linux-kernel - Re: [syzbot] INFO: task hung in io_sq_thread

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Message-ID: <98a5f4a7-428b-fc0c-3f8c-c368f98d79a2@kernel.dk>
Date:   Mon, 19 Jul 2021 12:38:37 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        syzbot <syzbot+ac957324022b7132accf@...kaller.appspotmail.com>,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...nel.org, mingo@...hat.com, peterz@...radead.org,
        rostedt@...dmis.org, syzkaller-bugs@...glegroups.com,
        will@...nel.org
Subject: Re: [syzbot] INFO: task hung in io_sq_thread_park (2)

On 7/19/21 11:28 AM, Pavel Begunkov wrote:
> On 7/19/21 6:13 PM, Jens Axboe wrote:
>> On 7/19/21 10:57 AM, Pavel Begunkov wrote:
>>> On 7/16/21 11:57 AM, syzbot wrote:
>>>> Hello,
>>>>
>>>> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
>>>> WARNING in io_uring_cancel_generic
>>>
>>> __arm_poll doesn't remove a second poll entry in case of failed
>>> __io_queue_proc(), it's most likely the cause here.
>>>
>>> #syz test: https://github.com/isilence/linux.git syztest_sqpoll_hang
>>
>> Was my thought on seeing the last debug run too. Haven't written a test
>> case, but my initial thought was catching this at the time that double
>> poll is armed, in __io_queue_proc(). Totally untested, just tossing
>> it out there.
> 
> Wouldn't help, unfortunately, the way syz triggers it is making a
> request to go through __io_queue_proc() three times.
> 
> Either it's 3 waitqueues or we need to extend the check below to
> the double poll entry.
> 
> if (poll_one->head == head)
> 	return;

Yes good point, that'd depend on single poll erroring first. Given
the variety of cases for it, catching it after the fact like in your
patch is likely the simplest/cleanest way.

-- 
Jens Axboe