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 03:20 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@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.
>
> 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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