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

On Mon, 2021-09-06 at 20:25 +0200, Rafael J. Wysocki wrote:

On Mon, Sep 6, 2021 at 8:14 PM Srinivas Pandruvada
[off-list ref] wrote:

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On Mon, 2021-09-06 at 19:54 +0200, Rafael J. Wysocki wrote:

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[...]

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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.

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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.