[14/67] smp_mb__{before,after}_atomic(): update Documentation

Commit Message

Andrew Morton Feb. 4, 2020, 1:34 a.m. UTC

From: Manfred Spraul <[e-mail]>
Subject: smp_mb__{before,after}_atomic(): update Documentation

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/[e-mail]
Fixes: 654672d4ba1a ("locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations")
Signed-off-by: Manfred Spraul <[e-mail]>
Acked-by: Waiman Long <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 Documentation/memory-barriers.txt |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

Patch

--- a/Documentation/memory-barriers.txt~smp_mb__beforeafter_atomic-update-documentation
+++ a/Documentation/memory-barriers.txt
@@ -1868,12 +1868,16 @@  There are some more advanced barrier fun
  (*) 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: