linux-kernel - [PATCH tip/core/rcu 2/3] memory-barriers: Retain barrier() in fold-to-zero example

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Date:	Thu, 28 Aug 2014 11:12:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/3] memory-barriers: Retain barrier() in fold-to-zero example

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

The transformation in the fold-to-zero example incorrectly omits the
barrier() directive.  This commit therefore adds it back in.

Reported-by: Pranith Kumar <pranith@...ech.edu>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index d67c508eb660..600b45c6e2ad 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -679,12 +679,15 @@ equal to zero, in which case the compiler is within its rights to
 transform the above code into the following:
 
 	q = ACCESS_ONCE(a);
+	barrier();
 	ACCESS_ONCE(b) = p;
 	do_something_else();
 
-This transformation loses the ordering between the load from variable 'a'
-and the store to variable 'b'.  If you are relying on this ordering, you
-should do something like the following:
+This transformation fails to require that the CPU respect the ordering
+between the load from variable 'a' and the store to variable 'b'.
+Yes, the barrier() is still there, but it affects only the compiler,
+not the CPU.  Therefore, if you are relying on this ordering, you should
+do something like the following:
 
 	q = ACCESS_ONCE(a);
 	BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
-- 
1.8.1.5

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