From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754149AbZDVNwV (ORCPT ); Wed, 22 Apr 2009 09:52:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753001AbZDVNwE (ORCPT ); Wed, 22 Apr 2009 09:52:04 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:40249 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752985AbZDVNwD (ORCPT ); Wed, 22 Apr 2009 09:52:03 -0400 Date: Wed, 22 Apr 2009 15:51:34 +0200 From: Ingo Molnar To: David Howells Cc: Oleg Nesterov , Andrew Morton , Trond.Myklebust@netapp.com, serue@us.ibm.com, steved@redhat.com, viro@zeniv.linux.org.uk, "Paul E. McKenney" , Nick Piggin , linux-kernel@vger.kernel.org Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier Message-ID: <20090422135134.GA5249@elte.hu> References: <20090416143351.GD6532@redhat.com> <20090415162712.342d4c07.akpm@linux-foundation.org> <1239649429.16771.9.camel@heimdal.trondhjem.org> <20090413181733.GA10424@redhat.com> <32260.1239658818@redhat.com> <20090413214852.GA1127@redhat.com> <1239659841.16771.26.camel@heimdal.trondhjem.org> <20090413222451.GA2758@redhat.com> <14561.1239873018@redhat.com> <21239.1240407420@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <21239.1240407420@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * David Howells wrote: > Oleg Nesterov wrote: > > > > That's an interesting question. Should wake_up() imply a barrier of any > > > sort, I wonder. Well, __wake_up() does impose a barrier as it uses a > > > spinlock, but I wonder if that's sufficient. > > > > wake_up() does imply the barrier. Note the smp_wmb() in try_to_wake_up(). > > And in fact this wmb() implies mb(), because spin_lock() itself is STORE, > > and the futher LOADs can't leak up before spin_lock(). > > > > But afaics, this doesn't matter? prepare_to_wait() sets > > task->state under wait_queue_head_t->lock and wake_up() takes > > this look too, so we can't miss the event. > > > > Or I completely misunderstood the issue... > > The problem is not what wake_up() and co. do, it's what you are > allowed to assume that they do. > > However, I think you're right, and that we can assume they imply a > full memory barrier. To this end, I've attached a patch to > document this. > > David > --- > From: David Howells > Subject: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier > > Add to the memory barriers document to note that wake_up(), complete() and > co. all imply a full memory barrier. No. They dont generally imply a full memory barrier versus any arbitrary prior (or following) memory access. try_to_wake_up() has an smp_wmb() so it is a write memory barrier (but not necessarily a read memory barrier). Otherwise there are spinlocks there but spinlocks are not explicit 'full memory barriers'. Also, there's a sub-detail wrt. the wake_up() variants in that they have a fastpath for the !q case - then they dont have any atomics at all - they just return straight away. Another sub-detail: wakeups using a special wakeup handler might not even call try_to_wake_up() - so in their case not even a write barrier can be assumed. So your patch is misleading in a number of areas here. Ingo