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Diffstat (limited to 'Documentation/RCU/rcubarrier.txt')
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diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt deleted file mode 100644 index a2782df69732..000000000000 --- a/Documentation/RCU/rcubarrier.txt +++ /dev/null @@ -1,325 +0,0 @@ -RCU and Unloadable Modules - -[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/] - -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 -kernels, generate no code whatsoever. - -This means that RCU writers are unaware of the presence of concurrent -readers, so that RCU updates to shared data must be undertaken quite -carefully, leaving an old version of the data structure in place until all -pre-existing readers have finished. These old versions are needed because -such readers might hold a reference to them. RCU updates can therefore be -rather expensive, and RCU is thus best suited for read-mostly situations. - -How can an RCU writer possibly determine when all readers are finished, -given that readers might well leave absolutely no trace of their -presence? There is a synchronize_rcu() primitive that blocks until all -pre-existing readers have completed. An updater wishing to delete an -element p from a linked list might do the following, while holding an -appropriate lock, of course: - - list_del_rcu(p); - synchronize_rcu(); - kfree(p); - -But the above code cannot be used in IRQ context -- the call_rcu() -primitive must be used instead. This primitive takes a pointer to an -rcu_head struct placed within the RCU-protected data structure and -another pointer to a function that may be invoked later to free that -structure. Code to delete an element p from the linked list from IRQ -context might then be as follows: - - list_del_rcu(p); - call_rcu(&p->rcu, p_callback); - -Since call_rcu() never blocks, this code can safely be used from within -IRQ context. The function p_callback() might be defined as follows: - - static void p_callback(struct rcu_head *rp) - { - struct pstruct *p = container_of(rp, struct pstruct, rcu); - - kfree(p); - } - - -Unloading Modules That Use call_rcu() - -But what if p_callback is defined in an unloadable module? - -If we unload the module while some RCU callbacks are pending, -the CPUs executing these callbacks are going to be severely -disappointed when they are later invoked, as fancifully depicted at -http://lwn.net/images/ns/kernel/rcu-drop.jpg. - -We could try placing a synchronize_rcu() in the module-exit code path, -but this is not sufficient. Although synchronize_rcu() does wait for a -grace period to elapse, it does not wait for the callbacks to complete. - -One might be tempted to try several back-to-back synchronize_rcu() -calls, but this is still not guaranteed to work. If there is a very -heavy RCU-callback load, then some of the callbacks might be deferred -in order to allow other processing to proceed. Such deferral is required -in realtime kernels in order to avoid excessive scheduling latencies. - - -rcu_barrier() - -We instead need the rcu_barrier() primitive. Rather than waiting for -a grace period to elapse, rcu_barrier() waits for all outstanding RCU -callbacks to complete. Please note that rcu_barrier() does -not- imply -synchronize_rcu(), in particular, if there are no RCU callbacks queued -anywhere, rcu_barrier() is within its rights to return immediately, -without waiting for a grace period to elapse. - -Pseudo-code using rcu_barrier() is as follows: - - 1. Prevent any new RCU callbacks from being posted. - 2. Execute rcu_barrier(). - 3. Allow the module to be unloaded. - -There is also an srcu_barrier() function for SRCU, and you of course -must match the flavor of rcu_barrier() with that of call_rcu(). If your -module uses multiple flavors of call_rcu(), then it must also use multiple -flavors of rcu_barrier() when unloading that module. For example, if -it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on -srcu_struct_2(), then the following three lines of code will be required -when unloading: - - 1 rcu_barrier(); - 2 srcu_barrier(&srcu_struct_1); - 3 srcu_barrier(&srcu_struct_2); - -The rcutorture module makes use of rcu_barrier() in its exit function -as