linux-kernel - Re: v5.14-rc3-rt1 losing wakeups?

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Date:   Fri, 30 Jul 2021 22:49:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mike Galbraith <efault@....de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: v5.14-rc3-rt1 losing wakeups?

Mike,

On Fri, Jul 30 2021 at 17:21, Mike Galbraith wrote:
> 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

KDE/Plasma points at the new fangled rtmutex based ww_mutex from
Peter. I tried to test the heck out of it...

Which graphics driver is in use on that machine?

> 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.

Ouch.

Thanks,

        tglx