linux-kernel - Re: [PATCH V2 11/19] csky: Atomic operations

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Date:   Sat, 7 Jul 2018 16:08:47 +0800
From:   Guo Ren <ren_guo@...ky.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, daniel.lezcano@...aro.org,
        jason@...edaemon.net, arnd@...db.de, c-sky_gcc_upstream@...ky.com,
        gnu-csky@...tor.com, thomas.petazzoni@...tlin.com,
        wbx@...ibc-ng.org, green.hu@...il.com,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH V2 11/19] csky: Atomic operations

On Fri, Jul 06, 2018 at 02:17:16PM +0200, Peter Zijlstra wrote:
> > 
> > 	CPU0			CPU1
> > 
> > 	r1 = READ_ONCE(x);	WRITE_ONCE(y, 1);
> > 	r2 = xchg(&y, 2);	smp_store_release(&x, 1);
> > 
> > must not allow: r1==1 && r2==0
> 
> Also, since you said "SYNC.IS" is a pipeline flush, those
> instruction-sync primitives normally do not imply a store-buffer flush,
> does yours? If not it is not a valid smp_mb() implementation.
Sync.is will flush pipeline and store-buffer.

"sync"	means completion memory barrier.
"i"	means flush cpu pipeline.
"s"	means sharable to other cpus.

> 
> Notably:
> 
> 	CPU0				CPU1
> 
> 	WRITE_ONCE(x, 1);		WRITE_ONCE(y, 1);
> 	smp_mb();			smp_mb();
> 	r0 = READ_ONCE(y);		r1 = READ_ONCE(x);
> 
> must not allow: r0==0 && r1==0
> 
> Which would be possible with a regular instruction-sync barrier, but
> must absolutely not be true with a full memory barrier.
> 
> (and you can replace the smp_mb(); r = READ_ONCE(); with r = xchg() to
> again see why you need that first smp_mb()).

	CPU0			CPU1

	WRITE_ONCE(x, 1)	WRITE_ONCE(y, 1)
	r0 = xchg(&y, 2)	r1 = xchg(&x, 2)

must not allow: r0==0 && r1==0
So we must add a smp_mb between WRITE_ONCE() and xchg(), right?

 Guo Ren