From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751923AbaHJVlz (ORCPT ); Sun, 10 Aug 2014 17:41:55 -0400 Received: from g2t2354.austin.hp.com ([15.217.128.53]:50289 "EHLO g2t2354.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751689AbaHJVln (ORCPT ); Sun, 10 Aug 2014 17:41:43 -0400 Message-ID: <53E7E712.9060201@hp.com> Date: Sun, 10 Aug 2014 17:41:38 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Davidlohr Bueso CC: Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, Jason Low , Scott J Norton , aswin@hp.com Subject: Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner before wakeup References: <1407450408-11679-1-git-send-email-Waiman.Long@hp.com> <1407450408-11679-2-git-send-email-Waiman.Long@hp.com> <1407458726.2513.12.camel@buesod1.americas.hpqcorp.net> <1407476387.2513.39.camel@buesod1.americas.hpqcorp.net> <53E5172A.7090508@hp.com> <1407524600.6583.6.camel@buesod1.americas.hpqcorp.net> In-Reply-To: <1407524600.6583.6.camel@buesod1.americas.hpqcorp.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/08/2014 03:03 PM, Davidlohr Bueso wrote: > On Fri, 2014-08-08 at 14:30 -0400, Waiman Long wrote: >> I have 2 issues about this. First of all, the timing windows between >> atomic_set() and mutex_has_owner() check is really small, I doubt it >> will be that effective. > That is true, which is why I didn't bother showing any performance data > in the changelog. However, more important than any performance, avoiding > bogus wakeups is the _right_ thing to do when allowing lock stealing. > >> Secondly, I think you may need to call >> mutex_release() and debug_mutex_unlock() to make the debugging code >> work, but they seems to be called only under the wait_lock. So I think >> there is more work that need to be done before this patch is ready. > When !DEBUG both mutex_release() and debug_mutex_unlock() should be > no-ops. So this allows us to do the mutex_has_owner() check *without* > holding the wait_lock. > > When DEBUG is set, we don't even bother calling mutex_has_owner(), so > nothing changes. > > I don't understand your concern. It is true I forgot the fact that MUTEX_SPIN_ON_OWNER is disabled when DEBUG_MUTEX is on. However, mutex_release is controlled by the LOCKDEP config variable which is independent of DEBUG_MUTEX. So it is still a concern. -Longman