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Date: Tue, 7 Feb 2017 11:35:52 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>, Dmitry Vyukov <dvyukov@google.com>,
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        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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On Tue 07-02-17 10:28:09, Mel Gorman wrote:
> On Tue, Feb 07, 2017 at 10:49:28AM +0100, Vlastimil Babka wrote:
> > On 02/07/2017 10:43 AM, Mel Gorman wrote:
> > > If I'm reading this right, a hot-remove will set the pool POOL_DISASSOCIATED
> > > and unbound. A workqueue queued for draining get migrated during hot-remove
> > > and a drain operation will execute twice on a CPU -- one for what was
> > > queued and a second time for the CPU it was migrated from. It should still
> > > work with flush_work which doesn't appear to block forever if an item
> > > got migrated to another workqueue. The actual drain workqueue function is
> > > using the CPU ID it's currently running on so it shouldn't get confused.
> > 
> > Is the worker that will process this migrated workqueue also guaranteed
> > to be pinned to a cpu for the whole work, though? drain_local_pages()
> > needs that guarantee.
> > 
> 
> It should be by running on a workqueue handler bound to that CPU (queued
> on wq->cpu_pwqs in __queue_work)

Are you sure? The comment in kernel/workqueue.c says
         * While DISASSOCIATED, the cpu may be offline and all workers have
         * %WORKER_UNBOUND set and concurrency management disabled, and may
         * be executing on any CPU.  The pool behaves as an unbound one.

I might be misreadig but an unbound pool can be handled by workers which
are not pinned on any cpu AFAIU.
-- 
Michal Hocko
SUSE Labs