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

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Date:   Sat, 7 Jul 2018 21:54:37 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Guo Ren <ren_guo@...ky.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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

Hi Guo,

On Sat, Jul 07, 2018 at 03:42:10PM +0800, Guo Ren wrote:
> On Fri, Jul 06, 2018 at 01:56:14PM +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
> CPU1 smp_store_release could be finished before WRITE_ONCE, so r1=1 &&
> r2=0?

The emphasis is on the "must": your implementation __must__ prevent this
from happening (say, by inserting memory barriers in smp_store_release());
if your implementation allows the state (r1==1 && r2==0), then the imple-
mentation is incorrect.

I'd suggest you have a look at the Linux-kernel memory consistency model
documentation and the associated tools, starting with:

  Documentation/memory-barriers.txt
  tools/memory-model/

(and please do not hesitate to ask questions about them, if something is
 unclear).

  Andrea