cpufreq: governor: Close dbs_data update race condition
authorRafael J. Wysocki <rafael.j.wysocki@intel.com>
Thu, 18 Feb 2016 01:19:00 +0000 (02:19 +0100)
committerRafael J. Wysocki <rafael.j.wysocki@intel.com>
Wed, 9 Mar 2016 13:41:06 +0000 (14:41 +0100)
commit574ef14d5dbcd2743326cc1b28e61a1e7733162a
tree7dbfd41fbfe7d59f2359d066fc823c119efaab59
parent8eb055d3f53e52805907ea54e2eec0885be91a50
cpufreq: governor: Close dbs_data update race condition

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>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
drivers/cpufreq/cpufreq_governor.c