[PATCH V3 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

[Keith Busch]

On Tue, Sep 08, 2020 at 04:15:36PM +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);
>  
>  	mutex_lock(&q->mq_quiesce_lock);
>  
> -	if (blk_queue_quiesced(q))
> -		goto exit;

Why remove the 'goto exit' on this condition? There shouldn't be a need
to synchronize dispatch again if a previous quiesce already did so.

> -	blk_mq_quiesce_queue_nowait(q);
> -
> -	queue_for_each_hw_ctx(q, hctx, i) {
> -		if (hctx->flags & BLK_MQ_F_BLOCKING)
> -			synchronize_srcu(hctx->srcu);
> -		else
> -			rcu = true;
> +	if (!blk_queue_quiesced(q)) {
> +		blk_mq_quiesce_queue_nowait(q);
> +		if (blocking)
> +			percpu_ref_kill(&q->dispatch_counter);
>  	}
> -	if (rcu)
> +
> +	if (blocking)
> +		wait_event(q->mq_quiesce_wq,
> +			   percpu_ref_is_zero(&q->dispatch_counter));
> +	else
>  		synchronize_rcu();
> - exit:
> +
>  	mutex_unlock(&q->mq_quiesce_lock);
>  }