From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752184AbaJBIH7 (ORCPT ); Thu, 2 Oct 2014 04:07:59 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:45384 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751471AbaJBIHz (ORCPT ); Thu, 2 Oct 2014 04:07:55 -0400 Date: Thu, 2 Oct 2014 09:37:39 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: mingo@kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de, ilya.dryomov@inktank.com, umgwanakikbuti@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 02/11] wait: Provide infrastructure to deal with nested blocking Message-ID: <20141002073739.GF2843@worktop.programming.kicks-ass.net> References: <20140924081845.572814794@infradead.org> <20140924082242.051202318@infradead.org> <20140929210221.GA12112@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140929210221.GA12112@redhat.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 29, 2014 at 11:02:21PM +0200, Oleg Nesterov wrote: > On 09/24, Peter Zijlstra wrote: > > > > There are a few places that call blocking primitives from wait loops, > > provide infrastructure to support this without the typical > > task_struct::state collision. > > > > We record the wakeup in wait_queue_t::flags which leaves > > task_struct::state free to be used by others. > > Sorry for delay. FWIW, > > Reviewed-by: Oleg Nesterov > > > +/* > > + * DEFINE_WAIT_FUNC(wait, woken_wait_func); > ^^^^^^^^^^^^^^^^ > woken_wake_function ;) > > > +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key) > > +{ > > + /* > > + * Although this function is called under waitqueue lock, LOCK > > + * doesn't imply write barrier and the users expects write > > + * barrier semantics on wakeup functions. The following > > + * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up() > > + * and is paired with set_mb() in wait_woken(). > > + */ > > + smp_wmb(); /* C */ > > + wait->flags |= WQ_FLAG_WOKEN; > > Perhaps it is just me, but I was a bit confused by the comment above wmb(). > Afaics, it is not that "users expects write barrier semantics", just we > need to ensure that > > CONDITION = true; > wait->flags |= WQ_FLAG_WOKEN; > > can't be reordered (and this differs from smp_wmb() in try_to_wake_up()). > Otherwise we can obviously race with > > // wait_woken() -> set_mb() > wait->flags &= ~WQ_FLAG_WOKEN; > mb(); > > if (CONDITION) > break; > Yes, that comment could be clearer. It is however, to me, the 'same' as a regular wakeup in that we need to separate whatever state changes before the wakeup (CONDITION=true typically) from whatever writes are required to affect the wakeup (->state=TASK_RUNNING typically, +optional enqueuing on runqueues and all that).