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



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