Commit-ID: e2c85cb12c86b080ee344928618eb918fa227ac8 Gitweb: http://git.kernel.org/tip/e2c85cb12c86b080ee344928618eb918fa227ac8 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Fri, 7 Oct 2016 10:33:24 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Mon, 14 Nov 2016 10:39:36 -0800 documentation: Present updated RCU guarantee Recent memory-model work deduces the relationships of RCU read-side critical sections and grace periods based on the relationships of accesses within a critical section and accesses preceding and following the grace period. This commit therefore adds this viewpoint. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- .../RCU/Design/Requirements/Requirements.html | 25 +++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index a4d3838..39bcb74 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> -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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