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

[Daniel Harding]

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.

--
Regards,

Daniel Harding