From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id C45E56B0033 for ; Tue, 7 Feb 2017 05:05:07 -0500 (EST) Received: by mail-wm0-f72.google.com with SMTP id x4so24416624wme.3 for ; Tue, 07 Feb 2017 02:05:07 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o15si4418967wrb.191.2017.02.07.02.05.06 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 07 Feb 2017 02:05:06 -0800 (PST) Date: Tue, 7 Feb 2017 11:05:03 +0100 From: Michal Hocko 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org