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    SubjectRe: [BUG][2.5.66bk9+] - tty hangings - patches, dmesg & sysctl+T info
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    I am completely convinced that the patch you are using (which I
    believe is the same as the one Andrew calls
    "tty-shutdown-race-fix.patch") is the problem. What happens is that
    release_dev() in tty_io.c calls cancel_delayed_work(), which calls
    del_timer_sync() without decrementing nr_queued for keventd_wq.

    When flush_scheduled_work() gets called it sleeps on the work_done
    waitqueue. The only place work_done gets woken up is in
    run_workqueue, and it only happens if
    atomic_dec_and_test(&cwq->nr_queued) decrements nr_queued to 0.
    But after calling cancel_delayed_work(), that can never happen (we
    deleted the timer that was going to add the work that we're waiting
    for).

    It seems to me that the implementation of cancel_delayed_work() is
    not quite right. We need to decrement nr_queued if we actually
    stopped the work from being added to the workqueue.

    Andrew, I've never seen a reply from you about this, can you tell me
    if I'm missing something here?

    By the way, I assume that the process below is the one that's hung:

    bash D C04CDC68 4233453816 8524 1 8387 (L-TLB)
    Call Trace:
    [<c010cd75>] do_IRQ+0x235/0x370
    [<c01394a5>] flush_workqueue+0x305/0x450
    [<c010ac18>] common_interrupt+0x18/0x20
    [<c011de30>] default_wake_function+0x0/0x20
    [<c011de30>] default_wake_function+0x0/0x20
    [<c0257a44>] release_dev+0x6a4/0x860
    [<c01566ab>] zap_pmd_range+0x4b/0x70
    [<c0258204>] tty_release+0x94/0x1b0
    [<c016dd7c>] __fput+0xac/0x100
    [<c0258170>] tty_release+0x0/0x1b0

    It would seem to be stuck in the flush_workqueue() called from
    release_dev(), just as I would expect.

    Shawn, can you try the patch below instead of Andrew's ttyfix2?

    - Roland

    ===== drivers/char/tty_io.c 1.72 vs edited =====
    --- 1.72/drivers/char/tty_io.c Thu Apr 3 10:20:22 2003
    +++ edited/drivers/char/tty_io.c Tue Apr 8 20:23:44 2003
    @@ -1286,8 +1286,15 @@
    }

    /*
    - * Make sure that the tty's task queue isn't activated.
    + * Prevent flush_to_ldisc() from rescheduling the work for later. Then
    + * kill any delayed work.
    */
    + clear_bit(TTY_DONT_FLIP, &tty->flags);
    + cancel_delayed_work(&tty->flip.work);
    +
    + /*
    + * Wait for ->hangup_work and ->flip.work handlers to terminate
    + */
    flush_scheduled_work();

    /*
    ===== include/linux/workqueue.h 1.4 vs edited =====
    --- 1.4/include/linux/workqueue.h Mon Nov 4 13:12:06 2002
    +++ edited/include/linux/workqueue.h Tue Apr 8 20:42:41 2003
    @@ -63,5 +63,12 @@

    extern void init_workqueues(void);

    +/*
    + * Kill off a pending schedule_delayed_work(). Note that the work callback
    + * function may still be running on return from cancel_delayed_work(). Run
    + * flush_scheduled_work() to wait on it.
    + */
    +extern int cancel_delayed_work(struct work_struct *work);
    +
    #endif

    ===== kernel/workqueue.c 1.6 vs edited =====
    --- 1.6/kernel/workqueue.c Tue Feb 11 14:57:54 2003
    +++ edited/kernel/workqueue.c Tue Apr 8 20:27:50 2003
    @@ -125,6 +125,24 @@
    return ret;
    }

    +int cancel_delayed_work(struct work_struct *work) {
    + struct cpu_workqueue_struct *cwq = work->wq_data;
    + int ret;
    +
    + ret = del_timer_sync(&work->timer);
    + if (ret) {
    + /*
    + * Wake up 'work done' waiters (flush) if we just
    + * removed the last thing on the workqueue.
    + */
    + if (atomic_dec_and_test(&cwq->nr_queued))
    + wake_up(&cwq->work_done);
    +
    + }
    +
    + return ret;
    +}
    +
    static inline void run_workqueue(struct cpu_workqueue_struct *cwq)
    {
    unsigned long flags;
    @@ -378,5 +396,5 @@

    EXPORT_SYMBOL(schedule_work);
    EXPORT_SYMBOL(schedule_delayed_work);
    +EXPORT_SYMBOL(cancel_delayed_work);
    EXPORT_SYMBOL(flush_scheduled_work);
    -
    -
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