Date:   Mon, 16 May 2022 19:13:05 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Daniel Harding <dharding@...ing180.net>
Cc:     regressions@...ts.linux.dev, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels

On 5/16/22 16:13, Daniel Harding wrote:
> On 5/16/22 16:57, Daniel Harding wrote:
>> On 5/16/22 16:25, Pavel Begunkov wrote:
>>> On 5/16/22 13:12, Pavel Begunkov wrote:
>>>> On 5/15/22 19:34, Daniel Harding wrote:
>>>>> On 5/15/22 11:20, Thorsten Leemhuis wrote:
>>>>>> On 04.05.22 08:54, Daniel Harding wrote:
>>>>>>> On 5/3/22 17:14, Pavel Begunkov wrote:
>>>>>>>> On 5/3/22 08:37, Daniel Harding wrote:
>>>>>>>>> [Resend with a smaller trace]
>>>>>>>>> On 5/3/22 02:14, Pavel Begunkov wrote:
>>>>>>>>>> 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
>>>>>>>>> Yes, that looks exactly like my bad trace.  I've attached good trace
>>>>>>>>> (captured with linux-5.16.19) and a bad trace (captured with
>>>>>>>>> linux-5.17.5).  These are the differences I noticed with just a
>>>>>>>>> visual scan:
>>>>>>>>>
>>>>>>>>> * Both traces have three io_uring_submit_sqe calls at the very
>>>>>>>>> beginning, but in the good trace, there are further
>>>>>>>>> io_uring_submit_sqe calls throughout the trace, while in the bad
>>>>>>>>> trace, there are none.
>>>>>>>>> * The good trace uses a mask of c3 for io_uring_task_add much more
>>>>>>>>> often than the bad trace:  the bad trace uses a mask of c3 only for
>>>>>>>>> the very last call to io_uring_task_add, but a mask of 41 for the
>>>>>>>>> other calls.
>>>>>>>>> * In the good trace, many of the io_uring_complete calls have a
>>>>>>>>> result of 195, while in the bad trace, they all have a result of 1.
>>>>>>>>>
>>>>>>>>> I don't know whether any of those things are significant or not, but
>>>>>>>>> that's what jumped out at me.
>>>>>>>>>
>>>>>>>>> I have also attached a copy of the script I used to generate the
>>>>>>>>> traces.  If there is anything further I can to do help debug, please
>>>>>>>>> let me know.
>>>>>>>> Good observations! thanks for traces.
>>>>>>>>
>>>>>>>> It sounds like multi-shot poll requests were getting downgraded
>>>>>>>> to one-shot, which is a valid behaviour and was so because we
>>>>>>>> didn't fully support some cases. If that's the reason, than
>>>>>>>> the userspace/lxc is misusing the ABI. At least, that's the
>>>>>>>> working hypothesis for now, need to check lxc.
>>>>>>> So, I looked at the lxc source code, and it appears to at least try to
>>>>>>> handle the case of multi-shot being downgraded to one-shot.  I don't
>>>>>>> know enough to know if the code is actually correct however:
>>>>>>>
>>>>>>> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L165-L189
>>>>>>> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L254
>>>>>>> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L288-L290
>>>>>> Hi, this is your Linux kernel regression tracker. Nothing happened here
>>>>>> for round about ten days now afaics; or did the discussion continue
>>>>>> somewhere else.
>>>>>>
>>>>>>  From what I gathered from this discussion is seems the root cause might
>>>>>> be in LXC, but it was exposed by kernel change. That makes it sill a
>>>>>> kernel regression that should be fixed; or is there a strong reason why
>>>>>> we should let this one slip?
>>>>>
>>>>> No, there hasn't been any discussion since the email you replied to. I've done a bit more testing on my end, but without anything conclusive.  The one thing I can say is that my testing shows that LXC does correctly handle multi-shot poll requests which were being downgraded to one-shot in 5.16.x kernels, which I think invalidates Pavel's theory.  In 5.17.x kernels, those same poll requests are no longer being downgraded to one-shot requests, and thus under 5.17.x LXC is no longer re-arming those poll requests (but also shouldn't need to, according to what is being returned by the kernel). I don't know if this change in kernel behavior is related to the hang, or if it is just a side effect of other io-uring changes that made it into 5.17.  Nothing in the LXC's usage of io-uring seems obviously incorrect to me, but I am far from an expert.  I also did some work toward creating a simpler reproducer, without success (I was able to get a simple program using io-uring 
>>>>> running, but never could get it to hang).  ISTM that this is still a kernel regression, unless someone can point out a definite fault in the way LXC is using io-uring.
>>>>
>>>> Haven't had time to debug it. Apparently LXC is stuck on
>>>> read(2) terminal fd. Not yet clear what is the reason.
>>>
>>> How it was with oneshots:
>>>
>>> 1: kernel: poll fires, add a CQE
>>> 2: kernel: remove poll
>>> 3: userspace: get CQE
>>> 4: userspace: read(terminal_fd);
>>> 5: userspace: add new poll
>>> 6: goto 1)
>>>
>>> What might happen and actually happens with multishot:
>>>
>>> 1: kernel: poll fires, add CQE1
>>> 2: kernel: poll fires again, add CQE2
>>> 3: userspace: get CQE1
>>> 4: userspace: read(terminal_fd); // reads all data, for both CQE1 and CQE2
>>> 5: userspace: get CQE2
>>> 6: userspace: read(terminal_fd); // nothing to read, hangs here
>>>
>>> It should be the read in lxc_terminal_ptx_io().
>>>
>>> IMHO, it's not a regression but a not perfect feature API and/or
>>> an API misuse.
>>>
>>> Cc: Christian Brauner
>>>
>>> Christian, in case you may have some input on the LXC side of things.
>>> Daniel reported an LXC problem when it uses io_uring multishot poll requests.
>>> Before aa43477b04025 ("io_uring: poll rework"), multishot poll requests for
>>> tty/pty and some other files were always downgraded to oneshots, which had
>>> been fixed by the commit and exposed the problem. I hope the example above
>>> explains it, but please let me know if it needs more details
>>
>> Pavel, I had actually just started a draft email with the same theory (although you stated it much more clearly than I could have).  I'm working on debugging the LXC side, but I'm pretty sure the issue is due to LXC using blocking reads and getting stuck exactly as you describe.  If I can confirm this, I'll go ahead and mark this regression as invalid and file an issue with LXC. Thanks for your help and patience.
> 
> Yes, it does appear that was the problem.  The attach POC patch against LXC fixes the hang.  The kernel is working as intended.

Daniel, that's great, thanks for confirming!

-- 
Pavel Begunkov

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