On Mon, Sep 24, 2018 at 12:51:07PM -0700, Bart Van Assche wrote:
> On Mon, 2018-09-24 at 13:13 -0600, Keith Busch wrote:
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 85a1c1a59c72..28d128450621 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -848,22 +848,6 @@ static void blk_mq_timeout_work(struct work_struct
> > *work)
> > struct blk_mq_hw_ctx *hctx;
> > int i;
> >
> > - /* A deadlock might occur if a request is stuck requiring a
> > - * timeout at the same time a queue freeze is waiting
> > - * completion, since the timeout code would not be able to
> > - * acquire the queue reference here.
> > - *
> > - * That's why we don't use blk_queue_enter here; instead, we use
> > - * percpu_ref_tryget directly, because we need to be able to
> > - * obtain a reference even in the short window between the queue
> > - * starting to freeze, by dropping the first reference in
> > - * blk_freeze_queue_start, and the moment the last request is
> > - * consumed, marked by the instant q_usage_counter reaches
> > - * zero.
> > - */
> > - if (!percpu_ref_tryget(&q->q_usage_counter))
> > - return;
> > -
> > blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
> >
> > if (next != 0) {
> > @@ -881,7 +865,6 @@ static void blk_mq_timeout_work(struct work_struct
> > *work)
> > blk_mq_tag_idle(hctx);
> > }
> > }
> > - blk_queue_exit(q);
> > }
>
> Hi Keith,
>
> The above introduces a behavior change: if the percpu_ref_tryget() call inside
> blk_mq_queue_tag_busy_iter() fails then blk_mq_timeout_work() will now call
> blk_mq_tag_idle(). I think that's wrong if the percpu_ref_tryget() call fails
> due to the queue having been frozen. Please make blk_mq_queue_tag_busy_iter()
> return a bool that indicates whether or not it has iterated over the request
> queue.
Good point, thanks for the feedback.
