Re: INFO: task hung in blk_queue_enter

[Tetsuo Handa]

On 2018/06/04 20:46, Dmitry Vyukov wrote:
> On Fri, Jun 1, 2018 at 12:10 PM, Tetsuo Handa
> <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>> Tetsuo Handa wrote:
>>> Since sum of percpu_count did not change after percpu_ref_kill(), this is
>>> not a race condition while folding percpu counter values into atomic counter
>>> value. That is, for some reason, someone who is responsible for calling
>>> percpu_ref_put(&q->q_usage_counter) (presumably via blk_queue_exit()) is
>>> unable to call percpu_ref_put().
>>> But I don't know how to find someone who is failing to call percpu_ref_put()...
>>
>> I found the someone. It was already there in the backtrace...
> 
> Nice!
> 
> Do I understand it correctly that this bug is probably the root cause
> of a whole lot of syzbot "task hung" reports? E.g. this one too?
> https://syzkaller.appspot.com/bug?id=cdc4add60bb95a4da3fec27c5fe6d75196b7f976
> I guess we will need to sweep close everything related to
> filesystems/block devices when this is committed?
> 

I can't tell it. We still have lockdep warning for loop module. Also, I suspect
that hung tasks waiting for page bit might be related to binder module.

Since we are about to merge "kernel/hung_task.c: show all hung tasks before panic"
patch, we might be able to get more hints after 4.18-rc1.