linux-kernel - Re: [PATCH v1 3/3] blk-mq: start to freeze queue just after setting dying

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Date:   Fri, 17 Mar 2017 23:26:26 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "tom.leiming@...il.com" <tom.leiming@...il.com>,
        "axboe@...com" <axboe@...com>
CC:     "yizhan@...hat.com" <yizhan@...hat.com>,
        "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH v1 3/3] blk-mq: start to freeze queue just after setting
 dying

On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote:
> Given blk_set_queue_dying() is always called in remove path
> of block device, and queue will be cleaned up later, we don't
> need to worry about undoing the counter.
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index d772c221cc17..62d4967c369f 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -500,9 +500,12 @@ void blk_set_queue_dying(struct request_queue *q)
>  	queue_flag_set(QUEUE_FLAG_DYING, q);
>  	spin_unlock_irq(q->queue_lock);
>  
> -	if (q->mq_ops)
> +	if (q->mq_ops) {
>  		blk_mq_wake_waiters(q);
> -	else {
> +
> +		/* block new I/O coming */
> +		blk_mq_freeze_queue_start(q);
> +	} else {
>  		struct request_list *rl;
>  
>  		spin_lock_irq(q->queue_lock);

Hello Ming,

The blk_freeze_queue() call in blk_cleanup_queue() waits until q_usage_counter
drops to zero. Since the above blk_mq_freeze_queue_start() call increases that
counter by one, how is blk_freeze_queue() expected to finish ever?

Bart.