linux-kernel - Re: [PATCH] seqlock: Use while instead of if+goto in __read_seqcount

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Date:   Tue, 14 Apr 2020 13:05:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     Muchun Song <songmuchun@...edance.com>, mingo@...hat.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] seqlock: Use while instead of if+goto in
 __read_seqcount_begin

On Fri, Apr 10, 2020 at 12:56:58PM +0100, Will Deacon wrote:
> On Thu, Apr 09, 2020 at 09:45:58PM +0800, Muchun Song wrote:
> > The creators of the C language gave us the while keyword. Let's use
> > that instead of synthesizing it from if+goto.
> > 
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > ---
> >  include/linux/seqlock.h | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > index 8b97204f35a77..7bdea019814ce 100644
> > --- a/include/linux/seqlock.h
> > +++ b/include/linux/seqlock.h
> > @@ -125,12 +125,8 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
> >  {
> >  	unsigned ret;
> >  
> > -repeat:
> > -	ret = READ_ONCE(s->sequence);
> > -	if (unlikely(ret & 1)) {
> > +	while (unlikely((ret = READ_ONCE(s->sequence)) & 1))
> >  		cpu_relax();
> > -		goto repeat;
> > -	}
> >  	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
> >  	return ret;
> 
> Patch looks fine to me, but I'll leave it to Peter as I don't have a
> preference either way.

Linus sometimes prefers the goto variant as that better expresses the
exception model. But like Will, I don't particularly care. That said,
Will, would it make sense to use smp_cond_load_relaxed() here ?