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Date:   Sat, 7 Jul 2018 22:10:24 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Guo Ren <ren_guo@c-sky.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        daniel.lezcano@linaro.org, jason@lakedaemon.net, arnd@arndb.de,
        c-sky_gcc_upstream@c-sky.com, gnu-csky@mentor.com,
        thomas.petazzoni@bootlin.com, wbx@uclibc-ng.org,
        green.hu@gmail.com, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH V2 11/19] csky: Atomic operations
Message-ID: <20180707201024.GB3225@andrea>
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On Sat, Jul 07, 2018 at 04:08:47PM +0800, Guo Ren wrote:
> On Fri, Jul 06, 2018 at 02:17:16PM +0200, Peter Zijlstra wrote:

> 	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?

The state (r0==0 && r1==0) _must_ not be allowed in the above snippet (so,
even without the additional smp_mb() between WRITE_ONCE() and xchg()).  In
informal terms, xchg() provides the smp_mb().

Compare implementations of xchg() and xchg_relaxed().  The following could
also be helpful (in addition to the references pointed out earlier):

  Documentation/atomic_t.txt

  Andrea


> 
>  Guo Ren
>