linux-kernel - [PATCH 1/5] smp_mb__{before,after}

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Message-Id: <20191020123305.14715-2-manfred@colorfullife.com>
Date:   Sun, 20 Oct 2019 14:33:01 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>
Cc:     1vier1@....de, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        Will Deacon <will.deacon@....com>
Subject: [PATCH 1/5] 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.

Fixes: 654672d4ba1a ("locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations")

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Acked-by: Waiman Long <longman@...hat.com>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Will Deacon <will.deacon@....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