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
