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

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Date:   Sun, 14 Apr 2019 21:09:43 +0800
From:   Bob Liu <bob.liu@...cle.com>
To:     Bart Van Assche <bvanassche@....org>, 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 4/13/19 11:42 AM, Bart Van Assche wrote:
> 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?
>
 
Yes, but this bug is difficult to reproduce. I'd prefer keep the change minimum.