linux-kernel - Re: [RFC][PATCH 1/5] mips/atomic: Fix cmpxchg64 barriers

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Date:   Thu, 25 Apr 2019 08:59:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paul Burton <paul.burton@...s.com>
Cc:     "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "andrea.parri@...rulasolutions.com" 
        <andrea.parri@...rulasolutions.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "paulmck@...ux.ibm.com" <paulmck@...ux.ibm.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 1/5] mips/atomic: Fix cmpxchg64 barriers

On Wed, Apr 24, 2019 at 09:00:25PM +0000, Paul Burton wrote:
> Hi Peter,
> 
> On Wed, Apr 24, 2019 at 02:36:57PM +0200, Peter Zijlstra wrote:
> > There were no memory barriers on cmpxchg64() _at_all_. Fix this.
> 
> This does looks problematic, but it's worth noting that this code path
> is only applicable to 32b kernels running on 64b CPUs which is pretty
> rare. The commit message as-is suggests to me that all configurations
> are broken, which isn't the case (at least, not in this respect :) ).

OK, I hadn't gone through the ifdef selection process. I just
encountered this cmpxchg implementation and noted a significant lack of
barriers.

> > 
> > Cc: Paul Burton <paul.burton@...s.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/mips/include/asm/cmpxchg.h |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > --- a/arch/mips/include/asm/cmpxchg.h
> > +++ b/arch/mips/include/asm/cmpxchg.h
> > @@ -290,9 +290,11 @@ static inline unsigned long __cmpxchg64(
> >  	 * will cause a build error unless cpu_has_64bits is a		\
> >  	 * compile-time constant 1.					\
> >  	 */								\
> > -	if (cpu_has_64bits && kernel_uses_llsc)				\
> > +	if (cpu_has_64bits && kernel_uses_llsc) {			\
> > +		smp_mb__before_llsc();					\
> >  		__res = __cmpxchg64((ptr), __old, __new);		\
> > -	else								\
> > +		smp_llsc_mb();						\
> > +	} else								\
> >  		__res = __cmpxchg64_unsupported();			\
> 
> It would be good to also add braces around the else block, and I believe
> checkpatch should be complaining about that ("braces {} should be used
> on all arms of this statement").

You're right, I'll fix up.