULL; - -out: - rcu_read_unlock(); - -2) Insert function : --------------------- - -/* - * Please note that new inserts are done at the head of list, - * not in the middle or end. - */ -obj = kmem_cache_alloc(cachep); -lock_chain(); // typically a spin_lock() -obj->key = key; -/* - * changes to obj->key must be visible before refcnt one - */ -smp_wmb(); -atomic_set(&obj->refcnt, 1); -/* - * insert obj in RCU way (readers might be traversing chain) - */ -hlist_nulls_add_head_rcu(&obj->obj_node, list); -unlock_chain(); // typically a spin_unlock() diff --git a/Documentation/RCU/rcuref.rst b/Documentation/RCU/rcuref.rst new file mode 100644 index 000000000000..b33aeb14fde3 --- /dev/null +++ b/Documentation/RCU/rcuref.rst @@ -0,0 +1,158 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==================================================================== +Reference-count design for elements of lists/arrays protected by RCU +==================================================================== + + +Please note that the percpu-ref feature is likely your first +stop if you need to combine reference counts and RCU. Please see +include/linux/percpu-refcount.h for more information. However, in +those unusual cases where percpu-ref would consume too much memory, +please read on. + +------------------------------------------------------------------------ + +Reference counting on elements of lists which are protected by traditional +reader/writer spinlocks or semaphores are straightforward: + +CODE LISTING A:: + + 1. 2. + add() search_and_reference() + { { + alloc_object read_lock(&list_lock); + ... search_for_element + atomic_set(&el->rc, 1); atomic_inc(&el->rc); + write_lock(&list_lock); ... + add_element read_unlock(&list_lock); + ... ... + write_unlock(&list_lock); } + } + + 3. 4. + release_referenced() delete() + { { + ... write_lock(&list_lock); + if(atomic_dec_and_test(&el->rc)) ... + kfree(el); + ... remove_element + } write_unlock(&list_lock); + ... + if (atomic_dec_and_test(&el->rc)) + kfree(el); + ... + } + +If this list/array is made lock free using RCU as in changing the +write_lock() in add() and delete() to spin_lock() and changing read_lock() +in search_and_reference() to rcu_read_lock(), the atomic_inc() in +search_and_reference() could potentially hold reference to an element which +has already been deleted from the list/array. Use atomic_inc_not_zero() +in this scenario as follows: + +CODE LISTING B:: + + 1. 2. + add() search_and_reference() + { { + alloc_object rcu_read_lock(); + ... search_for_element + atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) { + spin_lock(&list_lock); rcu_read_unlock(); + return FAIL; + add_element } + ... ... + spin_unlock(&list_lock); rcu_read_unlock(); + } } + 3. 4. + release_referenced() delete() + { { + ... spin_lock(&list_lock); + if (atomic_dec_and_test(&el->rc)) ... + call_rcu(&el->head, el_free); remove_element + ... spin_unlock(&list_lock); + } ... + if (atomic_dec_and_test(&el->rc)) + call_rcu(&el->head, el_free); + ... + } + +Sometimes, a reference to the element needs to be obtained in the +update (write) stream. In such cases, atomic_inc_not_zero() might be +overkill, since we hold the update-side spinlock. One might instead +use atomic_inc() in such cases. + +It is not always convenient to deal with "FAIL" in the +search_and_reference() code path. In such cases, the +atomic_dec_and_test() may be moved from delete() to el_free() +as follows: + +CODE LISTING C:: + + 1. 2. + add() search_and_reference() + { { + alloc_object rcu_read_lock(); + ... search_for_element + atomic_set(&el->rc, 1); atomic_inc(&el->rc); + spin_lock(&list_lock); ... + + add_element rcu_read_unlock(); + ... } + spin_unlock(&list_lock); 4. + } delete() + 3. { + release_referenced() spin_lock(&list_lock); + { ... + ... remove_element + if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock); + kfree(el); ... + ... call_rcu(&el->head, el_free); + } ... + 5. } + void el_free(struct rcu_head *rhp) + { + release_referenced(); + } + +The key point is that the initial reference added by add() is not removed +until after a grace period has elapsed following removal. This means that +search_and_reference() cannot find this element, which means that the value +of el->rc cannot increase. Thus, once it reaches zero, there are no +readers that can or ever will be able to reference the element. The +element can therefore safely be freed. This in turn guarantees that if +any reader finds the element, that reader may safely acquire a reference +without checking the value of the reference counter. + +A clear advantage of the RCU-based pattern in listing C over the one +in listing B is that any call to search_and_reference() that locates +a given object will succeed in obtaining a reference to that object, +even given a concurrent invocation of delete() for that same object. +Similarly, a clear advantage of both listings B and C over listing A is +that a call to delete() is not delayed even if there are an arbitrarily +large number of calls to search_and_reference() searching for the same +object that delete() was invoked on. Instead, all that is delayed is +the eventual invocation of kfree(), which is usually not a problem on +modern computer systems, even the small ones. + +In cases where delete() can sleep, synchronize_rcu() can be called from +delete(), so that el_free() can be subsumed into delete as follows:: + + 4. + delete() + { + spin_lock(&list_lock); + ... + remove_element + spin_unlock(&list_lock); + ... + synchronize_rcu(); + if (atomic_dec_and_test(&el->rc)) + kfree(el); + ... + } + +As additional examples in the kernel, the pattern in listing C is used by +reference counting of struct pid, while the pattern in listing B is used by +struct posix_acl. diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt deleted file mode 100644 index 5e6429d66c24..000000000000 --- a/Documentation/RCU/rcuref.txt +++ /dev/null @@ -1,151 +0,0 @@ -Reference-count design for elements of lists/arrays protected by RCU. - - -Please note that the percpu-ref feature is likely your first -stop if you need to combine reference counts and RCU. Please see -include/linux/percpu-refcount.h for more information. However, in -those unusual cases where percpu-ref would consume too much memory, -please read on. - ------------------------------------------------------------------------- - -Reference counting on elements of lists which are protected by traditional -reader/writer spinlocks or semaphores are straightforward: - -CODE LISTING A: -1. 2. -add() search_and_reference() -{ { - alloc_object read_lock(&list_lock); - ... search_for_element - atomic_set(&el->rc, 1); atomic_inc(&el->rc); - write_lock(&list_lock); ... - add_element read_unlock(&list_lock); - ... ... - write_unlock(&list_lock); } -} - -3. 4. -release_referenced() delete() -{ { - ... write_lock(&list_lock); - if(atomic_dec_and_test(&el->rc)) ... - kfree(el); - ... remove_element -} write_unlock(&list_lock); - ... - if (atomic_dec_and_test(&el->rc)) - kfree(el); - ... - } - -If this list/array is made lock free using RCU as in changing the -write_lock() in add() and delete() to spin_lock() and changing read_lock() -in search_and_reference() to rcu_read_lock(), the atomic_inc() in -search_and_reference() could potentially hold reference to an element which -has already been deleted from the list/array. Use atomic_inc_not_zero() -in this scenario as follows: - -CODE LISTING B: -1. 2. -add() search_and_reference() -{ { - alloc_object rcu_read_lock(); - ... search_for_element - atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) { - spin_lock(&list_lock); rcu_read_unlock(); - return FAIL; - add_element } - ... ... - spin_unlock(&list_lock); rcu_read_unlock(); -} } -3. 4. -release_referenced() delete() -{ { - ... spin_lock(&list_lock); - if (atomic_dec_and_test(&el->rc)) ... - call_rcu(&el->head, el_free); remove_element - ... spin_unlock(&list_lock); -} ... - if (atomic_dec_and_test(&el->rc)) - call_rcu(&el->head, el_free); - ... - } - -Sometimes, a reference to the element needs to be obtained in the -update (write) stream. In such cases, atomic_inc_not_zero() might be -overkill, since we hold the update-side spinlock. One might instead -use atomic_inc() in such cases. - -It is not always convenient to deal with "FAIL" in the -search_and_reference() code path. In such cases, the -atomic_dec_and_test() may be moved from delete() to el_free() -as follows: - -CODE LISTING C: -1. 