Re: [PATCH 1/4] block: add mq_ops->queue_rqs hook

[Keith Busch <kbusch@kernel.org>]

On Wed, Dec 15, 2021 at 09:24:18AM -0700, Jens Axboe wrote:
> +		/*
> +		 * Peek first request and see if we have a ->queue_rqs() hook.
> +		 * If we do, we can dispatch the whole plug list in one go. We
> +		 * already know at this point that all requests belong to the
> +		 * same queue, caller must ensure that's the case.
> +		 *
> +		 * Since we pass off the full list to the driver at this point,
> +		 * we do not increment the active request count for the queue.
> +		 * Bypass shared tags for now because of that.
> +		 */
> +		if (q->mq_ops->queue_rqs &&
> +		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
> +			blk_mq_run_dispatch_ops(q,
> +				q->mq_ops->queue_rqs(&plug->mq_list));

I think we still need to verify the queue isn't quiesced within
blk_mq_run_dispatch_ops()'s rcu protected area, prior to calling
.queue_rqs(). Something like below. Or is this supposed to be the
low-level drivers responsibility now?

---
+void __blk_mq_flush_plug_list(struct request_queue *q, struct blk_plug *plug)
+{
+	if (blk_queue_quiesced(q))
+		return;
+	q->mq_ops->queue_rqs(&plug->mq_list);
+}
+
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	struct blk_mq_hw_ctx *this_hctx;
@@ -2580,7 +2587,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 		if (q->mq_ops->queue_rqs &&
 		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
 			blk_mq_run_dispatch_ops(q,
-				q->mq_ops->queue_rqs(&plug->mq_list));
+				__blk_mq_flush_plug_list(q, plug));
 			if (rq_list_empty(plug->mq_list))
 				return;
 		}
--