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    SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
    On 01/04, Srivatsa Vaddagiri wrote:
    > On Thu, Jan 04, 2007 at 05:29:36PM +0300, Oleg Nesterov wrote:
    > > Thanks, I need to think about this.
    > >
    > > However I am not sure I fully understand the problem.
    > >
    > > First, this deadlock was not introduced by recent changes (including "single
    > > threaded flush_workqueue() takes workqueue_mutex too"), yes?
    > AFAIK this deadlock originated from Andrew's patch here:

    I don't think so. The core problem is not that we are doing unlock/sleep/lock
    with this patch. The thing is: work->func() can't take wq_mutex (and thus use
    flush_work/workqueue) because it is possible that CPU_DEAD holds this mutex
    and waits for us to complete(kthread_stop_info). I believe this bug is old.

    > (Yes, your patches didnt introduce this. I was just reiterating here my
    > earlier point that workqueue code is broken of late wrt cpu hotplug).
    > > Also, it seems to me we have a much more simple scenario for deadlock.
    > >
    > > events/0 runs run_workqueue(), work->func() sleeps or takes a preemtion. CPU 0
    > > dies, keventd thread migrates to another CPU. CPU_DEAD calls kthread_stop() under
    > > workqueue_mutex and waits for until kevents thread exits. Now, if this work (or
    > > another work pending on cwq->worklist) takes workqueue_mutex (for example, does
    > > flush_workqueue) we have a deadlock.
    > >
    > > No?
    > Yes, the above scenario also will cause a deadlock.

    Ok, thanks for acknowledgement.

    > I supposed one could avoid the deadlock by having a 'workqueue_mutex_held'
    > flag and avoid taking the mutex set under some conditions,

    I am thinking about the same right now. Probably we can do something like this:

    int xxx_lock(void)
    for (;;) {
    if (mutex_trylock(wq_mutex))
    return 1;

    // the owner of wq_mutex sleeps, we can proceed
    if (kthread_should_stop())
    return 0;
    void xxx_unlock(int yesno)
    if (yesno)

    and then do

    locked = xxx_lock();

    in flush_xxx() instead of plain lock/unlock.

    Yes, ugly. I'll try to do something else on weekend.

    > but IMHO a
    > more neater solution is to provide a cpu-hotplug lock which works under
    > all these corner cases. One such proposal was made here:

    I'll take a look later, thanks.


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