linux-kernel - Re: [PATCH] barriers: introduce smp_mb__release

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Date:	Wed, 16 Sep 2015 17:38:14 +0100
From:	Will Deacon <will.deacon@....com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] barriers: introduce smp_mb__release_acquire and update
 documentation

On Wed, Sep 16, 2015 at 12:49:18PM +0100, Boqun Feng wrote:
> Hi Will,

Hello,

> On Tue, Sep 15, 2015 at 05:13:30PM +0100, Will Deacon wrote:
> > +If necessary, ordering can be enforced by use of an
> > +smp_mb__release_acquire() barrier:
> > +
> > +	*A = a;
> > +	RELEASE M
> > +	smp_mb__release_acquire();
> 
> Should this barrier be placed after the ACQUIRE? Because we do actually
> want(?) and allow RELEASE and ACQUIRE operations to reorder in this
> case, like your following example, right?

I think it's a lot simpler to keep it where it is, in all honesty. The
relaxation for the RELEASE/ACQUIRE access ordering is mainly there to
allow architectures building those operations out of explicit barriers
to get away without a definition of smp_mb__release_acquire.

Will
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