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Date:   Mon, 2 May 2022 18:40:36 +0100
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Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels
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To:     Jens Axboe <axboe@kernel.dk>,
        Daniel Harding <dharding@living180.net>
Cc:     regressions@lists.linux.dev, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <7925e262-e0d4-6791-e43b-d37e9d693414@living180.net>
 <6ad38ecc-b2a9-f0e9-f7c7-f312a2763f97@kernel.dk>
 <ccf6cea1-1139-cd73-c4e5-dc9799708bdd@living180.net>
 <bb283ff5-6820-d096-2fca-ae7679698a50@kernel.dk>
 <371c01dd-258c-e428-7428-ff390b664752@kernel.dk>
From:   Pavel Begunkov <asml.silence@gmail.com>
In-Reply-To: <371c01dd-258c-e428-7428-ff390b664752@kernel.dk>
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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


> # 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.

-- 
Pavel Begunkov