linux-kernel - Re: [PATCH v2] barriers: introduce smp_mb__release

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Message-ID: <20151009111328.GA3816@twins.programming.kicks-ass.net>
Date:	Fri, 9 Oct 2015 13:13:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Boqun Feng <boqun.feng@...il.com>,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and
 update documentation

On Fri, Oct 09, 2015 at 10:40:39AM +0100, Will Deacon wrote:
> 
> >  - RELEASE -> ACQUIRE _chains_ (on shared variables) preserve causality,
> >    (because each link is fully ordered) but are not transitive.
> 
> Yup, and that's the same for UNLOCK -> LOCK, too.

Agreed, except RELEASE/ACQUIRE is more RCpc than UNLOCK/LOCK.

IFF we can get UNLOCK/LOCK as RCsc the chains are strongly transitive,
unlike the RELEASE/ACQUIRE chains, which will be weakly so.


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