From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752827AbbD3Sra (ORCPT ); Thu, 30 Apr 2015 14:47:30 -0400 Received: from cantor2.suse.de ([195.135.220.15]:35409 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751807AbbD3Sr3 (ORCPT ); Thu, 30 Apr 2015 14:47:29 -0400 Message-ID: <1430419619.2011.38.camel@stgolabs.net> Subject: Re: [PATCH v2] ipc/mqueue: remove STATE_PENDING From: Davidlohr Bueso To: Manfred Spraul Cc: Peter Zijlstra , Thomas Gleixner , Sebastian Andrzej Siewior , LKML , Ingo Molnar , Darren Hart , Steven Rostedt , fredrik.markstrom@windriver.com, "Paul E. McKenney" Date: Thu, 30 Apr 2015 11:46:59 -0700 In-Reply-To: <5541349C.5060000@colorfullife.com> References: <1428419030-20030-1-git-send-email-bigeasy@linutronix.de> <1428419030-20030-4-git-send-email-bigeasy@linutronix.de> <55241851.7060704@colorfullife.com> <20150410143726.GD3057@linutronix.de> <1430191493.2050.3.camel@stgolabs.net> <20150428123738.GY23123@twins.programming.kicks-ass.net> <1430239010.2004.6.camel@stgolabs.net> <20150428164329.GK5029@twins.programming.kicks-ass.net> <1430240361.2004.13.camel@stgolabs.net> <5541349C.5060000@colorfullife.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.11 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2015-04-29 at 21:44 +0200, Manfred Spraul wrote: > Hi Davidlohr, > > On 04/28/2015 06:59 PM, Davidlohr Bueso wrote: > > On Tue, 2015-04-28 at 18:43 +0200, Peter Zijlstra wrote: > >> Well, if you can 'guarantee' the cmpxchg will not fail, you can then > >> rely on the fact that cmpxchg implies a full barrier, which would > >> obviate the need for the wmb. > > Yes, assuming it implies barriers on both sides. And we could obviously > > remove the need for pairing. With wake_q being local to wq_sleep() I > > cannot see duplicate tasks trying to add themselves in the list. Failed > > cmpxchg should only occur when users start misusing the wake_q. > > > > Manfred, do you have any objections to this? Perhaps I've missed the > > real purpose of the barriers. > I don't remember the details either, so let's check what should happen: > > CPU1: sender copies message to kernel memory > aaaa > CPU1: sender does receiver->msg = message; > ** barrier 1 > CPU1: sender does receiver->state = STATE_READY; > > CPU2: receiver notices receiver->state = STATE_READY; > ** barrier 2 > CPU2: receiver reads receiver->msg > bbbb > CPU2: receiver reads *receiver->msg > > Failures would be: > - write to receiver->state is visible before the write to receiver->msg > or to *receiver->msg > ** barrier 1 needs to be an smp_wmb() > - cpu 2 reads receiver->msg before receiver->state > ** barrier 2 needs to be an smp_rmb(). > > As far as I can see, no barrier is needed in pos aaaa or bbbb. Thanks for confirming. > > With regards to failed cmpxchg(): > I don't see that mqueue could cause it by itself. Agreed. > > Who is allowed to use wake_q? > If it is permitted to use wake_q for e.g. timeout/signal delivery > wakeup, then that user might have a pending wakeup stored in the task > struct. No, this is not the case. All users are expected to do the wakeup right away. Thanks, Davidlohr