On 18-02-16, 02:19, Rafael J. Wysocki wrote: > From: Rafael J. Wysocki <[email protected]> > > 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 <[email protected]> > --- > 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
