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Diffstat (limited to 'Documentation/RCU/Design/Requirements/Requirements.html')
| -rw-r--r-- | Documentation/RCU/Design/Requirements/Requirements.html | 37 |
1 files changed, 30 insertions, 7 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index a4d3838130e4..21593496aca6 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -547,7 +547,7 @@ The <tt>rcu_access_pointer()</tt> on line 6 is similar to It could reuse a value formerly fetched from this same pointer. It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time manner, resulting in <i>load tearing</i>, in turn resulting a bytewise - mash-up of two distince pointer values. + mash-up of two distinct pointer values. It might even use value-speculation optimizations, where it makes a wrong guess, but by the time it gets around to checking the value, an update has changed the pointer to match the wrong guess. @@ -659,6 +659,29 @@ systems with more than one CPU: In other words, a given instance of <tt>synchronize_rcu()</tt> can avoid waiting on a given RCU read-side critical section only if it can prove that <tt>synchronize_rcu()</tt> started first. + + <p> + A related question is “When <tt>rcu_read_lock()</tt> + 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 + <tt>rcu_read_lock()</tt> 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, then a given RCU read-side critical + section begins before a given grace period when some access + preceding the grace period observes the effect of some access + within the critical section, in which case none of the accesses + within the critical section may observe the effects of any + access following the grace period. + + <p> + As of late 2016, mathematical models of RCU take this + viewpoint, for example, see slides 62 and 63 + of the + <a href="http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.2016.10.04c.LCE.pdf">2016 LinuxCon EU</a> + presentation. </font></td></tr> <tr><td> </td></tr> </table> @@ -1457,7 +1480,7 @@ speed-of-light delays if nothing else. <p> Furthermore, uncertainty about external state is inherent in many cases. -For example, a pair of veternarians might use heartbeat to determine +For example, a pair of veterinarians might use heartbeat to determine whether or not a given cat was alive. But how long should they wait after the last heartbeat to decide that the cat is in fact dead? @@ -1466,9 +1489,9 @@ mean that a relaxed cat would be considered to cycle between death and life more than 100 times per minute. Moreover, just as with human beings, a cat's heart might stop for some period of time, so the exact wait period is a judgment call. -One of our pair of veternarians might wait 30 seconds before pronouncing +One of our pair of veterinarians might wait 30 seconds before pronouncing the cat dead, while the other might insist on waiting a full minute. -The two veternarians would then disagree on the state of the cat during +The two veterinarians would then disagree on the state of the cat during the final 30 seconds of the minute following the last heartbeat. <p> @@ -1922,7 +1945,7 @@ guard against mishaps and misuse: <ol> <li> It is all too easy to forget to use <tt>rcu_read_lock()</tt> everywhere that it is needed, so kernels built with - <tt>CONFIG_PROVE_RCU=y</tt> will spat if + <tt>CONFIG_PROVE_RCU=y</tt> will splat if <tt>rcu_dereference()</tt> is used outside of an RCU read-side critical section. Update-side code can use <tt>rcu_dereference_protected()</tt>, @@ -2398,7 +2421,7 @@ However, there are some restrictions on the code placed within <li> Blocking is prohibited. In practice, this is not a serious restriction given that idle tasks are prohibited from blocking to begin with. -<li> Although nesting <tt>RCU_NONIDLE()</tt> is permited, they cannot +<li> Although nesting <tt>RCU_NONIDLE()</tt> 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 @@ -2862,7 +2885,7 @@ APIs for defining and initializing <tt>srcu_struct</tt> structures. <h3><a name="Tasks RCU">Tasks RCU</a></h3> <p> -Some forms of tracing use “tramopolines” to handle the +Some forms of tracing use “trampolines” to handle the binary rewriting 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. |