follows: - - 1 static void - 2 rcu_torture_cleanup(void) - 3 { - 4 int i; - 5 - 6 fullstop = 1; - 7 if (shuffler_task != NULL) { - 8 VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task"); - 9 kthread_stop(shuffler_task); -10 } -11 shuffler_task = NULL; -12 -13 if (writer_task != NULL) { -14 VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task"); -15 kthread_stop(writer_task); -16 } -17 writer_task = NULL; -18 -19 if (reader_tasks != NULL) { -20 for (i = 0; i < nrealreaders; i++) { -21 if (reader_tasks[i] != NULL) { -22 VERBOSE_PRINTK_STRING( -23 "Stopping rcu_torture_reader task"); -24 kthread_stop(reader_tasks[i]); -25 } -26 reader_tasks[i] = NULL; -27 } -28 kfree(reader_tasks); -29 reader_tasks = NULL; -30 } -31 rcu_torture_current = NULL; -32 -33 if (fakewriter_tasks != NULL) { -34 for (i = 0; i < nfakewriters; i++) { -35 if (fakewriter_tasks[i] != NULL) { -36 VERBOSE_PRINTK_STRING( -37 "Stopping rcu_torture_fakewriter task"); -38 kthread_stop(fakewriter_tasks[i]); -39 } -40 fakewriter_tasks[i] = NULL; -41 } -42 kfree(fakewriter_tasks); -43 fakewriter_tasks = NULL; -44 } -45 -46 if (stats_task != NULL) { -47 VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task"); -48 kthread_stop(stats_task); -49 } -50 stats_task = NULL; -51 -52 /* Wait for all RCU callbacks to fire. */ -53 rcu_barrier(); -54 -55 rcu_torture_stats_print(); /* -After- the stats thread is stopped! */ -56 -57 if (cur_ops->cleanup != NULL) -58 cur_ops->cleanup(); -59 if (atomic_read(&n_rcu_torture_error)) -60 rcu_torture_print_module_parms("End of test: FAILURE"); -61 else -62 rcu_torture_print_module_parms("End of test: SUCCESS"); -63 } - -Line 6 sets a global variable that prevents any RCU callbacks from -re-posting themselves. This will not be necessary in most cases, since -RCU callbacks rarely include calls to call_rcu(). However, the rcutorture -module is an exception to this rule, and therefore needs to set this -global variable. - -Lines 7-50 stop all the kernel tasks associated with the rcutorture -module. Therefore, once execution reaches line 53, no more rcutorture -RCU callbacks will be posted. The rcu_barrier() call on line 53 waits -for any pre-existing callbacks to complete. - -Then lines 55-62 print status and do operation-specific cleanup, and -then return, permitting the module-unload operation to be completed. - -Quick Quiz #1: Is there any other situation where rcu_barrier() might - be required? - -Your module might have additional complications. For example, if your -module invokes call_rcu() from timers, you will need to first cancel all -the timers, and only then invoke rcu_barrier() to wait for any remaining -RCU callbacks to complete. - -Of course, if you module uses call_rcu(), you will need to invoke -rcu_barrier() before unloading. Similarly, if your module uses -call_srcu(), you will need to invoke srcu_barrier() before unloading, -and on the same srcu_struct structure. If your module uses call_rcu() --and- call_srcu(), then you will need to invoke rcu_barrier() -and- -srcu_barrier(). - - -Implementing rcu_barrier() - -Dipankar Sarma's implementation of rcu_barrier() makes use of the fact -that RCU callbacks are never reordered once queued on one of the per-CPU -queues. His implementation queues an RCU callback on each of the per-CPU -callback queues, and then waits until they have all started executing, at -which point, all earlier RCU callbacks are guaranteed to have completed. - -The original code for rcu_barrier() was as follows: - - 1 void rcu_barrier(void) - 2 { - 3 BUG_ON(in_interrupt()); - 4 /* Take cpucontrol mutex to protect against CPU hotplug */ - 5 mutex_lock(&rcu_barrier_mutex); - 6 init_completion(&rcu_barrier_completion); - 7 atomic_set(&rcu_barrier_cpu_count, 0); - 8 on_each_cpu(rcu_barrier_func, NULL, 0, 1); - 9 wait_for_completion(&rcu_barrier_completion); -10 mutex_unlock(&rcu_barrier_mutex); -11 } - -Line 3 verifies that the caller is in process context, and lines 5 and 10 -use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the -global completion and counters at a time, which are initialized on lines -6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is -shown below. Note that the final "1" in on_each_cpu()'s argument list -ensures