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

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Date:   Tue, 7 Feb 2017 10:49:28 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>
Cc:     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 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.
>