linux-kernel - Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return

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Date:	Wed, 2 Sep 2015 11:36:09 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Waiman Long <waiman.long@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return_*
 variants

On Wed, Sep 2, 2015 at 11:23 AM, Pranith Kumar <bobby.prani@...il.com> wrote:
> Hi Will,
>
> On 09/02/2015 05:59 AM, Will Deacon wrote:
>> I just thought it was worth making this point, because it is prohibited
>> in SC and I don't want people to think that our RELEASE/ACQUIRE operations
>> are SC (even though they happen to be on arm64).
>
> This is interesting information. Does that mean that the following patch
> should work? (I am not proposing to use it, just trying to understand if
> REL+ACQ will act as a full barrier on ARM64, which you say it does).
>
> Thanks,
> Pranith.
>
> diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
> index d8c25b7..14a1b35 100644
> --- a/arch/arm64/include/asm/cmpxchg.h
> +++ b/arch/arm64/include/asm/cmpxchg.h
> @@ -68,8 +68,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
>                 BUILD_BUG();
>         }
>
> -       smp_mb();
> -       return ret;
> +       return smp_load_acquire(ret);

I meant 'smp_load_acquire(&ret);'

-- 
Pranith
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