From ed2de9f74ecbbf3063d29b2334e7b455d7f35189 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Thu, 16 Jul 2015 16:10:06 +0100 Subject: locking/Documentation: Clarify failed cmpxchg() memory ordering semantics A failed cmpxchg does not provide any memory ordering guarantees, a property that is used to optimise the cmpxchg implementations on Alpha, PowerPC and arm64. This patch updates atomic_ops.txt and memory-barriers.txt to reflect this. Signed-off-by: Will Deacon Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Davidlohr Bueso Cc: Douglas Hatch Cc: H. Peter Anvin Cc: Jonathan Corbet Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Scott J Norton Cc: Thomas Gleixner Cc: Waiman Long Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com Signed-off-by: Ingo Molnar --- Documentation/memory-barriers.txt | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) (limited to 'Documentation/memory-barriers.txt') diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 13feb697271f..18fc860df1be 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -2383,9 +2383,7 @@ about the state (old or new) implies an SMP-conditional general memory barrier explicit lock operations, described later). These include: xchg(); - cmpxchg(); atomic_xchg(); atomic_long_xchg(); - atomic_cmpxchg(); atomic_long_cmpxchg(); atomic_inc_return(); atomic_long_inc_return(); atomic_dec_return(); atomic_long_dec_return(); atomic_add_return(); atomic_long_add_return(); @@ -2398,7 +2396,9 @@ explicit lock operations, described later). These include: test_and_clear_bit(); test_and_change_bit(); - /* when succeeds (returns 1) */ + /* when succeeds */ + cmpxchg(); + atomic_cmpxchg(); atomic_long_cmpxchg(); atomic_add_unless(); atomic_long_add_unless(); These are used for such things as implementing ACQUIRE-class and RELEASE-class -- cgit v1.2.3