linux-kernel - Re: INFO: task hung in blk_queue

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Date:   Wed, 16 May 2018 17:16:40 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "syzbot+c4f9cebf9d651f6e54de@...kaller.appspotmail.com" 
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Subject: Re: INFO: task hung in blk_queue_enter

On Wed, May 16, 2018 at 4:56 PM, Bart Van Assche <Bart.VanAssche@....com> wrote:
> On Wed, 2018-05-16 at 22:05 +0900, Tetsuo Handa wrote:
>> One ore more threads are waiting for q->mq_freeze_depth to become 0. But the
>> thread who incremented q->mq_freeze_depth at blk_freeze_queue_start(q) from
>> blk_freeze_queue() is waiting at blk_mq_freeze_queue_wait(). Therefore,
>> atomic_read(&q->mq_freeze_depth) == 0 condition for wait_event() in
>> blk_queue_enter() will never be satisfied. But what does that wait_event()
>> want to do? Isn't "start freezing" a sort of blk_queue_dying(q) == true?
>> Since percpu_ref_tryget_live(&q->q_usage_counter) failed and the queue is
>> about to be frozen, shouldn't we treat atomic_read(&q->mq_freeze_depth) != 0
>> as if blk_queue_dying(q) == true? That is, something like below:
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 85909b4..59e2496 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -951,10 +951,10 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
>>               smp_rmb();
>>
>>               wait_event(q->mq_freeze_wq,
>> -                        (atomic_read(&q->mq_freeze_depth) == 0 &&
>> -                         (preempt || !blk_queue_preempt_only(q))) ||
>> +                        atomic_read(&q->mq_freeze_depth) ||
>> +                        (preempt || !blk_queue_preempt_only(q)) ||
>>                          blk_queue_dying(q));
>> -             if (blk_queue_dying(q))
>> +             if (atomic_read(&q->mq_freeze_depth) || blk_queue_dying(q))
>>                       return -ENODEV;
>>       }
>>  }
>
> That change looks wrong to me.

Hi Bart,

Why does it look wrong to you?

> Additionally, I think that you are looking in
> the wrong direction. Since blk_mq_freeze_queue_wait() and blk_queue_enter()
> work fine for all block drivers except the loop driver I think that you should
> have a closer look at how the loop driver uses this block layer functionality.
>
> Thanks,
>
> Bart.
>
>
>