linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Message-ID: <20150713131143.GY19282@twins.programming.kicks-ass.net>
Date:	Mon, 13 Jul 2015 15:11:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 13, 2015 at 01:15:04PM +0100, Will Deacon wrote:
> smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence
> into a full memory barrier.
> 
> However:

>   - The barrier only applies to UNLOCK + LOCK, not general
>     RELEASE + ACQUIRE operations

No it does too; note that on ppc both acquire and release use lwsync and
two lwsyncs do not make a sync.


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