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Date:   Mon, 16 May 2022 21:07:25 +0200
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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>,
        Pavel Begunkov <asml.silence@gmail.com>
Cc:     regressions@lists.linux.dev, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>
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From:   Thorsten Leemhuis <regressions@leemhuis.info>
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On 16.05.22 20:39, Jens Axboe wrote:
> On 5/16/22 12:34 PM, Thorsten Leemhuis wrote:
>> On 16.05.22 20:22, Jens Axboe wrote:
>>> On 5/16/22 12:17 PM, Thorsten Leemhuis wrote:
>>>>>> 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.
>>>>>
>>>>> #regzbot invalid:  userspace programming error
>>>>
>>>> Hmmm, not sure if I like this. So yes, this might be a bug in LXC, but
>>>> afaics it's a bug that was exposed by kernel change in 5.17 (correct me
>>>> if I'm wrong!). The problem thus still qualifies as a kernel regression
>>>> that normally needs to be fixed, as can be seen my some of the quotes
>>>> from Linus in this file:
>>>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>>
>>> Sorry, but that's really BS in this particularly case. This could always
>>> have triggered, it's the way multishot works. Will we count eg timing
>>> changes as potential regressions, because an application relied on
>>> something there? That does not make it ABI.
>>>
>>> In general I agree with Linus on this, a change in behavior breaking
>>> something should be investigated and figured out (and reverted, if need
>>> be). This is not that.
>>
>> Sorry, I have to deal with various subsystems and a lot of regressions
>> reports. I can't know the details of each of issue and there are
>> developers around that are not that familiar with all the practical
>> implications of the "no regressions". That's why I was just trying to
>> ensure that this is something safe to ignore. If you say it is, than I'm
>> totally happy and now rest my case. :-D
> 
> It's just a slippery slope that quickly leads to the fact that _any_
> kernel change is a potential regressions,

I know, don't worry, that's why I'm trying to be careful. But I also had
cases already where someone (even a proper subsystem maintainer) said
"this is not a regression, it's a userspace bug" and it clearly was a
kernel regression (and Linus wasn't happy when he found out). That why I
was trying to evaluate the situation to get an impression is this is
really something that can/should be ignored. But I guess by
approach/wording here might have not been the best and needs to be improved.

> as it may change something
> that an app unknowingly depends on. For this case, the multishot ended
> up being downgraded to single shot on older kernels, so you'd never see
> multiple triggers of it. And multiple triggers is a natural effect of
> the level triggered poll that io_uring does. The app didn't handle
> multiple events in between reading them, which was an oversight in how
> that was done.
> 
> Hence I do think this one can be safely closed.

Many thx for clarifying.

Ciao, Thorsten