Re: [PATCH] sched/numa: Prevent race on sysctl_numa_balancing static key

[chenjinghuang <chenjinghuang2@huawei.com>]

On 8/4/2026 1:09 PM, K Prateek Nayak wrote:
> Hell Chen,
> 
> On 8/3/2026 6:00 PM, Chen Jinghuang wrote:
>> +static DEFINE_MUTEX(numabalancing_mutex);
>> +
>>  void set_numabalancing_state(bool enabled)
>>  {
>> +	mutex_lock(&numabalancing_mutex);
>>  	if (enabled)
>>  		sysctl_numa_balancing_mode = NUMA_BALANCING_NORMAL;
>>  	else
>>  		sysctl_numa_balancing_mode = NUMA_BALANCING_DISABLED;
>>  	__set_numabalancing_state(enabled);
>> +	mutex_unlock(&numabalancing_mutex);
>>  }
> 
> Doesn't sysctl_numa_balancing() directly call __set_numabalancing_state()
> and this bit is only used by check_numabalancing_enable() on the mm side
> during early init?
> 
> I think you should move this serialization into sysctl_numa_balancing()
> because nothing can race during early init and we only need this for sysfs
> writes.
> 
You're right - my mistake, I'll move numabalancing_mutex into sysctl_numa_balancing()
and drop it from the early-init path. No one races during init, so the lock is only
needed for sysfs writes:

+static DEFINE_MUTEX(numabalancing_mutex);
+
 static int sysctl_numa_balancing(const struct ctl_table *table, int write,
 			  void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -4666,11 +4668,13 @@ static int sysctl_numa_balancing(const struct ctl_table *table, int write,
 	if (err < 0)
 		return err;
 	if (write) {
+		mutex_lock(&numabalancing_mutex);
 		if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
 		    (state & NUMA_BALANCING_MEMORY_TIERING))
 			reset_memory_tiering();
 		sysctl_numa_balancing_mode = state;
 		__set_numabalancing_state(state);
+		mutex_unlock(&numabalancing_mutex);
 	}
 	return err;
 }