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    SubjectRe: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update
    On Mon, Jan 08, 2007 at 08:06:35PM +0300, Oleg Nesterov wrote:
    > Ah, missed you point, thanks. Yet another old problem which was not introduced
    > by recent changes. And yet another indication we should avoid kthread_stop()
    > on CPU_DEAD event :) I believe this is easy to fix, but need to think more.

    I think the problem is not just with CPU_DEAD. Anyone who calls
    cleanup_workqueue_thread (say destroy_workqueue?) will see this race.

    Do you see any problems if cleanup_workqueue_thread is changed as:

    cleanup_workqueue_thread()
    {
    kthread_stop(p);
    spin_lock(cwq->lock);
    cwq->thread = NULL;
    spin_unlock(cwq->lock);
    }


    > run_workqueue:
    >
    > while (!list_empty(&cwq->worklist)) {
    > ...
    > // We hold lock_cpu_hotplug(), cpu event can't make
    > // progress.
    > ...
    > }

    Ok ..yes a cpu_event_waits_for_lock() helper will help here.

    > > I agree it minimizes the interactions. Maybe worth attempting. However I
    > > suspect it may not be as simple as it appears :)
    >
    > Yes, that is why this patch only does the first step: flush_workqueue() checks
    > the dead CPUs as well, this change is minimal.
    >
    > Do you see any problems this patch adds?

    I dont see as of now. I suspect we will know better when we implement
    the patch to eliminate CPU_DEAD handling in workqueue.c

    --
    Regards,
    vatsa
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