Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH tip/core/rcu 2/3] memory-barriers: Retain barrier() in fold-to-zero example | Date | Thu, 28 Aug 2014 11:12:29 -0700 |
| |
From: "Paul E. McKenney" <paulmck@linux.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@gatech.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.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
| |