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

[Keith Busch]

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.