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

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Message-ID: <20170207135846.usfrn7e4znjhmogn@techsingularity.net>
Date:   Tue, 7 Feb 2017 13:58:46 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        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 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