Re: [PATCH 1/2] cpufreq: qcom-hw: Use initialized cpumask for thermal pressure update

Viresh Kumar
2022-01-19T06:35:54Z

On 18-01-22, 10:56, Bjorn Andersson wrote:
> In the event that the SoC is under thermal pressure while booting it's
> possible for the dcvs notification to happen inbetween the cpufreq
> framework calling init and it actually updating the policy's
> related_cpus cpumask.
>
> Prior to the introduction of the thermal pressure update helper an empty
> cpumask would simply result in the thermal pressure of no cpus being
> updated, but the new code will attempt to dereference an invalid per_cpu
> variable.
>
> Avoid this problem by using the policy's cpus cpumask instead of the
> related_cpus mask, as this is initialized before the interrupt is
> registered.
>
> Fixes: 0258cb19c77d ("cpufreq: qcom-cpufreq-hw: Use new thermal pressure update function")
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
> drivers/cpufreq/qcom-cpufreq-hw.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/qcom-cpufreq-hw.c b/drivers/cpufreq/qcom-cpufreq-hw.c
> index 05f3d7876e44..866fba3ac6fc 100644
> --- a/drivers/cpufreq/qcom-cpufreq-hw.c
> +++ b/drivers/cpufreq/qcom-cpufreq-hw.c
> @@ -297,7 +297,7 @@ static void qcom_lmh_dcvs_notify(struct qcom_cpufreq_data *data)
> throttled_freq = freq_hz / HZ_PER_KHZ;
>
> /* Update thermal pressure (the boost frequencies are accepted) */
> - arch_update_thermal_pressure(policy->related_cpus, throttled_freq);
> + arch_update_thermal_pressure(policy->cpus, throttled_freq);

policy->cpus keeps on changing with CPU hotplug and this can leave
your platform in an inconsistent state. For example, in case where you
offline a CPU from policy, other CPUs get their thermal pressure
updated, online the CPU back and all CPUs of a policy don't have the
same settings anymore.

There are few things we can do here now:

- Check for empty related_cpus and return early. Since related_cpus is
updated only once, this shall work just fine and must not be racy.

While at it, I think we can also do something like this in
topology_update_thermal_pressure() instead:

cpu = cpumask_first(cpus);
if (unlikely(cpu >= NR_CPUS))
return;

- And while writing this email, I dropped all other ideas in favor of
change to topology_update_thermal_pressure() :)

--
viresh