linux-kernel - [PATCH 5/5] Documentation/memory-barriers.txt: Clarify cmpxchg()

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Message-Id: <20191011112009.2365-6-manfred@colorfullife.com>
Date:   Fri, 11 Oct 2019 13:20:09 +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>,
        Jonathan Corbet <corbet@....net>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 5/5] Documentation/memory-barriers.txt: Clarify cmpxchg()

The documentation in memory-barriers.txt claims that
smp_mb__{before,after}_atomic() are for atomic ops that do not return a
value.

This is misleading and doesn't match the example in atomic_t.txt,
and e.g. smp_mb__before_atomic() may and is used together with
cmpxchg_relaxed() in the wake_q code.

The purpose of e.g. smp_mb__before_atomic() is to "upgrade" a following
RMW atomic operation to a full memory barrier.
The return code of the atomic operation has no impact, so all of the
following examples are valid:

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();

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Cc: Waiman Long <longman@...hat.com>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 Documentation/memory-barriers.txt | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 1adbb8a371c7..52076b057400 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1873,12 +1873,13 @@ 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 (such as add, subtract,
+     increment, decrement, failed conditional operations, ...) that do
+     not imply memory barriers, but where the code needs a memory barrier,
+     for example when 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 imply a full
+     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