Re: [PATCH 3/4] blk-mq: use hw tag for scheduling if hw tag space is big enough

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Fri, 2017-04-28 at 23:15 +0800, Ming Lei wrote:
> +static inline bool blk_mq_sched_may_use_hw_tag(struct request_queue *q)
> +{
> +	if (q->tag_set->flags & BLK_MQ_F_TAG_SHARED)
> +		return false;
> +
> +	if (blk_mq_get_queue_depth(q) < q->nr_requests)
> +		return false;
> +
> +	return true;
> +}

The only user of shared tag sets I know of is scsi-mq. I think it's really
unfortunate that this patch systematically disables BLK_MQ_F_SCHED_USE_HW_T=
AG
for scsi-mq.

>  int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>  {
>  	struct blk_mq_tag_set *set =3D q->tag_set;
> @@ -2681,9 +2694,12 @@ int blk_mq_update_nr_requests(struct request_queue=
 *q, unsigned int nr)
>  			break;
>  	}
> =20
> -	if (!ret)
> +	if (!ret) {
>  		q->nr_requests =3D nr;
> =20
> +		blk_mq_update_sched_flag(q);
> +	}
> +
>  	blk_mq_unfreeze_queue(q);
>  	blk_mq_start_stopped_hw_queues(q, true);

If a queue is created with a low value of nr_requests that will cause
blk_mq_sched_alloc_tags() to skip allocation of .sched_tags. If nr_requests
is increased, can that cause this function to clear BLK_MQ_F_SCHED_USE_HW_T=
AG
while keeping hctx->sched_tags =3D=3D NULL and hence trigger a NULL pointer
dereference?

Bart.