Re: [PATCH] cpufreq: Fix race between sysfs writes and hotplug/policy update

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On 06/23/2013 11:08 PM, Viresh Kumar wrote:
Hi Saravana,

On 23 June 2013 08:32, Saravana Kannan <skannan@xxxxxxxxxxxxxx> wrote:
The sysfs store ops need to grab the policy write semaphore to avoid race
with hotplug and cpufreq_update_policy() calls. Without this, we could end
up with simultaneous calls to cpufreq_driver->target()

Interesting.

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 2d53f47..37db7f0 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -420,9 +420,13 @@ static ssize_t store_##file_name                                   \
         if (ret != 1)                                                   \
                 return -EINVAL;                                         \
                                                                         \
+       lock_policy_rwsem_write(policy->cpu);                           \
+                                                                       \
         ret = __cpufreq_set_policy(policy, &new_policy);                \
         policy->user_policy.object = policy->object;                    \
                                                                         \
+       unlock_policy_rwsem_write(policy->cpu);                         \
+                                                                       \
         return ret ? ret : count;                                       \
  }

As far as I know, this protection already exists. Check this
function, which eventually calls all **store() related to
struct freq_attr

static ssize_t store(struct kobject *kobj, struct attribute *attr,
		     const char *buf, size_t count)
{
	struct cpufreq_policy *policy = to_policy(kobj);
	struct freq_attr *fattr = to_attr(attr);
	ssize_t ret = -EINVAL;
	policy = cpufreq_cpu_get_sysfs(policy->cpu);
	if (!policy)
		goto no_policy;

	if (lock_policy_rwsem_write(policy->cpu) < 0)
		goto fail;

	if (fattr->store)
		ret = fattr->store(policy, buf, count);
	else
		ret = -EIO;

	unlock_policy_rwsem_write(policy->cpu);
fail:
	cpufreq_cpu_put_sysfs(policy);
no_policy:
	return ret;
}


You are right. I did look at this, but looks like I skimmed some code too fast. But the race is certainly happening. I'll have to dig deeper I guess. I do see some patches you have for serializing driver->target() calls. Haven't looked at them yet, but maybe they'll help.

I'll dig deeper.

Thanks,
Saravana

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