cpufreq: Avoid using inactive policies

There are two places in the cpufreq core in which low-level driver
callbacks may be invoked for an inactive cpufreq policy, which isn't
guaranteed to work in general.  Both are due to possible races with
CPU offline.

First, in cpufreq_get(), the policy may become inactive after
the check against policy->cpus in cpufreq_cpu_get() and before
policy->rwsem is acquired, in which case using it going forward may
not be correct.

Second, an analogous situation is possible in cpufreq_update_policy().

Avoid using inactive policies by adding policy_is_inactive() checks
to the code in the above places.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/cpufreq.c

@@ -1526,7 +1526,10 @@ unsigned int cpufreq_get(unsigned int cpu)
 
 	if (policy) {
 		down_read(&policy->rwsem);
-		ret_freq = __cpufreq_get(policy);
+
+		if (!policy_is_inactive(policy))
+			ret_freq = __cpufreq_get(policy);
+
 		up_read(&policy->rwsem);
 
 		cpufreq_cpu_put(policy);
@@ -2265,6 +2268,11 @@ int cpufreq_update_policy(unsigned int cpu)
 
 	down_write(&policy->rwsem);
 
+	if (policy_is_inactive(policy)) {
+		ret = -ENODEV;
+		goto unlock;
+	}
+
 	pr_debug("updating policy for CPU %u\n", cpu);
 	memcpy(&new_policy, policy, sizeof(*policy));
 	new_policy.min = policy->user_policy.min;