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Re: [PATCH V4 2/7] cpufreq: Call __cpufreq_governor() with policy->rwsem held

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  Re: [PATCH V4 2/7] cpufreq: Call __cpufreq_governor() with  policy->rwsem held "Rafael J. Wysocki" <rafael@kernel.org> - 2016-02-11 10:50 +0100
    Re: [PATCH V4 2/7] cpufreq: Call __cpufreq_governor() with  policy->rwsem held Viresh Kumar <viresh.kumar@linaro.org> - 2016-02-11 13:00 +0100

#1331790 — Re: [PATCH V4 2/7] cpufreq: Call __cpufreq_governor() with policy->rwsem held

On Tue, Feb 9, 2016 at 4:46 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> This isn't followed properly by all parts of the core code, some follow
> it, whereas others don't.

"The cpufreq core code is not consistent with respect to invoking
__cpufreq_governor() under policy->rwsem."

> Enforcing it will also enable us to remove cpufreq_governor_lock, that
> is used today because we can't guarantee that __cpufreq_governor() isn't
> executed in parallel.

"Changing all code to always hold policy->rwsem around
__cpufreq_governor() invocations will allow us to ..."

> We should also ensure that the lock is held across state changes to the
> governors.
>
> For example, while adding a CPU to the policy on cpu-online path, we
> need to stop the governor, change policy->cpus, start the governor and
> then refresh its limits. The complete sequence must be guaranteed to
> execute without any concurrent races. And that can be achieved using
> policy->rwsem around these use cases.
>
> Also note that cpufreq_driver->stop_cpu() and ->exit() can get called
> while policy->rwsem is held. That shouldn't have any side effects
> though.

The last paragraph is unclear.

Is it supposed to mean that the change will cause
cpufreq_driver->stop_cpu() and ->exit() to be called under
policy->rwsem sometimes?

Thanks,
Rafael

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#1331855

On 11-02-16, 10:48, Rafael J. Wysocki wrote:
> On Tue, Feb 9, 2016 at 4:46 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > Also note that cpufreq_driver->stop_cpu() and ->exit() can get called
> > while policy->rwsem is held. That shouldn't have any side effects
> > though.
> 
> The last paragraph is unclear.
> 
> Is it supposed to mean that the change will cause
> cpufreq_driver->stop_cpu() and ->exit() to be called under
> policy->rwsem sometimes?

Yeah, reworded it a bit ..

Also note that, after this patch cpufreq_driver->stop_cpu() and ->exit()
will get called while policy->rwsem is held, which wasn't the case
earlier. That shouldn't have any side effects though.

-- 
viresh

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