Re: [PATCH] cpufreq: intel_pstate: Fix for HWP interrupt before driver is ready

[Srinivas Pandruvada]

On Mon, 2021-09-06 at 20:25 +0200, Rafael J. Wysocki wrote:
> On Mon, Sep 6, 2021 at 8:14 PM Srinivas Pandruvada
> <srinivas.pandruvada@xxxxxxxxxxxxxxx> wrote:
> > 
> > On Mon, 2021-09-06 at 19:54 +0200, Rafael J. Wysocki wrote:
> > > 
[...]

> > > 
> > We are handling offline for other thermal interrupt sources from
> > same
> > interrupt in therm-throt.c, where we do similar in offline path (by
> > TGLX). If cpufreq offline can cause such issue of changing CPU,
> 
> This is not cpufreq offline, but intel_pstate_update_status() which
> may be triggered via sysfs.  And again, the theoretically problematic
> thing is dereferencing cpudata (which may be cleared by a remote CPU)
> from the interrupt handler without protection.

This will be a problem.

> 
> > I can call intel_pstate_disable_hwp_interrupt() via override from
> > https://elixir.bootlin.com/linux/latest/C/ident/thermal_throttle_offline
> > after masking APIC interrupt.
> 
> But why would using RCU be harder than this?
I think, this will require all_cpu_data and cpu_data to be rcu
protected. This needs to be well tested.

I think better to revert the patch for the next release.

Thanks,
Srinivas

> 
> Also please note that on RT kernels interrupt handlers are run in
> threads.