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

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Date:   Fri, 1 Nov 2019 16:49:48 +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 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.

[...]

>  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,

typo: "are memory barrier"

> +     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).

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.

Will