Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

[Saravana Kannan <skannan@codeaurora.org>]

On 08/05/2014 03:53 AM, Viresh Kumar wrote:
> On 5 August 2014 16:17, Prarit Bhargava <prarit@redhat.com> wrote:
>> Nope, not a stupid question.  After reproducing (finally!) yesterday I've been
>> wondering the same thing.
>
> Good to know that :)
>
>> I've been looking into *exactly* this.  On any platform where
>> cpu_weight(affected_cpus) == 1 for a particular cpu this lockdep trace should
>> happen.
>
>> That's what I'm wondering too.  I'm going to instrument the code to find out
>> this morning.  I'm wondering if this comes down to a lockdep class issue
>> (perhaps lockdep puts globally defined locks like cpufreq_global_kobject in a
>> different class?).
>
> Maybe, I tried this Hack to make this somewhat similar to the other case
> on my platform with just two CPUs:
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 6f02485..6b4abac 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -98,7 +98,7 @@ static DEFINE_MUTEX(cpufreq_governor_mutex);
>
>   bool have_governor_per_policy(void)
>   {
> -       return !!(cpufreq_driver->flags & CPUFREQ_HAVE_GOVERNOR_PER_POLICY);
> +       return !(cpufreq_driver->flags & CPUFREQ_HAVE_GOVERNOR_PER_POLICY);
>   }
>   EXPORT_SYMBOL_GPL(have_governor_per_policy);
>
>
> This should result in something similar to setting that per-policy-governor
> flag (Actually I could have done that too :)), and I couldn't see that crash :(
>
> That needs more investigation now, probably we can get some champ of
> sysfs stuff like Tejun/Greg into discussion now..

Stephen and I looked into this. This is not a sysfs framework 
difference. The reason we don't have this issue when we use global 
tunables is because we add the attribute group to the 
cpufreq_global_kobject and that kobject doesn't have a kobj_type ops 
similar to the per policy kobject. So, read/write to those attributes do 
NOT go through the generic show/store ops that wrap every other cpufreq 
framework attribute read/writes.

So, none of those read/write do any kind of locking. They don't race 
with POLICY_EXIT (because we remove the sysfs group first thing in 
POLICY_EXIT) but might still race with START/STOPs (not sure, haven't 
looked closely yet).

For example, writing to sampling_rate of ondemand governor might cause a 
race in update_sampling_rate(). It could race and happen between a STOP 
and POLICY_EXIT (triggered by hotplug, gov change, etc).

So, this might be a completely separate bug that needs fixing when we 
don't use per policy govs.

-Saravana

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