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locking/Documentation: Clarify failed cmpxchg() memory ordering semantics

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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 <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <waiman.long@hp.com>
Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2015-07-16 16:10:06 +01:00 committed by Ingo Molnar
parent 0b792bf519
commit ed2de9f74e
2 changed files with 6 additions and 4 deletions
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4
Documentation/atomic_ops.txt
View File

@ -266,7 +266,9 @@ with the given old and new values. Like all atomic_xxx operations,
atomic_cmpxchg will only satisfy its atomicity semantics as long as all
other accesses of *v are performed through atomic_xxx operations.
atomic_cmpxchg must provide explicit memory barriers around the operation.
atomic_cmpxchg must provide explicit memory barriers around the operation,
although if the comparison fails then no memory ordering guarantees are
required.
The semantics for atomic_cmpxchg are the same as those defined for 'cas'
below.

6
Documentation/memory-barriers.txt
View File

@ -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