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smp_mb__{before,after}_atomic(): update Documentation

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When adding the _{acquire|release|relaxed}() variants of some atomic
operations, it was forgotten to update Documentation/memory_barrier.txt:

smp_mb__{before,after}_atomic() is now intended for all RMW operations
that do not imply a memory barrier.

1)
	smp_mb__before_atomic();
	atomic_add();

2)
	smp_mb__before_atomic();
	atomic_xchg_relaxed();

3)
	smp_mb__before_atomic();
	atomic_fetch_add_relaxed();

Invalid would be:
	smp_mb__before_atomic();
	atomic_set();

In addition, the patch splits the long sentence into multiple shorter
sentences.

Link: http://lkml.kernel.org/r/20191020123305.14715-2-manfred@colorfullife.com
Fixes: 654672d4ba1a ("locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations")
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <1vier1@web.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Manfred Spraul 2020-02-03 17:34:29 -08:00 committed by Linus Torvalds
parent 9291799884
commit 39323c64b8
1 changed files with 9 additions and 5 deletions
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14
Documentation/memory-barriers.txt
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@ -1868,12 +1868,16 @@ There are some more advanced barrier functions:
(*) smp_mb__before_atomic();
(*) smp_mb__after_atomic();
These are for use with atomic (such as add, subtract, increment and
decrement) functions that don't return a value, especially when used for
reference counting. These functions do not imply memory barriers.
These are for use with atomic RMW functions that do not imply memory
barriers, but where the code needs a memory barrier. Examples for atomic
RMW functions that do not imply are memory barrier are e.g. add,
subtract, (failed) conditional operations, _relaxed functions,
but not atomic_read or atomic_set. A common example where a memory
barrier may be required is when atomic ops are used for reference
counting.
These are also used for atomic bitop functions that do not return a
value (such as set_bit and clear_bit).
These are also used for atomic RMW bitop functions that do not imply a
memory barrier (such as set_bit and clear_bit).
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count: