v5.14-rc3-rt1 losing wakeups?

[Mike Galbraith]

On Fri, 2021-07-30 at 13:07 +0200, Sebastian Andrzej Siewior wrote:
> Dear RT folks!
>
> I'm pleased to announce the v5.14-rc3-rt1 patch set.

Damn, I was hoping to figure out wth is going on before the 62 patch
version of tglx/rtmutex branch made its way out into the big wide
world, but alas, I was too slow.

I started meeting GUI hangs as soon as I merged the 62 patch series
into my 5.13-rt based master tree.  I took tglx/rtmutex (977db8e523f5)
back to 5.13-rt to make sure it wasn't some booboo I had made in the
rolled forward tree, but the hangs followed the backport, and I just
met them in virgin v5.14-rc3-rt1, so unfortunately it wasn't some local
booboo, there's a bug lurking.  Maybe a config sensitive one, as what
I'm seeing on my box seems unlikely to escape into the wild otherwise.

First symptom is KDE/Plasma's task manager going comatose.  Notice soon
enough with a !lockdep kernel, and I can crashdump, or rather I can
with my 5.13-rt based kernel, a dump from shiny new rt1 can't be loaded
by crash for some reason.  The local tree dumps I've looked at haven't
been helpful anyway, box at a glance looks fine.  With lockdep enabled,
a failing kernel gets so buggered it isn't even able to crashdump.

config attached.  Oh, a lockdep enabled kernel fails sooner, but both
fail here fairly quickly.

	-Mike

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