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    SubjectRe: mm: deadlock between get_online_cpus/pcpu_alloc
    Hello,

    Sorry about the delay.

    On Tue, Feb 07, 2017 at 04:34:59PM +0100, Michal Hocko wrote:
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index c3358d4f7932..b6411816787a 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -2343,7 +2343,16 @@ void drain_local_pages(struct zone *zone)
    >
    > static void drain_local_pages_wq(struct work_struct *work)
    > {
    > + /*
    > + * drain_all_pages doesn't use proper cpu hotplug protection so
    > + * we can race with cpu offline when the WQ can move this from
    > + * a cpu pinned worker to an unbound one. We can operate on a different
    > + * cpu which is allright but we also have to make sure to not move to
    > + * a different one.
    > + */
    > + preempt_disable();
    > drain_local_pages(NULL);
    > + preempt_enable();
    > }
    >
    > /*
    > @@ -2379,12 +2388,6 @@ void drain_all_pages(struct zone *zone)
    > }
    >
    > /*
    > - * As this can be called from reclaim context, do not reenter reclaim.
    > - * An allocation failure can be handled, it's simply slower
    > - */
    > - get_online_cpus();
    > -
    > - /*
    > * We don't care about racing with CPU hotplug event
    > * as offline notification will cause the notified
    > * cpu to drain that CPU pcps and on_each_cpu_mask
    > @@ -2423,7 +2426,6 @@ void drain_all_pages(struct zone *zone)
    > for_each_cpu(cpu, &cpus_with_pcps)
    > flush_work(per_cpu_ptr(&pcpu_drain, cpu));
    >
    > - put_online_cpus();
    > mutex_unlock(&pcpu_drain_mutex);

    I think this would work; however, a more canonical way would be
    something along the line of...

    drain_all_pages()
    {
    ...
    spin_lock();
    for_each_possible_cpu() {
    if (this cpu should get drained) {
    queue_work_on(this cpu's work);
    }
    }
    spin_unlock();
    ...
    }

    offline_hook()
    {
    spin_lock();
    this cpu should get drained = false;
    spin_unlock();
    queue_work_on(this cpu's work);
    flush_work(this cpu's work);
    }

    I think what workqueue should do is automatically flush in-flight CPU
    work items on CPU offline and erroring out on queue_work_on() on
    offline CPUs. And we now actually can do that because we have lifted
    the guarantee that queue_work() is local CPU affine some releases ago.
    I'll look into it soonish.

    For the time being, either approach should be fine. The more
    canonical one might be a bit less surprising but the
    preempt_disable/enable() change is short and sweet and completely fine
    for the case at hand.

    Please feel free to add

    Acked-by: Tejun Heo <tj@kernel.org>

    Thanks.

    --
    tejun

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