From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754297AbbKBRnu (ORCPT <rfc822;w@1wt.eu>);
	Mon, 2 Nov 2015 12:43:50 -0500
Received: from foss.arm.com ([217.140.101.70]:52010 "EHLO foss.arm.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752853AbbKBRnt (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 2 Nov 2015 12:43:49 -0500
Date: Mon, 2 Nov 2015 17:43:48 +0000
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, oleg@redhat.com, linux-kernel@vger.kernel.org,
        paulmck@linux.vnet.ibm.com, boqun.feng@gmail.com, corbet@lwn.net,
        mhocko@kernel.org, dhowells@redhat.com, torvalds@linux-foundation.org
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Message-ID: <20151102174347.GK29657@arm.com>
References: <20151102132901.157178466@infradead.org>
 <20151102134941.005198372@infradead.org>
 <20151102135726.GR17308@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151102135726.GR17308@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Nov 02, 2015 at 02:57:26PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 02, 2015 at 02:29:05PM +0100, Peter Zijlstra wrote:
> 
> > Note that while smp_cond_acquire() has an explicit
> > smp_read_barrier_depends() for Alpha, neither sites it gets used in
> > were actually buggy on Alpha for their lack of it. The first uses
> > smp_rmb(), which on Alpha is a full barrier too and therefore serves
> > its purpose. The second had an explicit full barrier.
> 
> > +/**
> > + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> > + * @cond: boolean expression to wait for
> > + *
> > + * Equivalent to using smp_load_acquire() on the condition variable but employs
> > + * the control dependency of the wait to reduce the barrier on many platforms.
> > + *
> > + * The control dependency provides a LOAD->STORE order, the additional RMB
> > + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
> > + * aka. ACQUIRE.
> > + */
> > +#define smp_cond_acquire(cond)	do {		\
> > +	while (!(cond))				\
> > +		cpu_relax();			\
> > +	smp_read_barrier_depends(); /* ctrl */	\
> > +	smp_rmb(); /* ctrl + rmb := acquire */	\
> > +} while (0)
> 
> So per the above argument we could leave out the
> smp_read_barrier_depends() for Alpha, although that would break
> consistency with all the other control dependency primitives we have. It
> would avoid issuing a double barrier.
> 
> Thoughts?

Do we even know that Alpha needs a barrier for control-dependencies in
the first place?

Will