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

[Viresh Kumar <viresh.kumar@linaro.org>]

On 18-02-16, 02:19, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> 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@intel.com>
> ---
>  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. And so usage_count can become zero
just after this check.

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);

-- 
viresh