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

From: Thorsten Leemhuis <regressions@leemhuis.info>
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>
Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels
Date: Mon, 16 May 2022 20:34:17 +0200	[thread overview]
Message-ID: <1ce76b24-9185-6b2e-844e-d6a0ce42bb0f@leemhuis.info> (raw)
In-Reply-To: <eaee4ea1-8e5a-dde8-472d-44241d992037@kernel.dk>

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

Ciao, Thorsten