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Date: Tue, 7 Feb 2017 13:48:35 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
Message-ID: <20170207124835.GP5065@dhcp22.suse.cz>
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To: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>, 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>

On Tue 07-02-17 13:43:39, Vlastimil Babka wrote:
[...]
> > Anyway, shouldn't be it sufficient to disable preemption
> > on drain_local_pages_wq? The CPU hotplug callback will not preempt us
> > and so we cannot work on the same cpus, right?
> 
> I thought the problem here was that the callback races with the work item
> that has been migrated to a different cpu. Once we are not working on the
> local cpu, disabling preempt/irq's won't help?

If the worker is racing with the callback than only one of can run on a
_particular_ cpu. So they cannot race. Or am I missing something?

-- 
Michal Hocko
SUSE Labs

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