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    SubjectRe: [BUG][2.5.66bk9+] - tty hangings - patches, dmesg & sysctl+T info
        Andrew> Well it does look like you've hit the flush_workqueue
    Andrew> livelock.

    I don't think it's really livelock. I think it's just the fact that
    his kernel (with your tty-shutdown-race-fix patch) does
    del_timer_sync() without decrementing nr_queued() and so
    flush_workqueue() never returns.

    Still, I like the idea of this patch, since it resolves the livelock.
    But I don't think the implementation is quite right. insert_sequence
    doesn't get incremented until delayed_work_timer_fn(). That means
    that a driver (tty_io.c, for example) could call
    schedule_delayed_work(), then call flush_scheduled_work() before
    delayed_work_timer_fn() has run for that work.

    In that case schedule_delayed_work() could return immediately because
    insert_sequence and remove_sequence are (probably) equal. Then
    delayed_work_timer_fn() runs after the driver exits, and we're back
    with the original problem (running a freed timer).

    It should be pretty easy to rejigger the patch so that it works
    correctly, just by moving the cwq->insert_sequence++ from
    delayed_work_timer_fn() into queue_delayed_work() (right before the
    add_timer(), say). I'm still not positive that this covers
    everything; I need to think a little harder.

    In any case, I think we still have to do something to fix
    release_dev() in tty_io.c. It seems we should at least add the
    clear_bit(TTY_DONT_FLIP, &tty->flags); however, I'm not familiar
    enough with how the tty driver works to know whether TTY_DONT_FLIP
    could get set again (while we're waiting for flush_scheduled_work()).
    If so we would also need something along the lines of

    Shawn, I think the patch I just posted a little while ago (with a
    fixed cancel_delayed_work() implementation) is more likely to cure
    your tty hanging right now. However, I think something along the
    lines of this patch from Andrew is a better solution in the long run.

    - Roland
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