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

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Message-ID: <20150713142418.GF2632@arm.com>
Date:	Mon, 13 Jul 2015 15:24:18 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Hurley <peter@...leysoftware.com>
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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 13, 2015 at 02:09:50PM +0100, Peter Hurley wrote:
> On 07/13/2015 08:15 AM, Will Deacon wrote:
> > smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence
> > into a full memory barrier.
> >
> > However:
> >
> >   - This ordering guarantee is already provided without the barrier on
> >     all architectures apart from PowerPC
> >
> >   - The barrier only applies to UNLOCK + LOCK, not general
> >     RELEASE + ACQUIRE operations
> 
> I'm unclear what you mean here: do you mean
> A) a memory barrier is not required between RELEASE M + ACQUIRE N when you
>    want to maintain distinct order between those operations on all arches
>    (with the possible exception of PowerPC), or,
> B) no one is using smp_mb__after_unlock_lock() in that way right now.

My understanding is (B), but Peter and I don't seem to agree yet!
I'll tighten up the text once we reach a conclusion.

Will
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