linux-kernel - Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

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Date:	Sun, 27 Sep 2015 22:06:05 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>,
	Ming Lei <tom.leiming@...il.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

Hi Tejun,

2015-09-27 2:32 GMT+09:00 Tejun Heo <tj@...nel.org>:
> Hello,
>
> On Sun, Sep 27, 2015 at 02:09:24AM +0900, Akinobu Mita wrote:
>> @@ -420,7 +420,9 @@ static void blk_mq_sysfs_init(struct request_queue *q)
>>  /* see blk_register_queue() */
>>  void blk_mq_finish_init(struct request_queue *q)
>>  {
>> +     mutex_lock(&q->mq_freeze_lock);
>>       percpu_ref_switch_to_percpu(&q->mq_usage_counter);
>> +     mutex_unlock(&q->mq_freeze_lock);
>
> This looks weird to me.  What can it race against at this point?

The possible scenario is described in commit log (1. ~ 7.).  In summary,
blk_mq_finish_init() and blk_mq_freeze_queue_start() can be executed
at the same time, so this is required to serialize the execution of
percpu_ref_switch_to_percpu() by blk_mq_finish_init() and
percpu_ref_kill() by blk_mq_freeze_queue_start().

>> @@ -115,11 +115,15 @@ void blk_mq_freeze_queue_start(struct request_queue *q)
>>  {
>>       int freeze_depth;
>>
>> +     mutex_lock(&q->mq_freeze_lock);
>> +
>>       freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
>
> It doesn't have to be an atomic anymore, right?

Yes, you are right.  I would like to make it in another patch in order to
simplify each change.
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