From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932673AbdJ3Lss (ORCPT ); Mon, 30 Oct 2017 07:48:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:34531 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752521AbdJ3Lsr (ORCPT ); Mon, 30 Oct 2017 07:48:47 -0400 Date: Mon, 30 Oct 2017 12:48:35 +0100 From: Michal Hocko To: Byungchul Park Cc: Dmitry Vyukov , syzbot , Andrew Morton , Dan Williams , Johannes Weiner , Jan Kara , jglisse@redhat.com, LKML , linux-mm@kvack.org, shli@fb.com, syzkaller-bugs@googlegroups.com, Thomas Gleixner , Vlastimil Babka , ying.huang@intel.com, kernel-team@lge.com Subject: Re: possible deadlock in lru_add_drain_all Message-ID: <20171030114835.ptlcxdwqizqeotpq@dhcp22.suse.cz> References: <089e0825eec8955c1f055c83d476@google.com> <20171027093418.om5e566srz2ztsrk@dhcp22.suse.cz> <20171027134234.7dyx4oshjwd44vqx@dhcp22.suse.cz> <20171030082203.4xvq2af25shfci2z@dhcp22.suse.cz> <20171030102619.GB18085@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171030102619.GB18085@X58A-UD3R> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 30-10-17 19:26:19, Byungchul Park wrote: > On Mon, Oct 30, 2017 at 09:22:03AM +0100, Michal Hocko wrote: > > [Cc Byungchul. The original full report is > > http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@google.com] > > > > Could you have a look please? This smells like a false positive to me. > > > > On Fri 27-10-17 15:42:34, Michal Hocko wrote: > > > On Fri 27-10-17 11:44:58, Dmitry Vyukov wrote: > > > > On Fri, Oct 27, 2017 at 11:34 AM, Michal Hocko wrote: > > > > > On Fri 27-10-17 02:22:40, syzbot wrote: > > > > >> Hello, > > > > >> > > > > >> syzkaller hit the following crash on > > > > >> a31cc455c512f3f1dd5f79cac8e29a7c8a617af8 > > > > >> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master > > > > >> compiler: gcc (GCC) 7.1.1 20170620 > > > > >> .config is attached > > > > >> Raw console output is attached. > > > > > > > > > > I do not see such a commit. My linux-next top is next-20171018 > > > > > > > > > > [...] > > > > >> Chain exists of: > > > > >> cpu_hotplug_lock.rw_sem --> &pipe->mutex/1 --> &sb->s_type->i_mutex_key#9 > > > > >> > > > > >> Possible unsafe locking scenario: > > > > >> > > > > >> CPU0 CPU1 > > > > >> ---- ---- > > > > >> lock(&sb->s_type->i_mutex_key#9); > > > > >> lock(&pipe->mutex/1); > > > > >> lock(&sb->s_type->i_mutex_key#9); > > > > >> lock(cpu_hotplug_lock.rw_sem); > > > > > > > > > > I am quite confused about this report. Where exactly is the deadlock? > > > > > I do not see where we would get pipe mutex from inside of the hotplug > > > > > lock. Is it possible this is just a false possitive due to cross release > > > > > feature? > > > > > > > > > > > > As far as I understand this CPU0/CPU1 scheme works only for simple > > > > cases with 2 mutexes. This seem to have larger cycle as denoted by > > > > "the existing dependency chain (in reverse order) is:" section. > > > > > > My point was that lru_add_drain_all doesn't take any external locks > > > other than lru_lock and that one is not anywhere in the chain AFAICS. > > I think lru_add_drain_all() takes cpu_hotplug_lock.rw_sem implicitly in > get_online_cpus(), which appears in the chain. Yes, but it doesn't take any _other_ locks which are externally visible except for lru_lock and that itself doesn't provide any further dependency AFAICS. So what exactly is the deadlock scenario? -- Michal Hocko SUSE Labs