Re: [PATCH V2 1/2] block: fix race between adding/removing rq qos and normal IO

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

On 6/8/21 12:19 AM, Ming Lei wrote:
>  static inline void rq_qos_add(struct request_queue *q, struct rq_qos *rqos)
>  {
> +	/*
> +	 * No IO can be in-flight when adding rqos, so freeze queue, which
> +	 * is fine since we only support rq_qos for blk-mq queue
> +	 */
> +	blk_mq_freeze_queue(q);
>  	rqos->next = q->rq_qos;
>  	q->rq_qos = rqos;
> +	blk_mq_unfreeze_queue(q);
>  
>  	if (rqos->ops->debugfs_attrs)
>  		blk_mq_debugfs_register_rqos(rqos);
> @@ -110,12 +117,18 @@ static inline void rq_qos_del(struct request_queue *q, struct rq_qos *rqos)
>  {
>  	struct rq_qos **cur;
>  
> +	/*
> +	 * No IO can be in-flight when removing rqos, so freeze queue,
> +	 * which is fine since we only support rq_qos for blk-mq queue
> +	 */
> +	blk_mq_freeze_queue(q);
>  	for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) {
>  		if (*cur == rqos) {
>  			*cur = rqos->next;
>  			break;
>  		}
>  	}
> +	blk_mq_unfreeze_queue(q);
>  
>  	blk_mq_debugfs_unregister_rqos(rqos);
>  }

Although this patch looks like an improvement to me, I think we also
need protection against concurrent rq_qos_add() and rq_qos_del() calls,
e.g. via a mutex.

Thanks,

Bart.