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Subject: Re: [PATCH 2/2] blk-mq: sync the update nr_hw_queues with
 part_in_flight
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Ming Lei <tom.leiming@gmail.com>, Jens Axboe <axboe@kernel.dk>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        Keith Busch <keith.busch@linux.intel.com>,
        linux-block <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <1534317915-5041-1-git-send-email-jianchao.w.wang@oracle.com>
 <1534317915-5041-3-git-send-email-jianchao.w.wang@oracle.com>
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 <8cd103e6-0986-2c45-00f2-8a2a540a2eb5@oracle.com>
 <20180816090334.GA7144@ming.t460p>
From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <f3fad2f5-9ab5-438a-d5a6-93c637921378@oracle.com>
Date:   Thu, 16 Aug 2018 17:20:50 +0800
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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. ；）

Thanks
Jianchao