Re: [PATCH] cpufreq: fix governor start/stop race condition

From: Xiaoguang Chen
Date: Thu May 16 2013 - 02:17:10 EST

On 05/13/2013 06:47 PM, Xiaoguang Chen wrote:
cpufreq governor stop and start should be kept in sequence.
If not, there will be unexpected behavior, for example:

we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
the normal sequence is as below:

1) Current governor is userspace, one application tries to set
governor to ondemand. it will call __cpufreq_set_policy in which it
will stop userspace governor and then start ondemand governor.

2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
call cpufreq_add_policy_cpu. on which it first stops userspace
governor, and then starts userspace governor.

Now if the sequence of above two cases interleaves, it becames
below sequence:

1) application stops userspace governor
2) hotplug stops userspace governor
3) application starts ondemand governor
4) hotplug starts a governor

in step 4, hotplug is supposed to start userspace governor, but now
the governor has been changed by application to ondemand, so hotplug
starts ondemand governor again !!!!

The solution is as below:
cpufreq policy has a rwsem to protect the read and write of policy.
make the scope of the rwsem to contain cpufreq governor stop/start
sequence, so that after the stop governor has started, other threads
will not stop governor, they have to wait the current thread starts
the governor and then do their job.

Change-Id: I054bb52789fc8abdcf80bdcc1caebd429c182bb0
Signed-off-by: Xiaoguang Chen <chenxg@xxxxxxxxxxx>
drivers/cpufreq/cpufreq.c | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 1b8a48e..935f750 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -811,14 +811,14 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling,
int ret = 0, has_target = !!cpufreq_driver->target;
unsigned long flags;
+ lock_policy_rwsem_write(sibling);
policy = cpufreq_cpu_get(sibling);
if (has_target)
__cpufreq_governor(policy, CPUFREQ_GOV_STOP);
- lock_policy_rwsem_write(sibling);
write_lock_irqsave(&cpufreq_driver_lock, flags);
cpumask_set_cpu(cpu, policy->cpus);
@@ -826,13 +826,13 @@ static int cpufreq_add_policy_cpu(unsigned int cpu, unsigned int sibling,
per_cpu(cpufreq_cpu_data, cpu) = policy;
write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- unlock_policy_rwsem_write(sibling);
if (has_target) {
__cpufreq_governor(policy, CPUFREQ_GOV_START);
__cpufreq_governor(policy, CPUFREQ_GOV_LIMITS);
+ unlock_policy_rwsem_write(sibling);
ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
if (ret) {
@@ -1028,6 +1028,8 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
return -EINVAL;
+ WARN_ON(lock_policy_rwsem_write(cpu));
if (cpufreq_driver->target)
__cpufreq_governor(data, CPUFREQ_GOV_STOP);
@@ -1037,12 +1039,10 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
data->governor->name, CPUFREQ_NAME_LEN);
- WARN_ON(lock_policy_rwsem_write(cpu));
cpus = cpumask_weight(data->cpus);
if (cpus > 1)
cpumask_clear_cpu(cpu, data->cpus);
- unlock_policy_rwsem_write(cpu);
if (cpu != data->cpu) {
sysfs_remove_link(&dev->kobj, "cpufreq");
@@ -1054,7 +1054,6 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
if (ret) {
pr_err("%s: Failed to move kobj: %d", __func__, ret);
- WARN_ON(lock_policy_rwsem_write(cpu));
cpumask_set_cpu(cpu, data->cpus);
write_lock_irqsave(&cpufreq_driver_lock, flags);
@@ -1068,9 +1067,7 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
return -EINVAL;
- WARN_ON(lock_policy_rwsem_write(cpu));
update_policy_cpu(data, cpu_dev->id);
- unlock_policy_rwsem_write(cpu);
pr_debug("%s: policy Kobject moved to cpu: %d from: %d\n",
__func__, cpu_dev->id, cpu);
@@ -1083,10 +1080,8 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
if (cpufreq_driver->target)
__cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);
- lock_policy_rwsem_read(cpu);
kobj = &data->kobj;
cmp = &data->kobj_unregister;
- unlock_policy_rwsem_read(cpu);
/* we need to make sure that the underlying kobj is actually
@@ -1108,6 +1103,8 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
__cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
+ unlock_policy_rwsem_write(cpu);
per_cpu(cpufreq_policy_cpu, cpu) = -1;
return 0;
Hi, Guys
What's your opinion about this patch?


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