Re: [PATCH V4 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

Keith Busch
2020-09-09T16:53:49Z

On Wed, Sep 09, 2020 at 06:41:14PM +0800, Ming Lei wrote:
> void blk_mq_quiesce_queue(struct request_queue *q)
> {
> - struct blk_mq_hw_ctx *hctx;
> - unsigned int i;
> - bool rcu = false;
> + bool blocking = !!(q->tag_set->flags & BLK_MQ_F_BLOCKING);
> + bool was_quiesced =__blk_mq_quiesce_queue_nowait(q);
>
> - __blk_mq_quiesce_queue_nowait(q);
> + if (!was_quiesced && blocking)
> + percpu_ref_kill(&q->dispatch_counter);
>
> - queue_for_each_hw_ctx(q, hctx, i) {
> - if (hctx->flags & BLK_MQ_F_BLOCKING)
> - synchronize_srcu(hctx->srcu);
> - else
> - rcu = true;
> - }
> - if (rcu)
> + if (blocking)
> + wait_event(q->mq_quiesce_wq,
> + percpu_ref_is_zero(&q->dispatch_counter));
> + else
> synchronize_rcu();
> }

In the previous version, you had ensured no thread can unquiesce a queue
while another is waiting for quiescence. Now that the locking is gone,
a thread could unquiesce the queue before percpu_ref reaches zero, so
the wait_event() may never complete on the resurrected percpu_ref.

I don't think any drivers do such a thing today, but it just looks like
the implementation leaves open the possibility.