From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753614AbdBGKFK (ORCPT ); Tue, 7 Feb 2017 05:05:10 -0500 Received: from mx2.suse.de ([195.135.220.15]:52435 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753235AbdBGKFH (ORCPT ); Tue, 7 Feb 2017 05:05:07 -0500 Date: Tue, 7 Feb 2017 11:05:03 +0100 From: Michal Hocko To: Vlastimil Babka Cc: Mel Gorman , 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: <20170207100503.GG5065@dhcp22.suse.cz> References: <20170206220530.apvuknbagaf2rdlw@techsingularity.net> <20170207084855.GC5065@dhcp22.suse.cz> <20170207094300.cuxfqi35wflk5nr5@techsingularity.net> <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz> 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 10:49:28, Vlastimil Babka wrote: > On 02/07/2017 10:43 AM, Mel Gorman wrote: > > If I'm reading this right, a hot-remove will set the pool POOL_DISASSOCIATED > > and unbound. A workqueue queued for draining get migrated during hot-remove > > and a drain operation will execute twice on a CPU -- one for what was > > queued and a second time for the CPU it was migrated from. It should still > > work with flush_work which doesn't appear to block forever if an item > > got migrated to another workqueue. The actual drain workqueue function is > > using the CPU ID it's currently running on so it shouldn't get confused. > > Is the worker that will process this migrated workqueue also guaranteed > to be pinned to a cpu for the whole work, though? drain_local_pages() > needs that guarantee. Yeah I guess you are right. This would mean that drain_local_pages_wq should to preempt_{disable,enable} around drain_local_pages > > > Tejun, did I miss anything? Does a workqueue item queued on a CPU being > > offline get unbound and a caller can still flush it safely? In this > > specific case, it's ok that the workqueue item does not run on the CPU it > > was queued on. I guess we need to do one more step and ensure that our (rebound) worker doesn't race with the page_alloc_cpu_notify. I guess we can just cmpxchg pcp->count in drain_pages_zone to ensure the exclusivity. Not as simple as I originally thought but doable I guess and definitely better than making a subtle dependency on the hotplug locks which is just a PITA to maintain. -- Michal Hocko SUSE Labs