Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit()

[Bart Van Assche <bvanassche@acm.org>]

On 3/5/21 7:14 AM, John Garry wrote:
> diff --git a/block/blk.h b/block/blk.h
> index 3b53e44b967e..1a948bfd91e4 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -201,10 +201,29 @@ void elv_unregister_queue(struct request_queue *q);
>  static inline void elevator_exit(struct request_queue *q,
>  		struct elevator_queue *e)
>  {
> +	struct blk_mq_tag_set *set = q->tag_set;
> +	struct request_queue *tmp;
> +
>  	lockdep_assert_held(&q->sysfs_lock);
>  
> +	mutex_lock(&set->tag_list_lock);
> +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> +		if (tmp == q)
> +			continue;
> +		blk_mq_freeze_queue(tmp);
> +		blk_mq_quiesce_queue(tmp);
> +	}
> +
>  	blk_mq_sched_free_requests(q);
>  	__elevator_exit(q, e);
> +
> +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> +		if (tmp == q)
> +			continue;
> +		blk_mq_unquiesce_queue(tmp);
> +		blk_mq_unfreeze_queue(tmp);
> +	}
> +	mutex_unlock(&set->tag_list_lock);
>  }

This patch introduces nesting of tag_list_lock inside sysfs_lock. The
latter is per request queue while the former can be shared across
multiple request queues. Has it been analyzed whether this is safe?

Thanks,

Bart.