2. -add() search_and_reference() -{ { - alloc_object rcu_read_lock(); - ... search_for_element - atomic_set(&el->rc, 1); atomic_inc(&el->rc); - spin_lock(&list_lock); ... - - add_element rcu_read_unlock(); - ... } - spin_unlock(&list_lock); 4. -} delete() -3. { -release_referenced() spin_lock(&list_lock); -{ ... - ... remove_element - if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock); - kfree(el); ... - ... call_rcu(&el->head, el_free); -} ... -5. } -void el_free(struct rcu_head *rhp) -{ - release_referenced(); -} - -The key point is that the initial reference added by add() is not removed -until after a grace period has elapsed following removal. This means that -search_and_reference() cannot find this element, which means that the value -of el->rc cannot increase. Thus, once it reaches zero, there are no -readers that can or ever will be able to reference the element. The -element can therefore safely be freed. This in turn guarantees that if -any reader finds the element, that reader may safely acquire a reference -without checking the value of the reference counter. - -A clear advantage of the RCU-based pattern in listing C over the one -in listing B is that any call to search_and_reference() that locates -a given object will succeed in obtaining a reference to that object, -even given a concurrent invocation of delete() for that same object. -Similarly, a clear advantage of both listings B and C over listing A is -that a call to delete() is not delayed even if there are an arbitrarily -large number of calls to search_and_reference() searching for the same -object that delete() was invoked on. Instead, all that is delayed is -the eventual invocation of kfree(), which is usually not a problem on -modern computer systems, even the small ones. - -In cases where delete() can sleep, synchronize_rcu() can be called from -delete(), so that el_free() can be subsumed into delete as follows: - -4. -delete() -{ - spin_lock(&list_lock); - ... - remove_element - spin_unlock(&list_lock); - ... - synchronize_rcu(); - if (atomic_dec_and_test(&el->rc)) - kfree(el); - ... -} - -As additional examples in the kernel, the pattern in listing C is used by -reference counting of struct pid, while the pattern in listing B is used by -struct posix_acl. diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst new file mode 100644 index 000000000000..e38c587067fc --- /dev/null +++ b/Documentation/RCU/stallwarn.rst @@ -0,0 +1,385 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== +Using RCU's CPU Stall Detector +============================== + +This document first discusses what sorts of issues RCU's CPU stall +detector can locate, and then discusses kernel parameters and Kconfig +options that can be used to fine-tune the detector's operation. Finally, +this document explains the stall detector's "splat" format. + + +What Causes RCU CPU Stall Warnings? +=================================== + +So your kernel printed an RCU CPU stall warning. The next question is +"What caused it?" The following problems can result in RCU CPU stall +warnings: + +- A CPU looping in an RCU read-side critical section. + +- A CPU looping with interrupts disabled. + +- A CPU looping with preemption disabled. + +- A CPU looping with bottom halves disabled. + +- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel + without invoking schedule(). If the looping in the kernel is + really expected and desirable behavior, you might need to add + some calls to cond_resched(). + +- Booting Linux using a console connection that is too slow to + keep up with the boot-time console-message rate. For example, + a 115Kbaud serial console can be *way* too slow to keep up + with boot-time message rates, and will frequently result in + RCU CPU stall warning messages. Especially if you have added + debug printk()s. + +- Anything that prevents RCU's grace-period kthreads from running. + This can result in the "All QSes seen" console-log message. + This message will include information on when the kthread last + ran and how often it should be expected to run. It can also + result in the ``rcu_.*kthread starved for`` console-log message, + which will include additional debugging information. + +- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might + happen to preempt a low-priority task in the middle of an RCU + read-side critical section. This is especially damaging if + that low-priority task is not permitted to run on any other CPU, + in which case the next RCU grace period can never complete, which + will eventually cause the system to run out of memory and hang. + While the system is in the process of running itself out of + memory, you might see stall-warning messages. + +- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that + is running at a higher priority than the RCU softirq threads. + This will prevent RCU callbacks from ever being invoked, + and in a CONFIG_PREEMPT_RCU kernel will further prevent + RCU grace periods from ever completing. Either way, the + system will eventually run out of memory and hang. In the + CONFIG_PREEMPT_RCU case, you might see stall-warning + messages. + + You can use the rcutree.kthread_prio kernel boot parameter to + increase the scheduling priority of RCU's kthreads, which can + help avoid this problem. However, please note that doing this + can increase your system's context-switch rate and thus degrade + performance. + +- A periodic interrupt whose handler takes longer than the time + interval between successive pairs of interrupts. This can + prevent RCU's kthreads and softirq handlers from running. + Note that certain high-overhead debugging options, for example + the function_graph tracer, can result in interrupt handler taking + considerably longer than normal, which can in turn result in + RCU CPU stall warnings. + +- Testing a workload on a fast system, tuning the stall-warning + timeout down to just barely avoid RCU CPU stall warnings, and then + running the same workload with the same stall-warning timeout on a + slow system. Note that thermal throttling and on-demand governors + can cause a single system to be sometimes fast and sometimes slow! + +- A hardware or software issue shuts off the scheduler-clock + interrupt on a CPU that is not in dyntick-idle mode. This + problem really has happened, and seems to be most likely to + result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. + +- A hardware or software issue that prevents time-based wakeups + from occurring. These issues can range from misconfigured or + buggy timer hardware through bugs in the interrupt or exception + path (whether hardware, firmware, or software) through bugs + in Linux's timer subsystem through bugs in the scheduler, and, + yes, even including bugs in RCU itself. It can also result in + the ``rcu_.*timer wakeup didn't happen for`` console-log message, + which will include additional debugging information. + +- A low-level kernel issue that either fails to invoke one of the + variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(), + ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one + hand, or that invokes one of them too many times on the other. + Historically, the most frequent issue has been an omission + of either irq_enter() or irq_exit(), which in turn invoke + ct_irq_enter() or ct_irq_exit(), respectively. Building your + kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types + of issues, which sometimes arise in architecture-specific code. + +- A bug in the RCU implementation. + +- A hardware failure. This is quite unlikely, but has occurred + at least once in real life. A CPU failed in a running system, + becoming unresponsive, but not causing an immediate crash. + This resulted in a series of RCU CPU stall warnings, eventually + leading the realization that the CPU had failed. + +The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. +Note that SRCU does *not* have CPU stall warnings. Please note that +RCU only detects CPU stalls when there is a grace period in progress. +No grace period, no CPU stall warnings. + +To diagnose the cause of the stall, inspect the stack traces. +The offending function will usually be near the top of the stack. +If you have a series of stall warnings from a single extended stall, +comparing the stack traces can often help determine where the stall +is occurring, which will usually be in the function nearest the top of +that portion of the stack which remains the same from trace to trace. +If you can reliably trigger the stall, ftrace can be quite helpful. + +RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE +and with RCU's event tracing. For information on RCU's event tracing, +see include/trace/events/rcu.h. + + +Fine-Tuning the RCU CPU Stall Detector +====================================== + +The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's +CPU stall detector, which detects conditions that unduly delay RCU grace +periods. This module parameter enables CPU stall detection by default, +but may be overridden via boot-time parameter or at runtime via sysfs. +The stall detector's idea of what constitutes "unduly delayed" is +controlled by a set of kernel configuration variables and cpp macros: + +CONFIG_RCU_CPU_STALL_TIMEOUT +---------------------------- + + This kernel configuration parameter defines the period of time + that RCU will wait from the beginning of a grace period until it + issues an RCU CPU stall warning. This time period is normally + 21 seconds. + + This configuration parameter may be changed at runtime via the + /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however + this parameter is checked only at the beginning of a cycle. + So if you are 10 seconds into a 40-second stall, setting this + sysfs parameter to (say) five will shorten the timeout for the + *next* stall, or the following warning for the current stall + (assuming the stall lasts long enough). It will not affect the + timing of the next warning for the current stall. + + Stall-warning messages may be enabled and disabled completely via + /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. + +CONFIG_RCU_EXP_CPU_STALL_TIMEOUT +-------------------------------- + + Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for + the expedited grace period. This parameter defines the period + of time that RCU will wait from the beginning of an expedited + grace period until it issues an RCU CPU stall warning. This time + period is normally 20 milliseconds on Android devices. A zero + value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used, + after conversion to milliseconds. + + This configuration parameter