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

[Saravana Kannan]

On 08/05/2014 03:20 PM, Prarit Bhargava wrote:
> On 08/05/2014 06:06 PM, Saravana Kannan wrote:
> > On 08/05/2014 03:53 AM, Viresh Kumar wrote:
> > > On 5 August 2014 16:17, Prarit Bhargava <prarit@xxxxxxxxxx> 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.
>
> Yeah, the show_one & store_one macros don't have any locking in them :/.
>
> Okay ... at least that isn't the issue.  I spent 1/2 the day trying to figure
> out why
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index fa11a7d..6297c76 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -745,12 +745,14 @@ static struct attribute *default_attrs[] = {
>   #define to_policy(k) container_of(k, struct cpufreq_policy, kobj)
>   #define to_attr(a) container_of(a, struct freq_attr, attr)
>
> +/* PRARIT - in the CPUFREQ_HAVE_GOVERNOR_PER_POLICY, this is used */
>   static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>   {
>          struct cpufreq_policy *policy = to_policy(kobj);
>          struct freq_attr *fattr = to_attr(attr);
>          ssize_t ret;
>
> +       printk("%s: kobject %p\n", __FUNCTION__, kobj);
>          if (!down_read_trylock(&cpufreq_rwsem))
>                  return -EINVAL;
>
> wasn't printing the kobject line when acpi-cpufreq didn't have the
> CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag.  And I agree ... it is a bug.

Wait, should I stop reporting bugs so that my patch series gets reviewed? :P

-Saravana


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