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Subject: Re: [PATCH 1/5] smp_mb__{before,after}_atomic(): Update Documentation
To:     Will Deacon <will@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Waiman Long <longman@redhat.com>, 1vier1@web.de,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>
References: <20191020123305.14715-1-manfred@colorfullife.com>
 <20191020123305.14715-2-manfred@colorfullife.com>
 <20191101164948.GD3603@willie-the-truck>
From:   Manfred Spraul <manfred@colorfullife.com>
Message-ID: <65a187c2-80de-2c6f-5f80-48c51f973d08@colorfullife.com>
Date:   Wed, 6 Nov 2019 20:23:03 +0100
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Hi Will,

On 11/1/19 5:49 PM, Will Deacon wrote:
> Hi Manfred,
>
> On Sun, Oct 20, 2019 at 02:33:01PM +0200, Manfred Spraul wrote:
>> 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.
> [...]
>
> Although I think this is correct, I really think we should instead refer to
> Documentation/atomic_t.txt and get rid of this out of memory-barriers.txt
> entirely. It's just duplication and is out of date.

So you would prefer the attached patch?

For me this would be fine, too.


--

     Manfred


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From 8d2b219794221e3ef1a1ec90e0f4fe344af9a55d Mon Sep 17 00:00:00 2001
From: Manfred Spraul <manfred@colorfullife.com>
Date: Fri, 11 Oct 2019 10:33:26 +0200
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_atomic and smp_mb__after_atomic can be combined with
all RMW operations that do not imply memory barriers.

In order to avoid that this happens again:
Remove the paragraph from Documentation/memory_barrier.txt, the functions
are sufficiently documented in Documentation/atomic_{t,bitops}.txt

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 | 20 +-------------------
 1 file changed, 1 insertion(+), 19 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 1adbb8a371c7..16dfb4cde1e1 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1873,25 +1873,7 @@ 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).
-
-     As an example, consider a piece of code that marks an object as being dead
-     and then decrements the object's reference count:
-
-	obj->dead = 1;
-	smp_mb__before_atomic();
-	atomic_dec(&obj->ref_count);
-
-     This makes sure that the death mark on the object is perceived to be set
-     *before* the reference counter is decremented.
-
-     See Documentation/atomic_{t,bitops}.txt for more information.
-
+     See Documentation/atomic_{t,bitops}.txt for information.
 
  (*) dma_wmb();
  (*) dma_rmb();
-- 
2.23.0


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