From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754756AbaHKTf2 (ORCPT ); Mon, 11 Aug 2014 15:35:28 -0400 Received: from g5t1626.atlanta.hp.com ([15.192.137.9]:2114 "EHLO g5t1626.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752825AbaHKTf1 (ORCPT ); Mon, 11 Aug 2014 15:35:27 -0400 Message-ID: <53E91AFC.1030307@hp.com> Date: Mon, 11 Aug 2014 15:35:24 -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> <53E7E712.9060201@hp.com> <1407714603.7594.10.camel@buesod1.americas.hpqcorp.net> In-Reply-To: <1407714603.7594.10.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 08/10/2014 07:50 PM, Davidlohr Bueso wrote: > On Sun, 2014-08-10 at 17:41 -0400, Waiman Long wrote: >> 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. > But afaict you cannot have LOCKDEP without enabling DEBUG_MUTEX (but not > necessarily vice-versa). Both are quite intertwined within other > debugging dependencies/options. > I think you are right. This will require comment in the code to avoid this kind of confusion. -Longman