may be changed at runtime via the + /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however + this parameter is checked only at the beginning of a cycle. If you + are in a current stall cycle, setting it to a new value will change + the timeout for the -next- stall. + + Stall-warning messages may be enabled and disabled completely via + /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. + +RCU_STALL_DELAY_DELTA +--------------------- + + Although the lockdep facility is extremely useful, it does add + some overhead. Therefore, under CONFIG_PROVE_RCU, the + RCU_STALL_DELAY_DELTA macro allows five extra seconds before + giving an RCU CPU stall warning message. (This is a cpp + macro, not a kernel configuration parameter.) + +RCU_STALL_RAT_DELAY +------------------- + + The CPU stall detector tries to make the offending CPU print its + own warnings, as this often gives better-quality stack traces. + However, if the offending CPU does not detect its own stall in + the number of jiffies specified by RCU_STALL_RAT_DELAY, then + some other CPU will complain. This delay is normally set to + two jiffies. (This is a cpp macro, not a kernel configuration + parameter.) + +rcupdate.rcu_task_stall_timeout +------------------------------- + + This boot/sysfs parameter controls the RCU-tasks stall warning + interval. A value of zero or less suppresses RCU-tasks stall + warnings. A positive value sets the stall-warning interval + in seconds. An RCU-tasks stall warning starts with the line: + + INFO: rcu_tasks detected stalls on tasks: + + And continues with the output of sched_show_task() for each + task stalling the current RCU-tasks grace period. + + +Interpreting RCU's CPU Stall-Detector "Splats" +============================================== + +For non-RCU-tasks flavors of RCU, when a CPU detects that some other +CPU is stalling, it will print a message similar to the following:: + + INFO: rcu_sched detected stalls on CPUs/tasks: + 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 + 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 + (detected by 32, t=2603 jiffies, g=7075, q=625) + +This message indicates that CPU 32 detected that CPUs 2 and 16 were both +causing stalls, and that the stall was affecting RCU-sched. This message +will normally be followed by stack dumps for each CPU. Please note that +PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that +the tasks will be indicated by PID, for example, "P3421". It is even +possible for an rcu_state stall to be caused by both CPUs *and* tasks, +in which case the offending CPUs and tasks will all be called out in the list. +In some cases, CPUs will detect themselves stalling, which will result +in a self-detected stall. + +CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with +the RCU core for the past three grace periods. In contrast, CPU 16's "(0 +ticks this GP)" indicates that this CPU has not taken any scheduling-clock +interrupts during the current stalled grace period. + +The "idle=" portion of the message prints the dyntick-idle state. +The hex number before the first "/" is the low-order 12 bits of the +dynticks counter, which will have an even-numbered value if the CPU +is in dyntick-idle mode and an odd-numbered value otherwise. The hex +number between the two "/"s is the value of the nesting, which will be +a small non-negative number if in the idle loop (as shown above) and a +very large positive number otherwise. + +The "softirq=" portion of the message tracks the number of RCU softirq +handlers that the stalled CPU has executed. The number before the "/" +is the number that had executed since boot at the time that this CPU +last noted the beginning of a grace period, which might be the current +(stalled) grace period, or it might be some earlier grace period (for +example, if the CPU might have been in dyntick-idle mode for an extended +time period). The number after the "/" is the number that have executed +since boot until the current time. If this latter number stays constant +across repeated stall-warning messages, it is possible that RCU's softirq +handlers are no longer able to execute on this CPU. This can happen if +the stalled CPU is spinning with interrupts are disabled, or, in -rt +kernels, if a high-priority process is starving RCU's softirq handler. + +The "fqs=" shows the number of force-quiescent-state idle/offline +detection passes that the grace-period kthread has made across this +CPU since the last time that this CPU noted the beginning of a grace +period. + +The "detected by" line indicates which CPU detected the stall (in this +case, CPU 32), how many jiffies have elapsed since the start of the grace +period (in this case 2603), the grace-period sequence number (7075), and +an estimate of the total number of RCU callbacks queued across all CPUs +(625 in this case). + +If the grace period ends just as the stall warning starts printing, +there will be a spurious stall-warning message, which will include +the following:: + + INFO: Stall ended before state dump start + +This is rare, but does happen from time to time in real life. It is also +possible for a zero-jiffy stall to be flagged in this case, depending +on how the stall warning and the grace-period initialization happen to +interact. Please note that it is not possible to entirely eliminate this +sort of false positive without resorting to things like stop_machine(), +which is overkill for this sort of problem. + +If all CPUs and tasks have passed through quiescent states, but the +grace period has nevertheless failed to end, the stall-warning splat +will include something like the following:: + + All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 + +The "23807" indicates that it has been more than 23 thousand jiffies +since the grace-period kthread ran. The "jiffies_till_next_fqs" +indicates how frequently that kthread should run, giving the number +of jiffies between force-quiescent-state scans, in this case three, +which is way less than 23807. Finally, the root rcu_node structure's +->qsmask field is printed, which will normally be zero. + +If the relevant grace-period kthread has been unable to run prior to +the stall warning, as was the case in the "All QSes seen" line above, +the following additional line is printed:: + + rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 + Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. + +Starving the grace-period kthreads of CPU time can of course result +in RCU CPU stall warnings even when all CPUs and tasks have passed +through the required quiescent states. The "g" number shows the current +grace-period sequence number, the "f" precedes the ->gp_flags command +to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the +kthread is waiting for a short timeout, the "state" precedes value of the +task_struct ->state field, and the "cpu" indicates that the grace-period +kthread last ran on CPU 5. + +If the relevant grace-period kthread does not wake from FQS wait in a +reasonable time, then the following additional line is printed:: + + kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 + +The "23804" indicates that kthread's timer expired more than 23 thousand +jiffies ago. The rest of the line has meaning similar to the kthread +starvation case. + +Additionally, the following line is printed:: + + Possible timer handling issue on cpu=4 timer-softirq=11142 + +Here "cpu" indicates that the grace-period kthread last ran on CPU 4, +where it queued the fqs timer. The number following the "timer-softirq" +is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not +change on successive RCU CPU stall warnings, there is further reason to +suspect a timer problem. + +These messages are usually followed by stack dumps of the CPUs and tasks +involved in the stall. These stack traces can help you locate the cause +of the stall, keeping in mind that the CPU detecting the stall will have +an interrupt frame that is mainly devoted to detecting the stall. + + +Multiple Warnings From One Stall +================================ + +If a stall lasts long enough, multiple stall-warning messages will +be printed for it. The second and subsequent messages are printed at +longer intervals, so that the time between (say) the first and second +message will be about three times the interval between the beginning +of the stall and the first message. It can be helpful to compare the +stack dumps for the different messages for the same stalled grace period. + + +Stall Warnings for Expedited Grace Periods +========================================== + +If an expedited grace period detects a stall, it will place a message +like the following in dmesg:: + + INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. + +This indicates that CPU 7 has failed to respond to a reschedule IPI. +The three periods (".") following the CPU number indicate that the CPU +is online (otherwise the first period would instead have been "O"), +that the CPU was online at the beginning of the expedited grace period +(otherwise the second period would have instead been "o"), and that +the CPU has been online at least once since boot (otherwise, the third +period would instead have been "N"). The number before the "jiffies" +indicates that the expedited grace period has been going on for 21,119 +jiffies. The number following the "s:" indicates that the expedited +grace-period sequence counter is 73. The fact that this last value is +odd indicates that an expedited grace period is in flight. The number +following "root:" is a bitmask