[lkml]   [2018]   [Oct]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
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.


Signed-off-by: Joel Fernandes (Google) <>
.../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.

-<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.
-<tr><th align="left">Quick Quiz:</th></tr>
- What? Where???
- I don't see any <tt>smp_mb()</tt> after the return from
- <tt>wait_for_completion()</tt>!!!
-<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.
<p>Tree RCU's grace--period memory-ordering guarantees rely most
heavily on the <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>
field, so much so that it is necessary to abbreviate this pattern
 \ /
  Last update: 2018-10-28 05:31    [W:0.077 / U:1.252 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site