On 6 August 2014 03:36, Saravana Kannan [off-list ref] wrote:

Stephen and I looked into this. This is not a sysfs framework difference.
The reason we don't have this issue when we use global tunables is because
we add the attribute group to the cpufreq_global_kobject and that kobject
doesn't have a kobj_type ops similar to the per policy kobject. So,
read/write to those attributes do NOT go through the generic show/store ops
that wrap every other cpufreq framework attribute read/writes.

So, none of those read/write do any kind of locking. They don't race with
POLICY_EXIT (because we remove the sysfs group first thing in POLICY_EXIT)
but might still race with START/STOPs (not sure, haven't looked closely
yet).

For example, writing to sampling_rate of ondemand governor might cause a
race in update_sampling_rate(). It could race and happen between a STOP and
POLICY_EXIT (triggered by hotplug, gov change, etc).

This sounds good but I couldn't prove it. Doing this on my dual core exynos
doesn't give me that crash report and it should?

diff --git a/drivers/cpufreq/exynos-cpufreq.c b/drivers/cpufreq/exynos-cpufreq.c
index 1e0ec57..027b6f7 100644
--- a/drivers/cpufreq/exynos-cpufreq.c
+++ b/drivers/cpufreq/exynos-cpufreq.c

@@ -139,7 +139,7 @@ static int exynos_cpufreq_cpu_init(struct

cpufreq_policy *policy)
 }

 static struct cpufreq_driver exynos_driver = {
-       .flags          = CPUFREQ_STICKY | CPUFREQ_NEED_INITIAL_FREQ_CHECK,
+       .flags          = CPUFREQ_STICKY |
CPUFREQ_NEED_INITIAL_FREQ_CHECK | CPUFREQ_HAVE_GOVERNOR_PER_POLICY,
        .verify         = cpufreq_generic_frequency_table_verify,
        .target_index   = exynos_target,
        .get            = cpufreq_generic_get,