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

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Message-ID: <20160127182551.GX6357@twins.programming.kicks-ass.net>
Date:	Wed, 27 Jan 2016 19:25:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
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 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?