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    SubjectRe: [patch 1/16] unplugging fix
    On Sun, Jun 02 2002, Jens Axboe wrote:
    > On Sun, Jun 02 2002, Andrew Morton wrote:
    > > Andrew Morton wrote:
    > > >
    > > > There's a plugging bug in 2.5.19. Once you start pushing several disks
    > > > hard, the new unplug code gets confused and queues are left in plugged
    > > > state, but not on the plug list. They never get unplugged and the
    > > > machine dies mysteriously.
    > > >
    > >
    > > This patch didn't fix it. It made it tons better, but I again have a
    > > wedged box. Same symptoms - against IDE this time.
    > >
    > > blk_plug_list is empty. queue_flags=0x03. Interestingly,
    > > q->plug_list is non-empty, non-zero, both list members pointing at
    > > a list which isn't either itself or blk_plug_list.
    > >
    > > I note that the code isn't taking queues off the plug list when the queue
    > > is destroyed. Guess that doesn't matter - we never destroy a plugged
    > > queue...
    > >
    > > This one is killing me.
    >
    > I've got a good handle on how to clean the whole plugging thing up, I
    > suspect it will make this case easier to fix. I'll be back with that
    > tomorrow, still got guests...

    Does this work? I can't poke holes in it, but then again...

    --- /opt/kernel/linux-2.5.20/drivers/block/ll_rw_blk.c 2002-06-03 10:35:35.000000000 +0200
    +++ linux/drivers/block/ll_rw_blk.c 2002-06-03 10:37:26.000000000 +0200
    @@ -821,7 +821,7 @@
    /*
    * not plugged
    */
    - if (!__test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
    + if (!test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
    return;

    if (test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags))
    @@ -893,6 +893,17 @@
    **/
    void blk_stop_queue(request_queue_t *q)
    {
    + /*
    + * remove from the plugged list, queue must not be called.
    + */
    + if (test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags)) {
    + unsigned long flags;
    +
    + spin_lock_irqsave(&blk_plug_lock, flags);
    + list_del(&q->plug_list);
    + spin_unlock_irqrestore(&blk_plug_lock, flags);
    + }
    +
    set_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
    }

    @@ -904,45 +915,36 @@
    * are currently stopped are ignored. This is equivalent to the older
    * tq_disk task queue run.
    **/
    +#define blk_plug_entry(entry) list_entry((entry), request_queue_t, plug_list)
    void blk_run_queues(void)
    {
    - struct list_head *n, *tmp, local_plug_list;
    - unsigned long flags;
    + struct list_head local_plug_list;

    INIT_LIST_HEAD(&local_plug_list);

    + spin_lock_irq(&blk_plug_lock);
    +
    /*
    * this will happen fairly often
    */
    - spin_lock_irqsave(&blk_plug_lock, flags);
    if (list_empty(&blk_plug_list)) {
    - spin_unlock_irqrestore(&blk_plug_lock, flags);
    + spin_unlock_irq(&blk_plug_lock);
    return;
    }

    list_splice(&blk_plug_list, &local_plug_list);
    INIT_LIST_HEAD(&blk_plug_list);
    - spin_unlock_irqrestore(&blk_plug_lock, flags);
    + spin_unlock_irq(&blk_plug_lock);
    +
    + while (!list_empty(&local_plug_list)) {
    + request_queue_t *q = blk_plug_entry(local_plug_list.next);

    - /*
    - * local_plug_list is now a private copy we can traverse lockless
    - */
    - list_for_each_safe(n, tmp, &local_plug_list) {
    - request_queue_t *q = list_entry(n, request_queue_t, plug_list);
    -
    - if (!test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags)) {
    - list_del(&q->plug_list);
    - generic_unplug_device(q);
    - }
    - }
    + BUG_ON(test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags));

    - /*
    - * add any remaining queue back to plug list
    - */
    - if (!list_empty(&local_plug_list)) {
    - spin_lock_irqsave(&blk_plug_lock, flags);
    - list_splice(&local_plug_list, &blk_plug_list);
    - spin_unlock_irqrestore(&blk_plug_lock, flags);
    + spin_lock_irq(q->queue_lock);
    + list_del(&q->plug_list);
    + __generic_unplug_device(q);
    + spin_unlock_irq(q->queue_lock);
    }
    }

    --
    Jens Axboe

    -
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