Re: [RFC PATCH 2/2] blk-mq: blk_mq_tag_to_rq() always return null for sched_tags

[Ming Lei <ming.lei@redhat.com>]

On Sun, Feb 28, 2021 at 09:14:44PM -0500, Yufen Yu wrote:
> We just set hctx->tags->rqs[tag] when get driver tag, but will
> not set hctx->sched_tags->rqs[tag] when get sched tag.
> So, blk_mq_tag_to_rq() always return NULL for sched_tags.

True, also blk_mq_tag_to_rq() seems an awkward API, and it needs
'struct blk_mq_tags *', but which is a block layer internal definition.

> 
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>
> ---
>  block/blk-mq.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 5362a7958b74..424afe112b58 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -846,6 +846,7 @@ static int blk_mq_test_tag_bit(struct blk_mq_tags *tags, unsigned int tag)
>  
>  struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>  {
> +	/* if tags is hctx->sched_tags, it always return NULL */
>  	if (tag < tags->nr_tags && blk_mq_test_tag_bit(tags, tag)) {
>  		prefetch(tags->rqs[tag]);
>  		return tags->rqs[tag];
> @@ -3845,17 +3846,8 @@ static bool blk_mq_poll_hybrid(struct request_queue *q,
>  
>  	if (!blk_qc_t_is_internal(cookie))
>  		rq = blk_mq_tag_to_rq(hctx->tags, blk_qc_t_to_tag(cookie));
> -	else {
> -		rq = blk_mq_tag_to_rq(hctx->sched_tags, blk_qc_t_to_tag(cookie));
> -		/*
> -		 * With scheduling, if the request has completed, we'll
> -		 * get a NULL return here, as we clear the sched tag when
> -		 * that happens. The request still remains valid, like always,
> -		 * so we should be safe with just the NULL check.
> -		 */
> -		if (!rq)
> -			return false;
> -	}
> +	else
> +		return false;
>  

I think the correct fix is to retrieve the request via:

	hctx->sched_tags->static_rqs[blk_qc_t_to_tag(cookie)]

since it is nice to run blk_mq_poll_hybrid_sleep() for one
non-started request in case of real scheduler.

-- 
Ming