Re: [PATCH 6/6] Documentation/memory-barriers.txt: Clarify cmpxchg()

From: Manfred Spraul
Date: Mon Oct 14 2019 - 13:50:04 EST

This is a multi-part message in MIME format. Hello Peter,

On 10/14/19 3:03 PM, Peter Zijlstra wrote:
On Sat, Oct 12, 2019 at 07:49:58AM +0200, Manfred Spraul wrote:
The documentation in memory-barriers.txt claims that
smp_mb__{before,after}_atomic() are for atomic ops that do not return a

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:
The value return of atomic ops is relevant in so far that
(traditionally) all value returning atomic ops already implied full
barriers. That of course changed when we added
_release/_acquire/_relaxed variants.
I've updated the Change description accordingly



Invalid would be:

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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
s/do/do not/ ?
Sorry, yes, of course
+ memory barrier (such as set_bit and clear_bit).