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

[Jens Axboe <axboe@kernel.dk>]

On 09/28/2015 08:48 AM, Tejun Heo wrote:
> Hello,
>
> On Sun, Sep 27, 2015 at 10:06:05PM +0900, Akinobu Mita wrote:
>>>>   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().
>
> Ah, you're right.  I was thinking that percpu_ref_switch_to_percpu()
> being called after blk_mq_freeze_queue_start() would be buggy and thus
> the above can't be enough but that is safe as long as the calls are
> properly synchronized.  Hmmm... maybe we should add synchronization to
> those operations from percpu_ref side.

I think that would be very useful, it seems sort of half-assed if the 
caller side has to provide serialization for that.

-- 
Jens Axboe