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

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Date:	Mon, 13 Jul 2015 17:56:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	"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 03:24:18PM +0100, Will Deacon wrote:
> 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.

I'm fairly sure (but I've not looked) that nobody does in fact rely on
this.

So I'm in agreement with B, and I'm quibbling on what exactly A means
;-)
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