[PATCH v2] cpufreq: schedutil: Serialize start against rate limit updates

[Hui Su]

rate_limit_us_store() is called with the governor attribute set
update_lock held, but sugov_start() updates freq_update_delay_ns from
the same tunable without taking that lock.

This allows the two paths to interleave as follows:

        sugov_start()             rate_limit_us_store()

        read old rate_limit_us
                                  write new rate_limit_us
                                  publish the new delay
        publish the old delay

The sysfs tunable then contains the new value while
freq_update_delay_ns contains the old one. Unlike a transient torn
read, the stale delay remains in effect until another rate limit update
or governor restart.

Serialize sugov_start() against sysfs stores by taking the existing
governor attribute set lock around sugov_update_rate_limit_us(). Use
gov_attr_set_lock() and gov_attr_set_unlock() to keep locking details
within the attribute set interface. This keeps the fix off the scheduler
hot path.

Fixes: 9bdcb44e391d ("cpufreq: schedutil: New governor based on scheduler utilization data")
Reviewed-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
Changes in v2:
- Wrap governor attribute set locking in helpers as suggested by
  Zhongqiu Han.

v1: https://lore.kernel.org/lkml/20260929103635.3780346-1-sh_def@xxxxxxx/

 include/linux/cpufreq.h          | 10 ++++++++++
 kernel/sched/cpufreq_schedutil.c |  4 ++++
 2 files changed, 14 insertions(+)

diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 35ce665edfd8..5448eb0a0885 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -694,6 +694,16 @@ static inline struct gov_attr_set *to_gov_attr_set(struct kobject *kobj)
 	return container_of(kobj, struct gov_attr_set, kobj);
 }
 
+static inline void gov_attr_set_lock(struct gov_attr_set *attr_set)
+{
+	mutex_lock(&attr_set->update_lock);
+}
+
+static inline void gov_attr_set_unlock(struct gov_attr_set *attr_set)
+{
+	mutex_unlock(&attr_set->update_lock);
+}
+
 void gov_attr_set_init(struct gov_attr_set *attr_set, struct list_head *list_node);
 void gov_attr_set_get(struct gov_attr_set *attr_set, struct list_head *list_node);
 unsigned int gov_attr_set_put(struct gov_attr_set *attr_set, struct list_head *list_node);
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 49ccd6f1c185..b847311c307d 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -861,7 +861,11 @@ static int sugov_start(struct cpufreq_policy *policy)
 	void (*uu)(struct update_util_data *data, u64 time, unsigned int flags);
 	unsigned int cpu;
 
+	/* Serialize against rate_limit_us_store(). */
+	gov_attr_set_lock(&sg_policy->tunables->attr_set);
 	sugov_update_rate_limit_us(sg_policy);
+	gov_attr_set_unlock(&sg_policy->tunables->attr_set);
+
 	sg_policy->last_freq_update_time	= 0;
 	sg_policy->next_freq			= 0;
 	sg_policy->work_in_progress		= false;

base-commit: 63de2367f6d280904a9c5e5bbc7c75bb91c5b3d8