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

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Message-ID: <20150903103123.GD877@arm.com>
Date:	Thu, 3 Sep 2015 11:31:23 +0100
From:	Will Deacon <will.deacon@....com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 02, 2015 at 04:36:09PM +0100, Pranith Kumar wrote:
> On Wed, Sep 2, 2015 at 11:23 AM, Pranith Kumar <bobby.prani@...il.com> wrote:
> > 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);'

Yes, I think that would work on arm64, but it's not portable between
architectures.

Will
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