that indicates which children of the root +rcu_node structure correspond to CPUs and/or tasks that are blocking the +current expedited grace period. If the tree had more than one level, +additional hex numbers would be printed for the states of the other +rcu_node structures in the tree. + +As with normal grace periods, PREEMPT_RCU builds can be stalled by +tasks as well as by CPUs, and that the tasks will be indicated by PID, +for example, "P3421". + +It is entirely possible to see stall warnings from normal and from +expedited grace periods at about the same time during the same run. diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt deleted file mode 100644 index a360a8796710..000000000000 --- a/Documentation/RCU/stallwarn.txt +++ /dev/null @@ -1,316 +0,0 @@ -Using RCU's CPU Stall Detector - -This document first discusses what sorts of issues RCU's CPU stall -detector can locate, and then discusses kernel parameters and Kconfig -options that can be used to fine-tune the detector's operation. Finally, -this document explains the stall detector's "splat" format. - - -What Causes RCU CPU Stall Warnings? - -So your kernel printed an RCU CPU stall warning. The next question is -"What caused it?" The following problems can result in RCU CPU stall -warnings: - -o A CPU looping in an RCU read-side critical section. - -o A CPU looping with interrupts disabled. - -o A CPU looping with preemption disabled. - -o A CPU looping with bottom halves disabled. - -o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel - without invoking schedule(). If the looping in the kernel is - really expected and desirable behavior, you might need to add - some calls to cond_resched(). - -o Booting Linux using a console connection that is too slow to - keep up with the boot-time console-message rate. For example, - a 115Kbaud serial console can be -way- too slow to keep up - with boot-time message rates, and will frequently result in - RCU CPU stall warning messages. Especially if you have added - debug printk()s. - -o Anything that prevents RCU's grace-period kthreads from running. - This can result in the "All QSes seen" console-log message. - This message will include information on when the kthread last - ran and how often it should be expected to run. It can also - result in the "rcu_.*kthread starved for" console-log message, - which will include additional debugging information. - -o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might - happen to preempt a low-priority task in the middle of an RCU - read-side critical section. This is especially damaging if - that low-priority task is not permitted to run on any other CPU, - in which case the next RCU grace period can never complete, which - will eventually cause the system to run out of memory and hang. - While the system is in the process of running itself out of - memory, you might see stall-warning messages. - -o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that - is running at a higher priority than the RCU softirq threads. - This will prevent RCU callbacks from ever being invoked, - and in a CONFIG_PREEMPT_RCU kernel will further prevent - RCU grace periods from ever completing. Either way, the - system will eventually run out of memory and hang. In the - CONFIG_PREEMPT_RCU case, you might see stall-warning - messages. - - You can use the rcutree.kthread_prio kernel boot parameter to - increase the scheduling priority of RCU's kthreads, which can - help avoid this problem. However, please note that doing this - can increase your system's context-switch rate and thus degrade - performance. - -o A periodic interrupt whose handler takes longer than the time - interval between successive pairs of interrupts. This can - prevent RCU's kthreads and softirq handlers from running. - Note that certain high-overhead debugging options, for example - the function_graph tracer, can result in interrupt handler taking - considerably longer than normal, which can in turn result in - RCU CPU stall warnings. - -o Testing a workload on a fast system, tuning the stall-warning - timeout down to just barely avoid RCU CPU stall warnings, and then - running the same workload with the same stall-warning timeout on a - slow system. Note that thermal throttling and on-demand governors - can cause a single system to be sometimes fast and sometimes slow! - -o A hardware or software issue shuts off the scheduler-clock - interrupt on a CPU that is not in dyntick-idle mode. This - problem really has happened, and seems to be most likely to - result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. - -o A bug in the RCU implementation. - -o A hardware failure. This is quite unlikely, but has occurred - at least once in real life. A CPU failed in a running system, - becoming unresponsive, but not causing an immediate crash. - This resulted in a series of RCU CPU stall warnings, eventually - leading the realization that the CPU had failed. - -The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. -Note that SRCU does -not- have CPU stall warnings. Please note that -RCU only detects CPU stalls when there is a grace period in progress. -No grace period, no CPU stall warnings. - -To diagnose the cause of the stall, inspect the stack traces. -The offending function will usually be near the top of the stack. -If you have a series of stall warnings from a single extended stall, -comparing the stack traces can often help determine where the stall -is occurring, which will usually be in the function nearest the top of -that portion of the stack which remains the same from trace to trace. -If you can reliably trigger the stall, ftrace can be quite helpful. - -RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE -and with RCU's event tracing. For information on RCU's event tracing, -see include/trace/events/rcu.h. - - -Fine-Tuning the RCU CPU Stall Detector - -The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's -CPU stall detector, which detects conditions that unduly delay RCU grace -periods. This module parameter enables CPU stall detection by default, -but may be overridden via boot-time parameter or at runtime via sysfs. -The stall detector's idea of what constitutes "unduly delayed" is -controlled by a set of kernel configuration variables and cpp macros: - -CONFIG_RCU_CPU_STALL_TIMEOUT - - This kernel configuration parameter defines the period of time - that RCU will wait from the beginning of a grace period until it - issues an RCU CPU stall warning. This time period is normally - 21 seconds. - - This configuration parameter may be changed at runtime via the - /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however - this parameter is checked only at the beginning of a cycle. - So if you are 10 seconds into a 40-second stall, setting this - sysfs parameter to (say) five will shorten the timeout for the - -next- stall, or the following warning for the current stall - (assuming the stall lasts long enough). It will not affect the - timing of the next warning for the current stall. - - Stall-warning messages may be enabled and disabled completely via - /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. - -RCU_STALL_DELAY_DELTA - - Although the lockdep facility is extremely useful, it does add - some overhead. Therefore, under CONFIG_PROVE_RCU, the - RCU_STALL_DELAY_DELTA macro allows five extra seconds before - giving an RCU CPU stall warning message. (This is a cpp - macro, not a kernel configuration parameter.) - -RCU_STALL_RAT_DELAY - - The CPU stall detector tries to make the offending CPU print its - own warnings, as this often gives better-quality stack traces. - However, if the offending CPU does not detect its own stall in - the number of jiffies specified by RCU_STALL_RAT_DELAY, then - some other CPU will complain. This delay is normally set to - two jiffies. (This is a cpp macro, not a kernel configuration - parameter.) - -rcupdate.rcu_task_stall_timeout - - This boot/sysfs parameter controls the RCU-tasks stall warning - interval. A value of zero or less suppresses RCU-tasks stall - warnings. A positive value sets the stall-warning interval - in seconds. An RCU-tasks stall warning starts with the line: - - INFO: rcu_tasks detected stalls on tasks: - - And continues with the output of sched_show_task() for each - task stalling the current RCU-tasks grace period. - - -Interpreting RCU's CPU Stall-Detector "Splats" - -For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling, -it will print a message similar to the following: - - INFO: rcu_sched detected stalls on CPUs/tasks: - 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 - 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 - (detected by 32, t=2603 jiffies, g=7075, q=625) - -This message indicates that CPU 32 detected that CPUs 2 and 16 were both -causing stalls, and that the stall was affecting RCU-sched. This message -will normally be followed by stack dumps for each CPU. Please note that -PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that -the tasks will be indicated by PID, for example, "P3421". It is even -possible for an rcu_state stall to be caused by both CPUs -and- tasks, -in which case the offending CPUs and tasks will all be called out in the list. - -CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with -the RCU core for the past three grace periods. In contrast, CPU 16's "(0 -ticks this GP)" indicates that this CPU has not taken any scheduling-clock -interrupts during the current stalled grace period. - -The "idle=" portion of the message prints the