cpufreq: unlock correct rwsem while updating policy->cpu

Current code looks like this:

        WARN_ON(lock_policy_rwsem_write(cpu));
        update_policy_cpu(policy, new_cpu);
        unlock_policy_rwsem_write(cpu);

{lock|unlock}_policy_rwsem_write(cpu) takes/releases policy->cpu's rwsem.
Because cpu is changing with the call to update_policy_cpu(), the
unlock_policy_rwsem_write() will release the incorrect lock.

The right solution would be to release the same lock as was taken earlier. Also
update_policy_cpu() was also called from cpufreq_add_dev() without any locks and
so its better if we move this locking to inside update_policy_cpu().

This patch fixes a regression introduced in 3.12 by commit f9ba680d23
(cpufreq: Extract the handover of policy cpu to a helper function).

Reported-and-tested-by: Jon Medhurst<tixy@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 11 insertions, 2 deletions

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index dbfe219667d3..82ecbe39dfb0 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -952,9 +952,20 @@ static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
         if (cpu == policy->cpu)
                 return;
 
+        /*
+         * Take direct locks as lock_policy_rwsem_write wouldn't work here.
+         * Also lock for last cpu is enough here as contention will happen only
+         * after policy->cpu is changed and after it is changed, other threads
+         * will try to acquire lock for new cpu. And policy is already updated
+         * by then.
+         */
+        down_write(&per_cpu(cpu_policy_rwsem, policy->cpu));
+
         policy->last_cpu = policy->cpu;
         policy->cpu = cpu;
 
+        up_write(&per_cpu(cpu_policy_rwsem, policy->last_cpu));
+
 #ifdef CONFIG_CPU_FREQ_TABLE
         cpufreq_frequency_table_update_policy_cpu(policy);
 #endif
@@ -1200,9 +1211,7 @@ static int __cpufreq_remove_dev_prepare(struct device *dev,
 
                 new_cpu = cpufreq_nominate_new_policy_cpu(policy, cpu, frozen);
                 if (new_cpu >= 0) {
-                        WARN_ON(lock_policy_rwsem_write(cpu));
                         update_policy_cpu(policy, new_cpu);
-                        unlock_policy_rwsem_write(cpu);
 
                         if (!frozen) {
                                 pr_debug("%s: policy Kobject moved to cpu: %d "