Subject: Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return_* variants

From: Will Deacon <[email-masked]>
Date: Thu, 3 Sep 2015 11:31:23 +0100

On Wed, Sep 02, 2015 at 04:36:09PM +0100, Pranith Kumar wrote:
> On Wed, Sep 2, 2015 at 11:23 AM, Pranith Kumar <[email-masked]> 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