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

Keith Busch
2021-12-20T20:36:57Z

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;
}
--