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Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
To: Michal Hocko <mhocko@kernel.org>
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Cc: Mel Gorman <mgorman@techsingularity.net>,
        Dmitry Vyukov <dvyukov@google.com>, Tejun Heo <tj@kernel.org>,
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        "linux-mm@kvack.org" <linux-mm@kvack.org>,
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        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>
From: Vlastimil Babka <vbabka@suse.cz>
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Date: Tue, 7 Feb 2017 14:57:56 +0100
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On 02/07/2017 01:48 PM, Michal Hocko wrote:
> 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?

Ah I forgot that migrated work item will in fact run on local cpu. So looks like 
nobody should race with the callback indeed (assuming that when the callback is 
called, the cpu in question already isn't executing workqueue workers).