linux-kernel - [PATCH V2 1/2] cpufreq: unlock correct rwsem while updating policy->cpu

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Date:	Tue, 17 Sep 2013 10:22:11 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	rjw@...k.pl
Cc:	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	tixy@...aro.org, Viresh Kumar <viresh.kumar@...aro.org>
Subject: [PATCH V2 1/2] 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().

Reported-and-Tested-by: Jon Medhurst<tixy@...aro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
Hi Rafael,

Only one patch is sent now as other one is unchanged.

 drivers/cpufreq/cpufreq.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 43c24aa..1479522 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
@@ -1203,9 +1214,7 @@ 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 "
-- 
1.7.12.rc2.18.g61b472e

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