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Date: Thu, 18 Feb 2016 02:19 +0100 From: "Rafael J. Wysocki" <rjw@...ysocki.net> To: Linux PM list <linux-pm@...r.kernel.org> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Viresh Kumar <viresh.kumar@...aro.org> Subject: [PATCH 1/12] cpufreq: governor: Close dbs_data update race condition From: Rafael J. Wysocki <rafael.j.wysocki@...el.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@...el.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) ret = gattr->store(dbs_data, buf, count); mutex_unlock(&dbs_data->mutex);
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