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Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz>
Date: Tue, 7 Feb 2017 10:49:28 +0100
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To: Mel Gorman <mgorman@techsingularity.net>, Michal Hocko <mhocko@kernel.org>
Cc: 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 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.

> Tejun, did I miss anything? Does a workqueue item queued on a CPU being
> offline get unbound and a caller can still flush it safely? In this
> specific case, it's ok that the workqueue item does not run on the CPU it
> was queued on.
> 

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