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    SubjectRe: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update
    On Mon, Jan 08, 2007 at 12:51:03AM +0300, Oleg Nesterov wrote:
    > Change flush_workqueue() to use for_each_possible_cpu(). This means that
    > flush_cpu_workqueue() may hit CPU which is already dead. However in that
    > case
    >
    > if (!list_empty(&cwq->worklist) || cwq->current_work != NULL)
    >
    > means that CPU_DEAD in progress, it will do kthread_stop() + take_over_work()
    > so we can proceed and insert a barrier. We hold cwq->lock, so we are safe.
    >
    > This patch replaces fix-flush_workqueue-vs-cpu_dead-race.patch which was
    > broken by switching to preempt_disable (now we don't need locking at all).
    > Note that migrate_sequence (was hotplug_sequence) is incremented under
    > cwq->lock. Since flush_workqueue does lock/unlock of cwq->lock on all CPUs,
    > it must see the new value if take_over_work() happened before we checked
    > this cwq, and this is the case we should worry about: otherwise we added
    > a barrier.
    >
    > Srivatsa?

    This is head-spinning :)

    Spotted atleast these problems:

    1. run_workqueue()->work.func()->flush_work()->mutex_lock(workqueue_mutex)
    deadlocks if we are blocked in cleanup_workqueue_thread()->kthread_stop()
    for the same worker thread to exit.

    Looks possible in practice to me.

    2.

    CPU_DEAD->cleanup_workqueue_thread->(cwq->thread = NULL)->kthread_stop() ..
    ^^^^^^^^^^^^^^^^^^^^
    |___ Problematic

    Now while we are blocked here, if a work->func() calls
    flush_workqueue->flush_cpu_workqueue, we clearly cant identify that event
    thread is trying to flush its own queue (cwq->thread == current test
    fails) and hence we will deadlock.

    A lock_cpu_hotplug(), or any other ability to block concurrent hotplug
    operations from happening, in run_workqueue would have avoided both the above
    races.

    Alternatively, for the second race, I guess we can avoid setting
    cwq->thread = NULL in cleanup_workqueue_thread() till the thread has exited,
    but I am not sure if that opens up any other race. The first race seems
    harder to fix ..

    I wonder if spin (spinroot.com) or some other formal model can make this job of
    spotting-races easier for us ..

    --
    Regards,
    vatsa
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