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Subject[PATCH 1/5] smp_mb__{before,after}_atomic(): Update Documentation
Date
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.

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>
---
Documentation/memory-barriers.txt | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 1adbb8a371c7..fe43f4b30907 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1873,12 +1873,16 @@ 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 also used for atomic bitop functions that do not return a
- value (such as set_bit and clear_bit).
+ 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 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:
--
2.21.0
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