linux-kernel - Re: [REGRESSION] lxc-stop hang on 5.17.x kernels

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Date:   Tue, 3 May 2022 00:14:23 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Daniel Harding <dharding@...ing180.net>,
        Jens Axboe <axboe@...nel.dk>
Cc:     regressions@...ts.linux.dev, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels

On 5/2/22 19:49, Daniel Harding wrote:
> On 5/2/22 20:40, Pavel Begunkov wrote:
>> On 5/2/22 18:00, Jens Axboe wrote:
>>> On 5/2/22 7:59 AM, Jens Axboe wrote:
>>>> On 5/2/22 7:36 AM, Daniel Harding wrote:
>>>>> On 5/2/22 16:26, Jens Axboe wrote:
>>>>>> On 5/2/22 7:17 AM, Daniel Harding wrote:
>>>>>>> I use lxc-4.0.12 on Gentoo, built with io-uring support
>>>>>>> (--enable-liburing), targeting liburing-2.1.  My kernel config is a
>>>>>>> very lightly modified version of Fedora's generic kernel config. After
>>>>>>> moving from the 5.16.x series to the 5.17.x kernel series, I started
>>>>>>> noticed frequent hangs in lxc-stop.  It doesn't happen 100% of the
>>>>>>> time, but definitely more than 50% of the time. Bisecting narrowed
>>>>>>> down the issue to commit aa43477b040251f451db0d844073ac00a8ab66ee:
>>>>>>> io_uring: poll rework. Testing indicates the problem is still present
>>>>>>> in 5.18-rc5. Unfortunately I do not have the expertise with the
>>>>>>> codebases of either lxc or io-uring to try to debug the problem
>>>>>>> further on my own, but I can easily apply patches to any of the
>>>>>>> involved components (lxc, liburing, kernel) and rebuild for testing or
>>>>>>> validation.  I am also happy to provide any further information that
>>>>>>> would be helpful with reproducing or debugging the problem.
>>>>>> Do you have a recipe to reproduce the hang? That would make it
>>>>>> significantly easier to figure out.
>>>>>
>>>>> I can reproduce it with just the following:
>>>>>
>>>>>      sudo lxc-create --n lxc-test --template download --bdev dir --dir /var/lib/lxc/lxc-test/rootfs -- -d ubuntu -r bionic -a amd64
>>>>>      sudo lxc-start -n lxc-test
>>>>>      sudo lxc-stop -n lxc-test
>>>>>
>>>>> The lxc-stop command never exits and the container continues running.
>>>>> If that isn't sufficient to reproduce, please let me know.
>>>>
>>>> Thanks, that's useful! I'm at a conference this week and hence have
>>>> limited amount of time to debug, hopefully Pavel has time to take a look
>>>> at this.
>>>
>>> Didn't manage to reproduce. Can you try, on both the good and bad
>>> kernel, to do:
>>
>> Same here, it doesn't reproduce for me
> OK, sorry it wasn't something simple.
>> # echo 1 > /sys/kernel/debug/tracing/events/io_uring/enable
>>>
>>> run lxc-stop
>>>
>>> # cp /sys/kernel/debug/tracing/trace ~/iou-trace
>>>
>>> so we can see what's going on? Looking at the source, lxc is just using
>>> plain POLL_ADD, so I'm guessing it's not getting a notification when it
>>> expects to, or it's POLL_REMOVE not doing its job. If we have a trace
>>> from both a working and broken kernel, that might shed some light on it.
> It's late in my timezone, but I'll try to work on getting those traces tomorrow.

I think I got it, I've attached a trace.

What's interesting is that it issues a multi shot poll but I don't
see any kind of cancellation, neither cancel requests nor task/ring
exit. Perhaps have to go look at lxc to see how it's supposed
to work

-- 
Pavel Begunkov
View attachment "uring_trace" of type "text/plain" (30396 bytes)