linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

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Message-ID: <20170207111355.lyqfbrc6akwzgy4d@techsingularity.net>
Date:   Tue, 7 Feb 2017 11:13:55 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

On Tue, Feb 07, 2017 at 10:42:49AM +0000, Mel Gorman wrote:
> On Tue, Feb 07, 2017 at 10:23:31AM +0100, Vlastimil Babka wrote:
> > > cpu offlining. I have to check the code but my impression was that WQ
> > > code will ignore the cpu requested by the work item when the cpu is
> > > going offline. If the offline happens while the worker function already
> > > executes then it has to wait as we run with preemption disabled so we
> > > should be safe here. Or am I missing something obvious?
> > 
> > Tejun suggested an alternative solution to avoiding get_online_cpus() in
> > this thread:
> > https://lkml.kernel.org/r/<20170123170329.GA7820@....duckdns.org>
> 
> But it would look like the following as it could be serialised against
> pcpu_drain_mutex as the cpu hotplug teardown callback is allowed to sleep.
> 

Bah, this is obviously unsafe. It's guaranteed to deadlock.

-- 
Mel Gorman
SUSE Labs