Re: [PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

[Christoph Hellwig <hch@lst.de>]

> -static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
> -	__releases(hctx->srcu)
> +static void hctx_unlock(struct blk_mq_hw_ctx *hctx)
>  {
>  	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
>  		rcu_read_unlock();
>  	else
> -		srcu_read_unlock(hctx->srcu, srcu_idx);
> +		percpu_ref_put(&hctx->queue->dispatch_counter);

While you're at it:  can we avoid the pointless inversion in the if
statement and just do:

 	if (hctx->flags & BLK_MQ_F_BLOCKING)
		percpu_ref_put(&hctx->queue->dispatch_counter);
	else
		rcu_read_unlock();

> +static inline bool hctx_lock(struct blk_mq_hw_ctx *hctx)
>  {
>  	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> -		/* shut up gcc false positive */
> -		*srcu_idx = 0;
>  		rcu_read_lock();
> +		return true;
>  	} else
> -		*srcu_idx = srcu_read_lock(hctx->srcu);
> +		return percpu_ref_tryget_live(&hctx->queue->dispatch_counter);
>  }

Same here.

Otherwise this looks good to me, but did you do a deep audit that all
the new hctx_lock() failure cases don't cause problems?