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SubjectRe: [PATCH v1 3/3] blk-mq: start to freeze queue just after setting dying
On Fri, Mar 17, 2017 at 11:26:26PM +0000, Bart Van Assche wrote:
> On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote:
> > Given blk_set_queue_dying() is always called in remove path
> > of block device, and queue will be cleaned up later, we don't
> > need to worry about undoing the counter.
> >
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index d772c221cc17..62d4967c369f 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -500,9 +500,12 @@ void blk_set_queue_dying(struct request_queue *q)
> > queue_flag_set(QUEUE_FLAG_DYING, q);
> > spin_unlock_irq(q->queue_lock);
> >
> > - if (q->mq_ops)
> > + if (q->mq_ops) {
> > blk_mq_wake_waiters(q);
> > - else {
> > +
> > + /* block new I/O coming */
> > + blk_mq_freeze_queue_start(q);
> > + } else {
> > struct request_list *rl;
> >
> > spin_lock_irq(q->queue_lock);
>
> Hello Ming,
>
> The blk_freeze_queue() call in blk_cleanup_queue() waits until q_usage_counter
> drops to zero. Since the above blk_mq_freeze_queue_start() call increases that
> counter by one, how is blk_freeze_queue() expected to finish ever?

It is q->mq_freeze_depth which is increased by blk_mq_freeze_queue_start(), not
q->q_usage_counter, otherwise blk_freeze_queue() would never return, :-)

Thanks,
Ming

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