Re: [PATCH 1/12] cpufreq: governor: Close dbs_data update race condition

[Rafael J. Wysocki]

On Thu, Feb 18, 2016 at 6:24 AM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> On 18-02-16, 02:19, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>>
>> It is possible for a dbs_data object to be updated after its
>> usage counter has become 0.  That may happen if governor_store()
>> runs (via a govenor tunable sysfs attribute write) in parallel
>> with cpufreq_governor_exit() called for the last cpufreq policy
>> associated with the dbs_data in question.  In that case, if
>> governor_store() acquires dbs_data->mutex right after
>> cpufreq_governor_exit() has released it, the ->store() callback
>> invoked by it may operate on dbs_data with no users.  Although
>> sysfs will cause the kobject_put() in cpufreq_governor_exit() to
>> block until governor_store() has returned, that situation may
>> lead to some unexpected results, depending on the implementation
>> of the ->store callback, and therefore it should be avoided.
>>
>> To that end, modify governor_store() to check the dbs_data's
>> usage count before invoking the ->store() callback and return
>> an error if it is 0 at that point.
>>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> ---
>>  drivers/cpufreq/cpufreq_governor.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
>> ===================================================================
>> --- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
>> +++ linux-pm/drivers/cpufreq/cpufreq_governor.c
>> @@ -112,7 +112,7 @@ static ssize_t governor_store(struct kob
>>
>>       mutex_lock(&dbs_data->mutex);
>>
>> -     if (gattr->store)
>> +     if (dbs_data->usage_count && gattr->store)
>
> That's not gonna be enough. The above lock doesn't guarantee
> protection with any such races.

I'm not really sure what you're talking about to be honest, so please
be more specific.  You can say "For example, function X decrements the
usage count without locking" or similar.

Such vague comments are quite difficult to address, especially if they
don't hold any water. :-)

> And so usage_count can become zero
> just after this check.

But how?

The only place it is decremented is cpufreq_governor_exit() and there
it is done under dbs_data->mutex (at my direct request, BTW).  So we
are guaranteed that it won't go down to zero while we're holding
dbs_data->mutex, aren't we?

> Btw, we should also kill the gattr->store checks here as well, as we
> did it in cpufreq-core.
>
>>               ret = gattr->store(dbs_data, buf, count);
>>
>>       mutex_unlock(&dbs_data->mutex);

Yeah, they are quite useless.  But not in this patch.

Thanks,
Rafael