From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754293AbdBGNsX (ORCPT ); Tue, 7 Feb 2017 08:48:23 -0500 Received: from mx2.suse.de ([195.135.220.15]:43203 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753451AbdBGNsU (ORCPT ); Tue, 7 Feb 2017 08:48:20 -0500 Date: Tue, 7 Feb 2017 14:48:18 +0100 From: Michal Hocko To: Mel Gorman Cc: Vlastimil Babka , Dmitry Vyukov , Tejun Heo , Christoph Lameter , "linux-mm@kvack.org" , LKML , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , syzkaller , Andrew Morton Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc Message-ID: <20170207134818.GQ5065@dhcp22.suse.cz> References: <20170206220530.apvuknbagaf2rdlw@techsingularity.net> <20170207084855.GC5065@dhcp22.suse.cz> <20170207094300.cuxfqi35wflk5nr5@techsingularity.net> <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz> <20170207102809.awh22urqmfrav5r6@techsingularity.net> <20170207103552.GH5065@dhcp22.suse.cz> <20170207113435.6xthczxt2cx23r4t@techsingularity.net> <20170207114327.GI5065@dhcp22.suse.cz> <20170207130350.njwuiq3uh6vhj5t2@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170207130350.njwuiq3uh6vhj5t2@techsingularity.net> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 07-02-17 13:03:50, Mel Gorman wrote: > On Tue, Feb 07, 2017 at 12:43:27PM +0100, Michal Hocko wrote: > > > Right. The unbind operation can set a mask that is any allowable CPU and > > > the final process_work is not done in a context that prevents > > > preemption. > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > index 3b93879990fd..7af165d308c4 100644 > > > --- a/mm/page_alloc.c > > > +++ b/mm/page_alloc.c > > > @@ -2342,7 +2342,14 @@ void drain_local_pages(struct zone *zone) > > > > > > static void drain_local_pages_wq(struct work_struct *work) > > > { > > > + /* > > > + * Ordinarily a drain operation is bound to a CPU but may be unbound > > > + * after a CPU hotplug operation so it's necessary to disable > > > + * preemption for the drain to stabilise the CPU ID. > > > + */ > > > + preempt_disable(); > > > drain_local_pages(NULL); > > > + preempt_enable_no_resched(); > > > } > > > > > > /* > > [...] > > > @@ -6711,7 +6714,16 @@ static int page_alloc_cpu_dead(unsigned int cpu) > > > { > > > > > > lru_add_drain_cpu(cpu); > > > + > > > + /* > > > + * A per-cpu drain via a workqueue from drain_all_pages can be > > > + * rescheduled onto an unrelated CPU. That allows the hotplug > > > + * operation and the drain to potentially race on the same > > > + * CPU. Serialise hotplug versus drain using pcpu_drain_mutex > > > + */ > > > + mutex_lock(&pcpu_drain_mutex); > > > drain_pages(cpu); > > > + mutex_unlock(&pcpu_drain_mutex); > > > > You cannot put sleepable lock inside the preempt disbaled section... > > We can make it a spinlock right? > > > > The CPU down callback can hold a mutex and at least he SLUB callback > already does so. That gives > > page_alloc_cpu_dead > mutex_lock > drain_pages > mutex_unlock > > drain_all_pages > mutex_lock > queue workqueue > drain_local_pages_wq > preempt_disable > drain_local_pages > drain_pages > preempt_enable > flush queues > mutex_unlock > > I must be blind or maybe it's rushing between multiple concerns but which > sleepable lock is of concern? I thought the cpu hotplug callback was non-preemptible. This is not the case as mentioned in other reply. The pcpu_drain_mutex in the hotplug callback is alright. Sorry about the confusion! I am still wondering whether the lock is really needed. See the other reply. -- Michal Hocko SUSE Labs