linux-kernel - Re: [PATCH v3] barriers: introduce smp_mb__release

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Message-ID: <20160127183923.GD25545@arm.com>
Date:	Wed, 27 Jan 2016 18:39:23 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] barriers: introduce smp_mb__release_acquire and
 update documentation

On Wed, Jan 27, 2016 at 07:25:51PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 27, 2016 at 06:22:04PM +0000, Will Deacon wrote:
> > As much as we'd like to live in a world where RELEASE -> ACQUIRE is
> > always cheaply ordered and can be used to construct UNLOCK -> LOCK
> > definitions with similar guarantees, the grim reality is that this isn't
> > even possible on x86 (thanks to Paul for bringing us crashing down to
> > Earth).
> > 
> > This patch handles the issue by introducing a new barrier macro,
> > smp_mb__after_release_acquire, that can be placed after an ACQUIRE that
> > either reads from a RELEASE or is in program-order after a RELEASE. The
> > barrier upgrades the RELEASE-ACQUIRE pair to a full memory barrier,
> > implying global transitivity. At the moment, it doesn't have any users,
> > so its existence serves mainly as a documentation aid and a potential
> > stepping stone to the reintroduction of smp_mb__after_unlock_lock() used
> > by RCU.
> > 
> > Documentation/memory-barriers.txt is updated to describe more clearly
> > the ACQUIRE and RELEASE ordering in this area and to show some examples
> > of the new barrier in action.
> 
> So the obvious question is: do we have a use-case?

We have a use-case for smp_mb__after_unlock_lock, so I think we should
either strengthen our locking guarantees so that smp_mb__after_unlock_lock
isn't needed or introduce smp_mb__after_release_acquire to close the gap.
As it stands, we've got an inconsistency (despite it being hidden inside
RCU).

The main advantage of this patch is a documentation aid, in my opinion
(hell, we talk about smp_mb__after_unlock_lock already when reasoning
about this stuff).

Will