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

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Message-ID: <20180706121716.GO2512@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Jul 2018 14:17:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <ren_guo@...ky.com>
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 01:56:14PM +0200, Peter Zijlstra wrote:

> > But I couldn't undertand what's wrong without the 1th smp_mb()?
> > 1th smp_mb will make all ld/st finish before ldex.w. Is it necessary?
> 
> Yes.
> 
> 	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.

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()).