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

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Message-ID: <20191014130321.GG2328@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Oct 2019 15:03:21 +0200
From:   Peter Zijlstra <peterz@...radead.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>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 6/6] Documentation/memory-barriers.txt: Clarify cmpxchg()

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
> 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:

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.

> 
> 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

s/do/do not/ ?

> +     memory barrier (such as set_bit and clear_bit).