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 S1753292AbbKBUks (ORCPT <rfc822;w@1wt.eu>);
	Mon, 2 Nov 2015 15:40:48 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:44638 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752533AbbKBUkp (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 2 Nov 2015 15:40:45 -0500
Date: Mon, 2 Nov 2015 21:40:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>, boqun.feng@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, Michal Hocko <mhocko@kernel.org>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Message-ID: <20151102204037.GX17308@twins.programming.kicks-ass.net>
References: <CA+55aFwMREaiVSZhTt3HB+Qrgj1pA5p9P6q8WD-dSa7=rHYPmQ@mail.gmail.com>
 <20151102132901.157178466@infradead.org>
 <20151102134941.005198372@infradead.org>
 <8722.1446496609@warthog.procyon.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <8722.1446496609@warthog.procyon.org.uk>
User-Agent: Mutt/1.5.21 (2012-12-30)
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 08:36:49PM +0000, David Howells wrote:
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > > +       smp_read_barrier_depends(); /* ctrl */  \
> > > +       smp_rmb(); /* ctrl + rmb := acquire */  \
> 
> Doesn't smp_rmb() imply an smp_read_barrier_depends() anyway?  In
> memory-barriers.txt, it says:
> 
>      Read memory barriers imply data dependency barriers, and so can
>      substitute for them.

Yes, I noted that in a follow up email, Alpha implements both as MB. So
just the smp_rmb() is sufficient.