From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753463AbZDVOoT (ORCPT ); Wed, 22 Apr 2009 10:44:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751433AbZDVOoJ (ORCPT ); Wed, 22 Apr 2009 10:44:09 -0400 Received: from mx2.redhat.com ([66.187.237.31]:44798 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750937AbZDVOoI (ORCPT ); Wed, 22 Apr 2009 10:44:08 -0400 Date: Wed, 22 Apr 2009 16:39:30 +0200 From: Oleg Nesterov To: Ingo Molnar Cc: David Howells , 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: <20090422143930.GA1212@redhat.com> References: <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> <20090422135134.GA5249@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090422135134.GA5249@elte.hu> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/22, Ingo Molnar wrote: > > * 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'. Yes. But please look at the changelog in "Add memory barrier semantics to wake_up() & co" 04e2f1741d235ba599037734878d72e57cb302b5 However, I must admit, I don't understand how to document the semantics correctly. This wmb() before spin_lock() ensures we don't read task->state before previous STOREs. This is what we care about, and this is what I meant when I said "this wmb() implies mb()". So, I think that try_to_wake_up() implies that the LOADS after it can't be reordered with STOREs before it (and wmb() of course). Oleg.