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 S1756002AbbH0Mnk (ORCPT <rfc822;w@1wt.eu>);
	Thu, 27 Aug 2015 08:43:40 -0400
Received: from casper.infradead.org ([85.118.1.10]:49622 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753733AbbH0Mnj (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 27 Aug 2015 08:43:39 -0400
Date: Thu, 27 Aug 2015 14:43:34 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: wake_up_process implied memory barrier clarification
Message-ID: <20150827124334.GY16853@twins.programming.kicks-ass.net>
References: <20150827122727.GC27052@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150827122727.GC27052@dhcp22.suse.cz>
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 Thu, Aug 27, 2015 at 02:27:27PM +0200, Michal Hocko wrote:
> Hi,
> I have just stumbled over the comment above wake_up_process which
> claims:
> "
>  * It may be assumed that this function implies a write memory barrier before
>  * changing the task state if and only if any tasks are woken up.
> "
> 
> but try_to_wake_up does smp_mb__before_spinlock and did smp_wmb
> since 04e2f1741d235 unconditionally. The comment was added when the
> smp_wmb was in place already so I am wondering whether the comment is
> wrong/misleading.
> 
> Could somebody clarify please?

Its true for wake_up(), since that bails early if the waitqueue list is
empty.

I suspect there was no exception made for wake_up_process() to simplify
the rules.