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

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Date:	Mon, 13 Jul 2015 15:09:16 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...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 02:11:43PM +0100, Peter Zijlstra wrote:
> 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.

Really? IIUC, that means smp_mb__after_unlock_lock needs to be a full
barrier on all architectures implementing smp_store_release as smp_mb() +
STORE, otherwise the following isn't ordered:

  RELEASE X
  smp_mb__after_unlock_lock()
  ACQUIRE Y

On 32-bit ARM (at least), the ACQUIRE can be observed before the RELEASE.

Will
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