dyntick-idle state. -The hex number before the first "/" is the low-order 12 bits of the -dynticks counter, which will have an even-numbered value if the CPU -is in dyntick-idle mode and an odd-numbered value otherwise. The hex -number between the two "/"s is the value of the nesting, which will be -a small non-negative number if in the idle loop (as shown above) and a -very large positive number otherwise. - -The "softirq=" portion of the message tracks the number of RCU softirq -handlers that the stalled CPU has executed. The number before the "/" -is the number that had executed since boot at the time that this CPU -last noted the beginning of a grace period, which might be the current -(stalled) grace period, or it might be some earlier grace period (for -example, if the CPU might have been in dyntick-idle mode for an extended -time period. The number after the "/" is the number that have executed -since boot until the current time. If this latter number stays constant -across repeated stall-warning messages, it is possible that RCU's softirq -handlers are no longer able to execute on this CPU. This can happen if -the stalled CPU is spinning with interrupts are disabled, or, in -rt -kernels, if a high-priority process is starving RCU's softirq handler. - -The "fqs=" shows the number of force-quiescent-state idle/offline -detection passes that the grace-period kthread has made across this -CPU since the last time that this CPU noted the beginning of a grace -period. - -The "detected by" line indicates which CPU detected the stall (in this -case, CPU 32), how many jiffies have elapsed since the start of the grace -period (in this case 2603), the grace-period sequence number (7075), and -an estimate of the total number of RCU callbacks queued across all CPUs -(625 in this case). - -In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed -for each CPU: - - 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1 - -The "last_accelerate:" prints the low-order 16 bits (in hex) of the -jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() -from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from -rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle -processing is enabled. - -If the grace period ends just as the stall warning starts printing, -there will be a spurious stall-warning message, which will include -the following: - - INFO: Stall ended before state dump start - -This is rare, but does happen from time to time in real life. It is also -possible for a zero-jiffy stall to be flagged in this case, depending -on how the stall warning and the grace-period initialization happen to -interact. Please note that it is not possible to entirely eliminate this -sort of false positive without resorting to things like stop_machine(), -which is overkill for this sort of problem. - -If all CPUs and tasks have passed through quiescent states, but the -grace period has nevertheless failed to end, the stall-warning splat -will include something like the following: - - All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 - -The "23807" indicates that it has been more than 23 thousand jiffies -since the grace-period kthread ran. The "jiffies_till_next_fqs" -indicates how frequently that kthread should run, giving the number -of jiffies between force-quiescent-state scans, in this case three, -which is way less than 23807. Finally, the root rcu_node structure's -->qsmask field is printed, which will normally be zero. - -If the relevant grace-period kthread has been unable to run prior to -the stall warning, as was the case in the "All QSes seen" line above, -the following additional line is printed: - - kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 - -Starving the grace-period kthreads of CPU time can of course result -in RCU CPU stall warnings even when all CPUs and tasks have passed -through the required quiescent states. The "g" number shows the current -grace-period sequence number, the "f" precedes the ->gp_flags command -to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the -kthread is waiting for a short timeout, the "state" precedes value of the -task_struct ->state field, and the "cpu" indicates that the grace-period -kthread last ran on CPU 5. - - -Multiple Warnings From One Stall - -If a stall lasts long enough, multiple stall-warning messages will be -printed for it. The second and subsequent messages are printed at -longer intervals, so that the time between (say) the first and second -message will be about three times the interval between the beginning -of the stall and the first message. - - -Stall Warnings for Expedited Grace Periods - -If an expedited grace period detects a stall, it will place a message -like the following in dmesg: - - INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. - -This indicates that CPU 7 has failed to respond to a reschedule IPI. -The three periods (".") following the CPU number indicate that the CPU -is online (otherwise the first period would instead have been "O"), -that the CPU was online at the beginning of the expedited grace period -(otherwise the second period would have instead been "o"), and that -the CPU has been online at least once since boot (otherwise, the third -period would instead have been "N"). The number before the "jiffies" -indicates that the expedited grace period has been going on for 21,119 -jiffies. The number following the "s:" indicates that the expedited -grace-period sequence counter is 73. The fact that this last value is -odd indicates that an expedited grace period is in flight. The number -following "root:" is a bitmask that indicates which children of the root -rcu_node structure correspond to CPUs and/or tasks that are blocking the -current expedited grace period. If the tree had more than one level, -additional hex numbers would be printed for the states of the other -rcu_node structures in the tree. - -As with normal grace periods, PREEMPT_RCU builds can be stalled by -tasks as well as by CPUs, and that the tasks will be indicated by PID, -for example, "P3421". - -It is entirely possible to see stall warnings from normal and from -expedited grace periods at about the same time during the same run. diff --git a/Documentation/RCU/torture.rst b/Documentation/RCU/torture.rst new file mode 100644 index 000000000000..a90147713062 --- /dev/null +++ b/Documentation/RCU/torture.rst @@ -0,0 +1,293 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +RCU Torture Test Operation +========================== + + +CONFIG_RCU_TORTURE_TEST +======================= + +The CONFIG_RCU_TORTURE_TEST config option is available for all RCU +implementations. It creates an rcutorture kernel module that can +be loaded to run a torture test. The test periodically outputs +status messages via printk(), which can be examined via the dmesg +command (perhaps grepping for "torture"). The test is started +when the module is loaded, and stops when the module is unloaded. + +Module parameters are prefixed by "rcutorture." in +Documentation/admin-guide/kernel-parameters.txt. + +Output +====== + +The statistics output is as follows:: + + rcu-torture:--- Start of test: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 + rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767 + rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 + rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 + rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 + rcu-torture:--- End of test: SUCCESS: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 + +The command "dmesg | grep torture:" will extract this information on +most systems. On more esoteric configurations, it may be necessary to +use other commands to access the output of the printk()s used by +the RCU torture test. The printk()s use KERN_ALERT, so they should +be evident. ;-) + +The first and last lines show the rcutorture module parameters, and the +last line shows either "SUCCESS" or "FAILURE", based on rcutorture's +automatic determination as to whether RCU operated correctly. + +The entries are as follows: + +* "rtc": The hexadecimal address of the structure currently visible + to readers. + +* "ver": The number of times since boot that the RCU writer task + has changed the structure visible to readers. + +* "tfle": If non-zero, indicates that the "torture freelist" + containing structures to be placed into the "rtc" area is empty. + This condition is important, since it can fool you into thinking + that RCU is working when it is not. :-/ + +* "rta": Number of structures allocated from the torture freelist. + +* "rtaf": Number of allocations from the torture freelist that have + failed due to the list being empty. It is not unusual for this + to be non-zero, but it is bad for it to be a large fraction of + the value indicated by "rta". + +* "rtf": Number of frees into the torture freelist. + +* "rtmbe": A non-zero value indicates that rcutorture believes that + rcu_assign_pointer() and rcu_dereference() are not working + correctly. This value should be zero. + +* "rtbe": A non-zero value indicates that one of the rcu_barrier() + family of functions is not working correctly. + +* "rtbke": rcutorture was unable to create the real-time kthreads + used to force RCU priority inversion. This value should be zero. + +* "rtbre": Although rcutorture successfully created the kthreads + used to force RCU priority inversion, it was unable to set them + to the real-time priority level of 1. This value should be zero. + +* "rtbf": The number of times that RCU priority boosting failed + to