Re: [PATCH] cpufreq: Fix RCU reboot regression on x86 PIC machines

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Oct 4, 2019 at 2:30 PM Ville Syrjälä
<ville.syrjala@linux.intel.com> wrote:
>
> On Thu, Oct 03, 2019 at 05:05:28PM +0200, Rafael J. Wysocki wrote:
> > On Thursday, October 3, 2019 4:08:28 PM CEST Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >
> > > Since 4.20-rc1 my PIC machines no longer reboot/shutdown.
> > > I bisected this down to commit 45975c7d21a1 ("rcu: Define RCU-sched
> > > API in terms of RCU for Tree RCU PREEMPT builds").
> > >
> > > I traced the hang into
> > > -> cpufreq_suspend()
> > >  -> cpufreq_stop_governor()
> > >   -> cpufreq_dbs_governor_stop()
> > >    -> gov_clear_update_util()
> > >     -> synchronize_sched()
> > >      -> synchronize_rcu()
> > >
> > > Only PREEMPT=y is affected for obvious reasons. The problem
> > > is limited to PIC machines since they mask off interrupts
> > > in i8259A_shutdown() (syscore_ops.shutdown() registered
> > > from device_initcall()).
> >
> > Let me treat this as a fresh bug report. :-)
> >
> > > I reported this long ago but no better fix has surfaced,
> >
> > So I don't recall seeing the original report or if I did, I had not understood
> > the problem then.
> >
> > > hence sending out my initial workaround which I've been
> > > carrying around ever since. I just move cpufreq_core_init()
> > > to late_initcall() so the syscore_ops get registered in the
> > > oppsite order and thus the .shutdown() hooks get executed
> > > in the opposite order as well. Not 100% convinced this is
> > > safe (especially moving the cpufreq_global_kobject creation
> > > to late_initcall()) but I've not had any problems with it
> > > at least.
> >
> > The problem is a bug in cpufreq that shouldn't point its syscore shutdown
> > callback pointer to cpufreq_suspend(), because the syscore stage is generally
> > too lat to call that function and I'm not sure why this has not been causing
> > any other issues to trigger (or maybe it did, but they were not reported).
> >
> > Does the patch below work for you?
>
> It does. Thanks.
>
> Feel free to slap on
> Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> if you want to go with that.

I will, thank you!