Re: [PATCH 2/2] blk-mq: sync the update nr_hw_queues with part_in_flight

[jianchao.wang]

Hi Ming

On 08/16/2018 05:03 PM, Ming Lei wrote:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b42a2c9ba00e..fbc5534f8178 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -113,6 +113,10 @@ void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
>  	struct mq_inflight mi = { .part = part, .inflight = inflight, };
>  
>  	inflight[0] = inflight[1] = 0;
> +
> +	if (percpu_ref_is_dying(&q->q_usage_counter))
> +		return;
> +
>  	blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
>  }

That's a good idea to use q->q_usage_counter.
But I think we could do following modification:
1. use percpu_ref_is_zero, then we will not miss any in-flight request here.
2. use rcu to ensure the user of blk_mq_in_flight has gone out of the critical section.

Like following patch:
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 89904cc..cd9878e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -113,7 +113,12 @@ void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
 
        inflight[0] = inflight[1] = 0;
 
+       rcu_read_lock();
+       if (percpu_ref_is_zero(&q->q_usage_counter))
+               return;
+
        blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
+       rcu_read_unlock();
 }
 
 static void blk_mq_check_inflight_rw(struct blk_mq_hw_ctx *hctx,
@@ -2907,6 +2912,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
        list_for_each_entry(q, &set->tag_list, tag_set_list)
                blk_mq_freeze_queue(q);
 
+       synchronize_rcu();
        /*
         * switch io scheduler to NULL to clean up the data in it.
         * will get it back after update mapping between cpu and hw queues.

And also, some comment is needed to describe them. ïï

Thanks
Jianchao