resolve RCU priority inversion. + +* "rtb": The number of times that rcutorture attempted to force + an RCU priority inversion condition. If you are testing RCU + priority boosting via the "test_boost" module parameter, this + value should be non-zero. + +* "nt": The number of times rcutorture ran RCU read-side code from + within a timer handler. This value should be non-zero only + if you specified the "irqreader" module parameter. + +* "Reader Pipe": Histogram of "ages" of structures seen by readers. + If any entries past the first two are non-zero, RCU is broken. + And rcutorture prints the error flag string "!!!" to make sure + you notice. The age of a newly allocated structure is zero, + it becomes one when removed from reader visibility, and is + incremented once per grace period subsequently -- and is freed + after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. + + The output displayed above was taken from a correctly working + RCU. If you want to see what it looks like when broken, break + it yourself. ;-) + +* "Reader Batch": Another histogram of "ages" of structures seen + by readers, but in terms of counter flips (or batches) rather + than in terms of grace periods. The legal number of non-zero + entries is again two. The reason for this separate view is that + it is sometimes easier to get the third entry to show up in the + "Reader Batch" list than in the "Reader Pipe" list. + +* "Free-Block Circulation": Shows the number of torture structures + that have reached a given point in the pipeline. The first element + should closely correspond to the number of structures allocated, + the second to the number that have been removed from reader view, + and all but the last remaining to the corresponding number of + passes through a grace period. The last entry should be zero, + as it is only incremented if a torture structure's counter + somehow gets incremented farther than it should. + +Different implementations of RCU can provide implementation-specific +additional information. For example, Tree SRCU provides the following +additional line:: + + srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6) + +This line shows the per-CPU counter state, in this case for Tree SRCU +using a dynamically allocated srcu_struct (hence "srcud-" rather than +"srcu-"). The numbers in parentheses are the values of the "old" and +"current" counters for the corresponding CPU. The "idx" value maps the +"old" and "current" values to the underlying array, and is useful for +debugging. The final "T" entry contains the totals of the counters. + +Usage on Specific Kernel Builds +=============================== + +It is sometimes desirable to torture RCU on a specific kernel build, +for example, when preparing to put that kernel build into production. +In that case, the kernel should be built with CONFIG_RCU_TORTURE_TEST=m +so that the test can be started using modprobe and terminated using rmmod. + +For example, the following script may be used to torture RCU:: + + #!/bin/sh + + modprobe rcutorture + sleep 3600 + rmmod rcutorture + dmesg | grep torture: + +The output can be manually inspected for the error flag of "!!!". +One could of course create a more elaborate script that automatically +checked for such errors. The "rmmod" command forces a "SUCCESS", +"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first +two are self-explanatory, while the last indicates that while there +were no RCU failures, CPU-hotplug problems were detected. + + +Usage on Mainline Kernels +========================= + +When using rcutorture to test changes to RCU itself, it is often +necessary to build a number of kernels in order to test that change +across a broad range of combinations of the relevant Kconfig options +and of the relevant kernel boot parameters. In this situation, use +of modprobe and rmmod can be quite time-consuming and error-prone. + +Therefore, the tools/testing/selftests/rcutorture/bin/kvm.sh +script is available for mainline testing for x86, arm64, and +powerpc. By default, it will run the series of tests specified by +tools/testing/selftests/rcutorture/configs/rcu/CFLIST, with each test +running for 30 minutes within a guest OS using a minimal userspace +supplied by an automatically generated initrd. After the tests are +complete, the resulting build products and console output are analyzed +for errors and the results of the runs are summarized. + +On larger systems, rcutorture testing can be accelerated by passing the +--cpus argument to kvm.sh. For example, on a 64-CPU system, "--cpus 43" +would use up to 43 CPUs to run tests concurrently, which as of v5.4 would +complete all the scenarios in two batches, reducing the time to complete +from about eight hours to about one hour (not counting the time to build +the sixteen kernels). The "--dryrun sched" argument will not run tests, +but rather tell you how the tests would be scheduled into batches. This +can be useful when working out how many CPUs to specify in the --cpus +argument. + +Not all changes require that all scenarios be run. For example, a change +to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the +--configs argument to kvm.sh as follows: "--configs 'SRCU-N SRCU-P'". +Large systems can run multiple copies of of the full set of scenarios, +for example, a system with 448 hardware threads can run five instances +of the full set concurrently. To make this happen:: + + kvm.sh --cpus 448 --configs '5*CFLIST' + +Alternatively, such a system can run 56 concurrent instances of a single +eight-CPU scenario:: + + kvm.sh --cpus 448 --configs '56*TREE04' + +Or 28 concurrent instances of each of two eight-CPU scenarios:: + + kvm.sh --cpus 448 --configs '28*TREE03 28*TREE04' + +Of course, each concurrent instance will use memory, which can be +limited using the --memory argument, which defaults to 512M. Small +values for memory may require disabling the callback-flooding tests +using the --bootargs parameter discussed below. + +Sometimes additional debugging is useful, and in such cases the --kconfig +parameter to kvm.sh may be used, for example, ``--kconfig 'CONFIG_KASAN=y'``. + +Kernel boot arguments can also be supplied, for example, to control +rcutorture's module parameters. For example, to test a change to RCU's +CPU stall-warning code, use "--bootargs 'rcutorture.stall_cpu=30'". +This will of course result in the scripting reporting a failure, namely +the resuling RCU CPU stall warning. As noted above, reducing memory may +require disabling rcutorture's callback-flooding tests:: + + kvm.sh --cpus 448 --configs '56*TREE04' --memory 128M \ + --bootargs 'rcutorture.fwd_progress=0' + +Sometimes all that is needed is a full set of kernel builds. This is +what the --buildonly argument does. + +Finally, the --trust-make argument allows each kernel build to reuse what +it can from the previous kernel build. + +There are additional more arcane arguments that are documented in the +source code of the kvm.sh script. + +If a run contains failures, the number of buildtime and runtime failures +is listed at the end of the kvm.sh output, which you really should redirect +to a file. The build products and console output of each run is kept in +tools/testing/selftests/rcutorture/res in timestamped directories. A +given directory can be supplied to kvm-find-errors.sh in order to have +it cycle you through summaries of errors and full error logs. For example:: + + tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh \ + tools/testing/selftests/rcutorture/res/2020.01.20-15.54.23 + +However, it is often more convenient to access the files directly. +Files pertaining to all scenarios in a run reside in the top-level +directory (2020.01.20-15.54.23 in the example above), while per-scenario +files reside in a subdirectory named after the scenario (for example, +"TREE04"). If a given scenario ran more than once (as in "--configs +'56*TREE04'" above), the directories corresponding to the second and +subsequent runs of that scenario include a sequence number, for example, +"TREE04.2", "TREE04.3", and so on. + +The most frequently used file in the top-level directory is testid.txt. +If the test ran in a git repository, then this file contains the commit +that was tested and any uncommitted changes in diff format. + +The most frequently used files in each per-scenario-run directory are: + +.config: + This file contains the Kconfig options. + +Make.out: + This contains build output for a specific scenario. + +console.log: + This contains the console output for a specific scenario. + This file may be examined once the kernel has booted, but + it might not exist if the build failed. + +vmlinux: + This contains the kernel, which can be useful with tools like + objdump and gdb. + +A number of additional files are available, but are less frequently used. +Many are intended for debugging of rcutorture itself or of its scripting. + +As of v5.4, a successful run with the default set of scenarios produces +the following summary at the end of the run on a 12-CPU system:: + + SRCU-N ------- 804233 GPs (148.932/s) [srcu: g10008272 f0x0 ] + SRCU-P ------- 202320 GPs (37.4667/s) [srcud: g1809476 f0x0 ] + SRCU-t ------- 1122086 GPs (207.794/s) [srcu: g0 f0x0 ] + SRCU-u ------- 1111285 GPs (205.794/s) [srcud: g1 f0x0 ] + TASKS01 ------- 19666 GPs (3.64185/s) [tasks: g0 f0x0 ] + TASKS02 ------- 20541 GPs (3.80389/s) [tasks: g0 f0x0 ] + TASKS03 ------- 19416 GPs (3.59556/s) [tasks: g0 f0x0 ] + TINY01 ------- 836134 GPs (154.84/s) [rcu: g0 f0x0 ] n_max_cbs: 34198 + TINY02 ------- 