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Date: Sat, 27 Oct 2018 21:30:46 -0700 From: "Joel Fernandes (Google)" <joel@...lfernandes.org> To: linux-kernel@...r.kernel.org Cc: "Joel Fernandes (Google)" <joel@...lfernandes.org>, "Paul E. McKenney" <paulmck@...ux.ibm.com> Subject: [RFC] doc: rcu: remove note on smp_mb during synchronize_rcu As per this thread [1], it seems this smp_mb isn't needed anymore: "So the smp_mb() that I was trying to add doesn't need to be there." So let us remove this part from the memory ordering documentation. [1] https://lkml.org/lkml/2017/10/6/707 Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org> --- .../Tree-RCU-Memory-Ordering.html | 32 +------------------ 1 file changed, 1 insertion(+), 31 deletions(-) diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html index a346ce0116eb..0fb1511763d4 100644 --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html @@ -77,7 +77,7 @@ The key point is that the lock-acquisition functions, including <tt>smp_mb__after_unlock_lock()</tt> immediately after successful acquisition of the lock. -<p>Therefore, for any given <tt>rcu_node</tt> struction, any access +<p>Therefore, for any given <tt>rcu_node</tt> structure, any access happening before one of the above lock-release functions will be seen by all CPUs as happening before any access happening after a later one of the above lock-acquisition functions. @@ -162,36 +162,6 @@ an <tt>atomic_add_return()</tt> of zero) to detect idle CPUs. <tr><td> </td></tr> </table> -<p>The approach must be extended to handle one final case, that -of waking a task blocked in <tt>synchronize_rcu()</tt>. -This task might be affinitied to a CPU that is not yet aware that -the grace period has ended, and thus might not yet be subject to -the grace period's memory ordering. -Therefore, there is an <tt>smp_mb()</tt> after the return from -<tt>wait_for_completion()</tt> in the <tt>synchronize_rcu()</tt> -code path. - -<table> -<tr><th> </th></tr> -<tr><th align="left">Quick Quiz:</th></tr> -<tr><td> - What? Where??? - I don't see any <tt>smp_mb()</tt> after the return from - <tt>wait_for_completion()</tt>!!! -</td></tr> -<tr><th align="left">Answer:</th></tr> -<tr><td bgcolor="#ffffff"><font color="ffffff"> - That would be because I spotted the need for that - <tt>smp_mb()</tt> during the creation of this documentation, - and it is therefore unlikely to hit mainline before v4.14. - Kudos to Lance Roy, Will Deacon, Peter Zijlstra, and - Jonathan Cameron for asking questions that sensitized me - to the rather elaborate sequence of events that demonstrate - the need for this memory barrier. -</font></td></tr> -<tr><td> </td></tr> -</table> - <p>Tree RCU's grace--period memory-ordering guarantees rely most heavily on the <tt>rcu_node</tt> structure's <tt>->lock</tt> field, so much so that it is necessary to abbreviate this pattern -- 2.19.1.568.g152ad8e336-goog
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