From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9A50DC433EF for ; Mon, 16 May 2022 19:07:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345282AbiEPTHa (ORCPT ); Mon, 16 May 2022 15:07:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49848 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241889AbiEPTH3 (ORCPT ); Mon, 16 May 2022 15:07:29 -0400 Received: from wp530.webpack.hosteurope.de (wp530.webpack.hosteurope.de [IPv6:2a01:488:42:1000:50ed:8234::]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A524B2559C; Mon, 16 May 2022 12:07:27 -0700 (PDT) Received: from [2a02:8108:963f:de38:eca4:7d19:f9a2:22c5]; authenticated by wp530.webpack.hosteurope.de running ExIM with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) id 1nqg3x-0008CN-GQ; Mon, 16 May 2022 21:07:25 +0200 Message-ID: <735012d3-bbda-54df-11fb-8e7b561c598d@leemhuis.info> Date: Mon, 16 May 2022 21:07:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels Content-Language: en-US To: Jens Axboe , Daniel Harding , Pavel Begunkov Cc: regressions@lists.linux.dev, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, Christian Brauner References: <7925e262-e0d4-6791-e43b-d37e9d693414@living180.net> <371c01dd-258c-e428-7428-ff390b664752@kernel.dk> <2436d42c-85ca-d060-6508-350c769804f1@gmail.com> <12a57dd9-4423-a13d-559b-2b1dd2fb0ef3@living180.net> <897dc597-fc0a-34ec-84b8-7e1c4901e0fc@leemhuis.info> <41c86189-0d1f-60f0-ca8e-f80b3ccf5130@gmail.com> <3fc08243-f9e0-9cec-4207-883c55ccff78@living180.net> <13028ff4-3565-f09e-818c-19e5f95fa60f@living180.net> <469e5a9b-c7e0-6365-c353-d831ff1c5071@leemhuis.info> <1ce76b24-9185-6b2e-844e-d6a0ce42bb0f@leemhuis.info> <55ddaa3c-57a5-f17e-13a7-7427c5f1bb88@kernel.dk> From: Thorsten Leemhuis In-Reply-To: <55ddaa3c-57a5-f17e-13a7-7427c5f1bb88@kernel.dk> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-bounce-key: webpack.hosteurope.de;regressions@leemhuis.info;1652728047;80ec63e8; X-HE-SMSGID: 1nqg3x-0008CN-GQ Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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