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

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Date:   Mon, 2 May 2022 11:00:38 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Daniel Harding <dharding@...ing180.net>,
        Pavel Begunkov <asml.silence@...il.com>
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 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:

# 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.

-- 
Jens Axboe