Re: [PATCH] blk-mq: recover from stale cached request in blk_mq_submit_bio

[Christoph Hellwig <hch@lst.de>]

On Fri, Apr 24, 2026 at 11:45:51AM -0700, Keith Busch wrote:
> -static void blk_mq_use_cached_rq(struct request *rq, struct blk_plug *plug,
> +static bool blk_mq_use_cached_rq(struct request *rq, struct blk_plug *plug,
>  		struct bio *bio)
>  {
> -	if (rq_list_pop(&plug->cached_rqs) != rq)
> -		WARN_ON_ONCE(1);
> -
>  	/*
> -	 * If any qos ->throttle() end up blocking, we will have flushed the
> -	 * plug and hence killed the cached_rq list as well. Pop this entry
> -	 * before we throttle.
> +	 * We will have flushed the plug and hence killed the cached_rq list as
> +	 * well if anything had blocked. Pop this entry before we throttle if
> +	 * the entry is still valid.
>  	 */
> +	if (rq_list_pop(&plug->cached_rqs) != rq)
> +		return false;
> +
>  	rq_qos_throttle(rq->q, bio);
>  
>  	blk_mq_rq_time_init(rq, blk_time_get_ns());
>  	rq->cmd_flags = bio->bi_opf;
>  	INIT_LIST_HEAD(&rq->queuelist);
> +	return true;
>  }
>  
>  static bool bio_unaligned(const struct bio *bio, struct request_queue *q)
> @@ -3211,8 +3212,13 @@ void blk_mq_submit_bio(struct bio *bio)
>  
>  new_request:
>  	if (rq) {
> -		blk_mq_use_cached_rq(rq, plug, bio);
> -	} else {
> +		if (!blk_mq_use_cached_rq(rq, plug, bio)) {
> +			rq = NULL;
> +			if (unlikely(bio_queue_enter(bio)))
> +				return;
> +		}

This means we now did the checks earlier in the function without
q_usage_counter protection, which looks wrong.  I think we have
to redo all of that here.