Re: NMI hardlock stacktrace deadlock [was Re: Linux 5.2-rc5]

From: Thomas Gleixner
Date: Fri Jun 21 2019 - 11:30:58 EST

On Wed, 19 Jun 2019, Linus Torvalds wrote:

> On Wed, Jun 19, 2019 at 12:19 PM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Do you have the oops itself at all?
> >
> > An example at
> >
> >

The second once contains a lockdep splat which warns about a potential
deadlock in the iommu code.

> > The bug causing the oops is clearly a driver problem. The rc5 fallout
> > just seems to be because of some shrinker changes affecting some object
> > reaping that were unfortunately still active. What perturbed the CI
> > team was the machine failed to panic & reboot.
> Hmm. It's hard to guess at the cause of that. The oopses themselves
> don't look like they are happening in any particularly bad context, so
> all the normal reboot-on-oops etc stuff _should_ work.
> So it would help a lot if you could bisect the bad problem at least a
> bit, if it is at all reproducible. Because with no other clues, it's
> hard to even guess at what might be up.
> The fact that you say "NMI watchdog firing as we dumped the ftrace"
> means that maybe it might be some ftrace / stacktrace issue where the
> dumping itself leads to some endless loop, but who knows.
> For example, one thing that has happened during this development cycle
> is the stacktrace common infrastructure changes (arch_stack_walk() and
> friends). I'm, not seeing why that would cause your issues, but I'm
> adding a few random people for ftrace / stacktrace changes.

/me whistels innocently.

Chris, do you have the actual NMI lockup detector splats somewhere?