From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756900Ab0JYQDs (ORCPT ); Mon, 25 Oct 2010 12:03:48 -0400 Received: from hera.kernel.org ([140.211.167.34]:37380 "EHLO hera.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756707Ab0JYQDr (ORCPT ); Mon, 25 Oct 2010 12:03:47 -0400 Message-ID: <4CC5AA5F.9060909@kernel.org> Date: Mon, 25 Oct 2010 18:03:43 +0200 From: Tejun Heo User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.2.11) Gecko/20101013 Lightning/1.0b2 Thunderbird/3.1.5 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: linux-kernel@vger.kernel.org Subject: Re: Question about synchronize_sched_expedited() References: <20101025154358.GA6919@linux.vnet.ibm.com> In-Reply-To: <20101025154358.GA6919@linux.vnet.ibm.com> X-Enigmail-Version: 1.1.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.3 (hera.kernel.org [127.0.0.1]); Mon, 25 Oct 2010 16:03:45 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Paul. On 10/25/2010 05:43 PM, Paul E. McKenney wrote: > Hello, Tejun, > > I was taking another look at synchronize_sched_expedited(), and was > concerned about the scenario listed out in the following commit. > Is this scenario a real problem, or am I missing the synchronization > that makes it safe? > > (If my concerns are valid, I should also be able to change this > to non-atomically increment synchronize_sched_expedited_count, but > one step at a time...) I think your concern is valid and this can happen w/o preemption given enough cpus and perfect timing. Was the original code free from this problem? IMHO the counter based mechanism is a bit too difficult to ponder and verify. Can we do more conventional double queueing (ie. flipping pending and executing queues so that multiple sync calls can get coalesced while another one is in progress)? That's what the code is trying to achieve anyway, right? Thanks. -- tejun