850371 GPs (157.476/s) [rcu: g0 f0x0 ] n_max_cbs: 2631 + TREE01 ------- 162625 GPs (30.1157/s) [rcu: g1124169 f0x0 ] + TREE02 ------- 333003 GPs (61.6672/s) [rcu: g2647753 f0x0 ] n_max_cbs: 35844 + TREE03 ------- 306623 GPs (56.782/s) [rcu: g2975325 f0x0 ] n_max_cbs: 1496497 + CPU count limited from 16 to 12 + TREE04 ------- 246149 GPs (45.5831/s) [rcu: g1695737 f0x0 ] n_max_cbs: 434961 + TREE05 ------- 314603 GPs (58.2598/s) [rcu: g2257741 f0x2 ] n_max_cbs: 193997 + TREE07 ------- 167347 GPs (30.9902/s) [rcu: g1079021 f0x0 ] n_max_cbs: 478732 + CPU count limited from 16 to 12 + TREE09 ------- 752238 GPs (139.303/s) [rcu: g13075057 f0x0 ] n_max_cbs: 99011 diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt deleted file mode 100644 index a41a0384d20c..000000000000 --- a/Documentation/RCU/torture.txt +++ /dev/null @@ -1,149 +0,0 @@ -RCU Torture Test Operation - - -CONFIG_RCU_TORTURE_TEST - -The CONFIG_RCU_TORTURE_TEST config option is available for all RCU -implementations. It creates an rcutorture kernel module that can -be loaded to run a torture test. The test periodically outputs -status messages via printk(), which can be examined via the dmesg -command (perhaps grepping for "torture"). The test is started -when the module is loaded, and stops when the module is unloaded. - -Module parameters are prefixed by "rcutorture." in -Documentation/admin-guide/kernel-parameters.txt. - -OUTPUT - -The statistics output is as follows: - - rcu-torture:--- Start of test: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 - rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767 - rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 - rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 - rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 - rcu-torture:--- End of test: SUCCESS: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 - -The command "dmesg | grep torture:" will extract this information on -most systems. On more esoteric configurations, it may be necessary to -use other commands to access the output of the printk()s used by -the RCU torture test. The printk()s use KERN_ALERT, so they should -be evident. ;-) - -The first and last lines show the rcutorture module parameters, and the -last line shows either "SUCCESS" or "FAILURE", based on rcutorture's -automatic determination as to whether RCU operated correctly. - -The entries are as follows: - -o "rtc": The hexadecimal address of the structure currently visible - to readers. - -o "ver": The number of times since boot that the RCU writer task - has changed the structure visible to readers. - -o "tfle": If non-zero, indicates that the "torture freelist" - containing structures to be placed into the "rtc" area is empty. - This condition is important, since it can fool you into thinking - that RCU is working when it is not. :-/ - -o "rta": Number of structures allocated from the torture freelist. - -o "rtaf": Number of allocations from the torture freelist that have - failed due to the list being empty. It is not unusual for this - to be non-zero, but it is bad for it to be a large fraction of - the value indicated by "rta". - -o "rtf": Number of frees into the torture freelist. - -o "rtmbe": A non-zero value indicates that rcutorture believes that - rcu_assign_pointer() and rcu_dereference() are not working - correctly. This value should be zero. - -o "rtbe": A non-zero value indicates that one of the rcu_barrier() - family of functions is not working correctly. - -o "rtbke": rcutorture was unable to create the real-time kthreads - used to force RCU priority inversion. This value should be zero. - -o "rtbre": Although rcutorture successfully created the kthreads - used to force RCU priority inversion, it was unable to set them - to the real-time priority level of 1. This value should be zero. - -o "rtbf": The number of times that RCU priority boosting failed - to resolve RCU priority inversion. - -o "rtb": The number of times that rcutorture attempted to force - an RCU priority inversion condition. If you are testing RCU - priority boosting via the "test_boost" module parameter, this - value should be non-zero. - -o "nt": The number of times rcutorture ran RCU read-side code from - within a timer handler. This value should be non-zero only - if you specified the "irqreader" module parameter. - -o "Reader Pipe": Histogram of "ages" of structures seen by readers. - If any entries past the first two are non-zero, RCU is broken. - And rcutorture prints the error flag string "!!!" to make sure - you notice. The age of a newly allocated structure is zero, - it becomes one when removed from reader visibility, and is - incremented once per grace period subsequently -- and is freed - after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. - - The output displayed above was taken from a correctly working - RCU. If you want to see what it looks like when broken, break - it yourself. ;-) - -o "Reader Batch": Another histogram of "ages" of structures seen - by readers, but in terms of counter flips (or batches) rather - than in terms of grace periods. The legal number of non-zero - entries is again two. The reason for this separate view is that - it is sometimes easier to get the third entry to show up in the - "Reader Batch" list than in the "Reader Pipe" list. - -o "Free-Block Circulation": Shows the number of torture structures - that have reached a given point in the pipeline. The first element - should closely correspond to the number of structures allocated, - the second to the number that have been removed from reader view, - and all but the last remaining to the corresponding number of - passes through a grace period. The last entry should be zero, - as it is only incremented if a torture structure's counter - somehow gets incremented farther than it should. - -Different implementations of RCU can provide implementation-specific -additional information. For example, Tree SRCU provides the following -additional line: - - srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6) - -This line shows the per-CPU counter state, in this case for Tree SRCU -using a dynamically allocated srcu_struct (hence "srcud-" rather than -"srcu-"). The numbers in parentheses are the values of the "old" and -"current" counters for the corresponding CPU. The "idx" value maps the -"old" and "current" values to the underlying array, and is useful for -debugging. The final "T" entry contains the totals of the counters. - - -USAGE - -The following script may be used to torture RCU: - - #!/bin/sh - - modprobe rcutorture - sleep 3600 - rmmod rcutorture - dmesg | grep torture: - -The output can be manually inspected for the error flag of "!!!". -One could of course create a more elaborate script that automatically -checked for such errors. The "rmmod" command forces a "SUCCESS", -"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first -two are self-explanatory, while the last indicates that while there -were no RCU failures, CPU-hotplug problems were detected. - -However, the tools/testing/selftests/rcutorture/bin/kvm.sh script -provides better automation, including automatic failure analysis. -It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd. -See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions -on setting up such an initrd. diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst index c7f147b8034f..1c747ac3f2c8 100644 --- a/Documentation/RCU/whatisRCU.rst +++ b/Documentation/RCU/whatisRCU.rst @@ -6,13 +6,15 @@ What is RCU? -- "Read, Copy, Update" Please note that the "What is RCU?" LWN series is an excellent place to start learning about RCU: -| 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ -| 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ -| 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ -| 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ -| 2010 Big API Table http://lwn.net/Articles/419086/ -| 5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/ -| 2014 Big API Table http://lwn.net/Articles/609973/ +| 1. What is RCU, Fundamentally? https://lwn.net/Articles/262464/ +| 2. What is RCU? Part 2: Usage https://lwn.net/Articles/263130/ +| 3. RCU part 3: the RCU API https://lwn.net/Articles/264090/ +| 4. The RCU API, 2010 Edition https://lwn.net/Articles/418853/ +| 2010 Big API Table https://lwn.net/Articles/419086/ +| 5. The RCU API, 2014 Edition https://lwn.net/Articles/609904/ +| 2014 Big API Table https://lwn.net/Articles/609973/ +| 6. The RCU API, 2019 Edition https://lwn.net/Articles/777036/ +| 2019 Big API Table https://lwn.net/Articles/777165/ What is RCU? @@ -39,9 +41,11 @@ different paths, as follows: :ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>` -:ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>` +:ref:`7. ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>` -:ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>` +:ref:`8. FULL LIST OF RCU APIs <8_whatisRCU>` + +:ref:`9. ANSWERS TO QUICK QUIZZES <9_whatisRCU>` People who prefer starting with a conceptual overview should focus on Section 1, though most readers will profit by reading this section at @@ -222,7 +226,7 @@ synchronize_rcu() be delayed. This property results in system resilience in face of denial-of-service attacks. Code using call_rcu() should limit update rate in order to gain this same sort of resilience. See - checklist.txt for some approaches to limiting the update rate. + checklist.rst for some approaches to limiting the update rate. rcu_assign_pointer() ^^^^^^^^^^^^^^^^^^^^ @@ -316,7 +320,7 @@ rcu_dereference() must prohibit. The rcu_dereference_protected() variant takes a lockdep expression to indicate which locks must be acquired by the caller. If the indicated protection is not provided, - a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst + a lockdep splat is emitted. See Design/Requirements/Requirements.rst and the API's code comments for more details and example usage. .. [2] If the list_for_each_entry_rcu() instance might be used by @@ -360,7 +364,7 @@ order to amortize their overhead over many uses of the corresponding APIs. There are at least three flavors of RCU usage in the Linux kernel. The diagram above shows the most common one. On the updater side, the rcu_assign_pointer(), -sychronize_rcu() and call_rcu() primitives used are the same for all three +synchronize_rcu() and call_rcu() primitives used are the same for all three flavors. However for protection (on the reader side), the primitives used vary depending on the flavor: @@ -397,8 +401,7 @@ for specialized uses, but are relatively uncommon. This section shows a simple use of the core RCU API to protect a global pointer to a dynamically allocated structure. More-typical -uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`, -:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`. +uses of RCU may be found in listRCU.rst, arrayRCU.rst, and NMI-RCU.rst. :: struct foo { @@ -480,10 +483,9 @@ So, to sum up: RCU read-side critical sections that might be referencing that data item. -See checklist.txt for additional rules to follow when using RCU. -And again, more-typical uses of RCU may be found in :ref:`listRCU.rst -<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst -<NMI_rcu_doc>`. +See checklist.rst for additional rules to follow when using RCU. +And again, more-typical uses of RCU may be found in listRCU.rst, +arrayRCU.rst, and NMI-RCU.rst. .. _4_whatisRCU: @@ -497,8 +499,7 @@ long -- there might be other high-priority work to be done. In such cases, one uses call_rcu() rather than synchronize_rcu(). The call_rcu() API is as follows:: - void call_rcu(struct rcu_head * head, - void (*func)(struct rcu_head *head)); + void call_rcu(struct rcu_head *head, rcu_callback_t func); This function invokes func(head) after a grace period has elapsed. This invocation might happen from either softirq or process context, @@ -578,7 +579,7 @@ to avoid having to write your own callback:: kfree_rcu(old_fp, rcu); -Again, see checklist.txt for additional rules governing the use of RCU. +Again, see checklist.rst for additional rules governing the use of RCU. .. _5_whatisRCU: @@ -662,7 +663,7 @@ been able to write-acquire the lock otherwise. The smp_mb__after_spinlock() promotes synchronize_rcu() to a full memory barrier in compliance with the "Memory-Barrier Guarantees" listed in: - Documentation/RCU/Design/Requirements/Requirements.rst + Design/Requirements/Requirements.rst It is possible to nest rcu_read_lock(), since reader-writer locks may be recursively acquired. Note also that rcu_read_lock() is immune @@ -678,13 +679,13 @@ Quick Quiz #1: occur when using this algorithm in a real-world Linux kernel? How could this deadlock be avoided? -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` 5B. "TOY" EXAMPLE #2: CLASSIC RCU ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and -on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT +on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. :: @@ -733,16 +734,16 @@ Quick Quiz #2: Give an example where Classic RCU's read-side overhead is **negative**. -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` .. _quiz_3: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` .. _6_whatisRCU: @@ -873,7 +874,84 @@ be used in place of synchronize_rcu(). .. _7_whatisRCU: -7. FULL LIST OF RCU APIs +7. ANALOGY WITH REFERENCE COUNTING +----------------------------------- + +The reader-writer analogy (illustrated by the previous section) is not +always the best way to think about using RCU. Another helpful analogy +considers RCU an effective reference count on everything which is +protected by RCU. + +A reference count typically does not prevent the referenced object's +values from changing, but does prevent changes to type -- particularly the +gross change of type that happens when that object's memory is freed and +re-allocated for some other purpose. Once a type-safe reference to the +object is obtained, some other mechanism is needed to ensure consistent +access to the data in the object. This could involve taking a spinlock, +but with RCU the typical approach is to perform reads with SMP-aware +operations such as smp_load_acquire(), to perform updates with atomic +read-modify-write operations, and to provide the necessary ordering. +RCU provides a number of support functions that embed the required +operations and ordering, such as the list_for_each_entry_rcu() macro +used in the previous section. + +A more focused view of the reference counting behavior is that, +between rcu_read_lock() and rcu_read_unlock(), any reference taken with +rcu_dereference() on a pointer marked as ``__rcu`` can be treated as +though a reference-count on that object has been temporarily increased. +This prevents the object from changing type. Exactly what this means +will depend on normal expectations of objects of that type, but it +typically includes that spinlocks can still be safely locked, normal +reference counters can be safely manipulated, and ``__rcu`` pointers +can be safely dereferenced. + +Some operations that one might expect to see on an object for +which an RCU reference is held include: + + - Copying out data that is guaranteed to be stable by the object's type. + - Using kref_get_unless_zero() or similar to get a longer-term + reference. This may fail of course. + - Acquiring a spinlock in the object, and checking if the object still + is the expected object and if so, manipulating it freely. + +The understanding that RCU provides a reference that only prevents a +change of type is particularly visible with objects allocated from a +slab cache marked ``SLAB_TYPESAFE_BY_RCU``. RCU operations may yield a +reference to an object from such a cache that has been concurrently freed +and the memory reallocated to a completely different object, though of +the same type. In this case RCU doesn't even protect the identity of the +object from changing, only its type. So the object found may not be the +one expected, but it will be one where it is safe to take a reference +(and then potentially acquiring a spinlock), allowing subsequent code +to check whether the identity matches expectations. It is tempting +to simply acquire the spinlock without first taking the reference, but +unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be +initialized after each and every call to kmem_cache_alloc(), which renders +reference-free spinlock acquisition completely unsafe. Therefore, when +using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter. + +With traditional reference counting -- such as that implemented by the +kref library in Linux -- there is typically code that runs when the last +reference to an object is dropped. With kref, this is the function +passed to kref_put(). When RCU is being used, such finalization code +must not be run until all ``__rcu`` pointers referencing the object have +been updated, and then a grace period has passed. Every remaining +globally visible pointer to the object must be considered to be a +potential counted reference, and the finalization code is typically run +using call_rcu() only after all those pointers have been changed. + +To see how to choose between these two analogies -- of RCU as a +reader-writer lock and RCU as a reference counting system -- it is useful +to reflect on the scale of the thing being protected. The reader-writer +lock analogy looks at larger multi-part objects such as a linked list +and shows how RCU can facilitate concurrency while elements are added +to, and removed from, the list. The reference-count analogy looks at +the individual objects and looks at how they can be accessed safely +within whatever whole they are a part of. + +.. _8_whatisRCU: + +8. FULL LIST OF RCU APIs ------------------------- The RCU APIs are documented in docbook-format header comments in the @@ -986,14 +1064,20 @@ SRCU: Initialization/cleanup:: init_srcu_struct cleanup_srcu_struct -All: lockdep-checked RCU-protected pointer access:: +All: lockdep-checked RCU utility APIs:: - rcu_access_pointer - rcu_dereference_raw RCU_LOCKDEP_WARN rcu_sleep_check RCU_NONIDLE +All: Unchecked RCU-protected pointer access:: + + rcu_dereference_raw + +All: Unchecked RCU-protected pointer access with dereferencing prohibited:: + + rcu_access_pointer + See the comment headers in the source code (or the docbook generated from them) for more information. @@ -1036,9 +1120,9 @@ g. Otherwise, use RCU. Of course, this all assumes that you have determined that RCU is in fact the right tool for your job. -.. _8_whatisRCU: +.. _9_whatisRCU: -8. ANSWERS TO QUICK QUIZZES +9. ANSWERS TO QUICK QUIZZES ---------------------------- Quick Quiz #1: @@ -1094,7 +1178,7 @@ Quick Quiz #2: overhead is **negative**. Answer: - Imagine a single-CPU system with a non-CONFIG_PREEMPT + Imagine a single-CPU system with a non-CONFIG_PREEMPTION kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling @@ -1121,10 +1205,10 @@ Answer: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? Answer: - Just as PREEMPT_RT permits preemption of spinlock + Just as CONFIG_PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical |
