From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: INFO: task hung in blk_queue_enter
Date: Mon, 4 Jun 2018 22:13:23 +0900
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Cc: Bart Van Assche <Bart.VanAssche@wdc.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-block@vger.kernel.org,
        Johannes Thumshirn <jthumshirn@suse.de>,
        Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        syzbot <syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Jens Axboe <axboe@kernel.dk>,
        Dan Williams <dan.j.williams@intel.com>,
        Christoph Hellwig <hch@lst.de>, oleksandr@natalenko.name,
        ming.lei@redhat.com, martin@lichtvoll.de,
        Hannes Reinecke <hare@suse.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        keith.busch@intel.com, linux-ext4@vger.kernel.org
To: Dmitry Vyukov <dvyukov@google.com>
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On 2018/06/04 20:46, Dmitry Vyukov wrote:
> On Fri, Jun 1, 2018 at 12:10 PM, Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> 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.