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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2016-04-12 08:52:49 -0700
committerIngo Molnar <mingo@kernel.org>2016-04-13 08:52:21 +0200
commita5052657c164107032d521f0d9e92703d78845f2 (patch)
tree3f9ed953ae89b091ddca52a14bb1e4bcc21a6b9b /Documentation/memory-barriers.txt
parent5c8a010c2411729a07cb1b90c09fa978ac0ac6c0 (diff)
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locking/Documentation: Clarify relationship of barrier() to control dependencies
The current documentation claims that the compiler ignores barrier(), which is not the case. Instead, the compiler carefully pays attention to barrier(), but in a creative way that still manages to destroy the control dependency. This commit sets the story straight. Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: bobby.prani@gmail.com Cc: dhowells@redhat.com Cc: dipankar@in.ibm.com Cc: dvhart@linux.intel.com Cc: edumazet@google.com Cc: fweisbec@gmail.com Cc: jiangshanlai@gmail.com Cc: josh@joshtriplett.org Cc: oleg@redhat.com Cc: rostedt@goodmis.org Link: http://lkml.kernel.org/r/1460476375-27803-1-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'Documentation/memory-barriers.txt')
-rw-r--r--Documentation/memory-barriers.txt7
1 files changed, 4 insertions, 3 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 3729cbe..ec12890 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -813,9 +813,10 @@ In summary:
the same variable, then those stores must be ordered, either by
preceding both of them with smp_mb() or by using smp_store_release()
to carry out the stores. Please note that it is -not- sufficient
- to use barrier() at beginning of each leg of the "if" statement,
- as optimizing compilers do not necessarily respect barrier()
- in this case.
+ to use barrier() at beginning of each leg of the "if" statement
+ because, as shown by the example above, optimizing compilers can
+ destroy the control dependency while respecting the letter of the
+ barrier() law.
(*) Control dependencies require at least one run-time conditional
between the prior load and the subsequent store, and this
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