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

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Message-ID: <20191107112200.GA11587@willie-the-truck>
Date:   Thu, 7 Nov 2019 11:22:01 +0000
From:   Will Deacon <will@...nel.org>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>, 1vier1@....de,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 1/5] smp_mb__{before,after}_atomic(): Update Documentation

Hi Manfred,

On Wed, Nov 06, 2019 at 08:23:03PM +0100, Manfred Spraul wrote:
> From 8d2b219794221e3ef1a1ec90e0f4fe344af9a55d Mon Sep 17 00:00:00 2001
> From: Manfred Spraul <manfred@...orfullife.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@...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 | 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();

Thanks, I much prefer this approach:

Acked-by: Will Deacon <will@...nel.org>

Will