From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1754348AbdBGN6u (ORCPT <rfc822;w@1wt.eu>);
        Tue, 7 Feb 2017 08:58:50 -0500
Received: from outbound-smtp03.blacknight.com ([81.17.249.16]:45414 "EHLO
        outbound-smtp03.blacknight.com" rhost-flags-OK-OK-OK-OK)
        by vger.kernel.org with ESMTP id S1753825AbdBGN6t (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 7 Feb 2017 08:58:49 -0500
Date: Tue, 7 Feb 2017 13:58:46 +0000
From: Mel Gorman <mgorman@techsingularity.net>
To: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>, Dmitry Vyukov <dvyukov@google.com>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
Message-ID: <20170207135846.usfrn7e4znjhmogn@techsingularity.net>
References: <CACT4Y+Z-juavN8s+5sc-PB0rbqy4zmsRpc6qZBg3C7z3topLTw@mail.gmail.com>
 <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>
 <20170207123708.GO5065@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-15
Content-Disposition: inline
In-Reply-To: <20170207123708.GO5065@dhcp22.suse.cz>
User-Agent: Mutt/1.6.2 (2016-07-01)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Feb 07, 2017 at 01:37:08PM +0100, Michal Hocko wrote:
> > You cannot put sleepable lock inside the preempt disbaled section...
> > We can make it a spinlock right?
> 
> Scratch that! For some reason I thought that cpu notifiers are run in an
> atomic context. Now that I am checking the code again it turns out I was
> wrong. __cpu_notify uses __raw_notifier_call_chain so this is not an
> atomic context.

Indeed.

> Anyway, shouldn't be it sufficient to disable preemption
> on drain_local_pages_wq?

That would be sufficient for a hot-removed CPU moving the drain request
to another CPU and avoiding any scheduling events.

> The CPU hotplug callback will not preempt us
> and so we cannot work on the same cpus, right?
> 

I don't see a specific guarantee that it cannot be preempted and it
would depend on an the exact cpu hotplug implementation which is subject
to quite a lot of change. Hence, the mutex provides a guantee that the
hot-removed CPU teardown cannot run on the same CPU as a workqueue drain
running on a CPU it was not originally scheduled for.

-- 
Mel Gorman
SUSE Labs