linux-kernel - Re: [PATCH] asm-generic/atomic: Add try

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Message-ID: <20200220103934.GA14459@willie-the-truck>
Date:   Thu, 20 Feb 2020 10:39:35 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Carter Li 李通洲 <carter.li@...tek.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring <io-uring@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks

On Thu, Feb 20, 2020 at 11:37:27AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 20, 2020 at 10:30:45AM +0000, Will Deacon wrote:
> > On Tue, Feb 18, 2020 at 03:27:00PM +0100, Peter Zijlstra wrote:
> > > diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
> > > index b6c6f5d306a7..3c9be8d550e0 100755
> > > --- a/scripts/atomic/gen-atomic-fallback.sh
> > > +++ b/scripts/atomic/gen-atomic-fallback.sh
> > > @@ -140,6 +140,32 @@ cat <<EOF
> > >  EOF
> > >  }
> > >  
> > > +gen_try_cmpxchg_fallback()
> > > +{
> > > +	local order="$1"; shift;
> > > +
> > > +cat <<EOF
> > > +#ifndef try_cmpxchg${order}
> > > +#define try_cmpxchg${order}(_ptr, _oldp, _new) \\
> > > +({ \\
> > > +	typeof(*ptr) ___r, ___o = *(_oldp); \\
> > > +	___r = cmpxchg${order}((_ptr), ___o, (_new)); \\
> > > +	if (unlikely(___r != ___o)) \\
> > > +		*(_old) = ___r; \\
> > 
> > This doesn't compile because _old isn't declared. Probably best to avoid
> > evaluating _oldp twice though.
> 
> Compiler had already spotted that, I'll make it something like:
> 
> 	typeof(*ptr) *___op = (_oldp), ___o = *___op;
> 
> 	...
> 
> 		*___op = ___r;
> 
> Which avoids the double eval.

Cool, you can stick my Ack on the patch with that change.

Will