that all the calls to rcu_barrier_func() will have completed -before on_each_cpu() returns. Line 9 then waits for the completion. - -This code was rewritten in 2008 and several times thereafter, but this -still gives the general idea. - -The rcu_barrier_func() runs on each CPU, where it invokes call_rcu() -to post an RCU callback, as follows: - - 1 static void rcu_barrier_func(void *notused) - 2 { - 3 int cpu = smp_processor_id(); - 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu); - 5 struct rcu_head *head; - 6 - 7 head = &rdp->barrier; - 8 atomic_inc(&rcu_barrier_cpu_count); - 9 call_rcu(head, rcu_barrier_callback); -10 } - -Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure, -which contains the struct rcu_head that needed for the later call to -call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line -8 increments a global counter. This counter will later be decremented -by the callback. Line 9 then registers the rcu_barrier_callback() on -the current CPU's queue. - -The rcu_barrier_callback() function simply atomically decrements the -rcu_barrier_cpu_count variable and finalizes the completion when it -reaches zero, as follows: - - 1 static void rcu_barrier_callback(struct rcu_head *notused) - 2 { - 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count)) - 4 complete(&rcu_barrier_completion); - 5 } - -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes - immediately (thus incrementing rcu_barrier_cpu_count to the - value one), but the other CPU's rcu_barrier_func() invocations - are delayed for a full grace period? Couldn't this result in - rcu_barrier() returning prematurely? - -The current rcu_barrier() implementation is more complex, due to the need -to avoid disturbing idle CPUs (especially on battery-powered systems) -and the need to minimally disturb non-idle CPUs in real-time systems. -However, the code above illustrates the concepts. - - -rcu_barrier() Summary - -The rcu_barrier() primitive has seen relatively little use, since most -code using RCU is in the core kernel rather than in modules. However, if -you are using RCU from an unloadable module, you need to use rcu_barrier() -so that your module may be safely unloaded. - - -Answers to Quick Quizzes - -Quick Quiz #1: Is there any other situation where rcu_barrier() might - be required? - -Answer: Interestingly enough, rcu_barrier() was not originally - implemented for module unloading. Nikita Danilov was using - RCU in a filesystem, which resulted in a similar situation at - filesystem-unmount time. Dipankar Sarma coded up rcu_barrier() - in response, so that Nikita could invoke it during the - filesystem-unmount process. - - Much later, yours truly hit the RCU module-unload problem when - implementing rcutorture, and found that rcu_barrier() solves - this problem as well. - -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes - immediately (thus incrementing rcu_barrier_cpu_count to the - value one), but the other CPU's rcu_barrier_func() invocations - are delayed for a full grace period? Couldn't this result in - rcu_barrier() returning prematurely? - -Answer: This cannot happen. The reason is that on_each_cpu() has its last - argument, the wait flag, set to "1". This flag is passed through - 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, - 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. - - Therefore, on_each_cpu() disables preemption across its call - to smp_call_function() and also across the local call to - rcu_barrier_func(). This prevents the local CPU from context - switching, again preventing grace periods from completing. This - means that all CPUs have executed rcu_barrier_func() before - the first rcu_barrier_callback() can possibly execute, in turn - preventing rcu_barrier_cpu_count from prematurely reaching zero. - - Currently, -rt implementations of RCU keep but a single global - queue for RCU callbacks, and thus do not suffer from this - problem. However, when the -rt RCU eventually does have per-CPU - callback queues, things will have to change. One simple change - is to add an rcu_read_lock() before line 8 of rcu_barrier() - and an rcu_read_unlock() after line 8 of this same function. If - you can think of a better change, please let me know! |