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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-08-04 11:49:34 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-09-07 16:15:52 -0700
commitefdcd51a4d5bd355796b1a757ff0355bb09ed394 (patch)
treec8b39a7b01980753ff864c7a28bc9ac0ae180701
parent5646f7acc95f14873f1ec715380c1c493b4243ce (diff)
memory-barriers: Retain barrier() in fold-to-zero example
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@gatech.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
-rw-r--r--Documentation/memory-barriers.txt9
1 files 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. */