linux-kernel - Re: [RESEND PATCH] blk-mq: fix hang caused by freeze/unfreeze sequence

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Date:   Fri, 12 Apr 2019 20:42:03 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Bob Liu <bob.liu@...cle.com>, linux-block@...r.kernel.org
Cc:     shirley.ma@...cle.com, martin.petersen@...cle.com,
        Roman Pen <roman.penyaev@...fitbricks.com>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] blk-mq: fix hang caused by freeze/unfreeze
 sequence

On 4/9/19 2:08 AM, Bob Liu wrote:
>  void blk_freeze_queue_start(struct request_queue *q)
>  {
> -	int freeze_depth;
> -
> -	freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
> -	if (freeze_depth == 1) {
> +	mutex_lock(&q->mq_freeze_lock);
> +	if (++q->mq_freeze_depth == 1) {
>  		percpu_ref_kill(&q->q_usage_counter);
> +		mutex_unlock(&q->mq_freeze_lock);
>  		if (queue_is_mq(q))
>  			blk_mq_run_hw_queues(q, false);
> +	} else {
> +		mutex_unlock(&q->mq_freeze_lock);
>  	}
>  }
Have you considered to move the mutex_unlock() call to the end of the function
such that there is only one mutex_unlock() call instead of two? In case you
would be worried about holding the mutex around the code that runs the queue,
how about changing the blk_mq_run_hw_queues() call such that the queues are
run async?

Bart.