linux-kernel - Re: [PATCH v2 6/8] thermal/sysfs: Update instance->weight under tz lock

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Message-ID: <65b879ad-aff2-4f7e-bad1-ac85dc5fd146@arm.com>
Date: Wed, 20 Dec 2023 16:23:59 +0000
From: Lukasz Luba <lukasz.luba@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
 linux-pm@...r.kernel.org, rui.zhang@...el.com
Subject: Re: [PATCH v2 6/8] thermal/sysfs: Update instance->weight under tz
 lock



On 12/20/23 14:53, Rafael J. Wysocki wrote:
> On Tue, Dec 12, 2023 at 2:48 PM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> The user-space can change thermal instance weight value while the
>> throtte() callback is running for a governor.
> 
> This needs to be more precise IMV.  Something like
> 
> "User space can change the weight of a thermal instance via sysfs
> while the .throttle() callback is running for a governor, because
> weight_store() does not use the zone lock."

Agree, I'll use it.

> 
>> The IPA governor uses the
>> 'weight' value for power calculation and also keeps it in 'total_weight'.
>> Therefore, the 'weight' value must not change during the throttle()
>> callback. Use 'tz->lock' mutex which also guards the throttle() to make
>> the update value safe.
> 
> "The IPA governor uses instance weight values for power calculations
> and caches the sum of them as total_weight, so it gets confused when
> one of them changes while its .throttle() callback is running.
> 
> To prevent that from happening, use thermal zone locking in weight_store()."

Agree, thanks.

> 
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>> ---
>>   drivers/thermal/thermal_sysfs.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
>> index eef40d4f3063..df85df7d4a88 100644
>> --- a/drivers/thermal/thermal_sysfs.c
>> +++ b/drivers/thermal/thermal_sysfs.c
>> @@ -961,6 +961,7 @@ ssize_t weight_store(struct device *dev, struct device_attribute *attr,
>>                       const char *buf, size_t count)
>>   {
>>          struct thermal_instance *instance;
>> +       struct thermal_zone_device *tz;
> 
> IMO the local var doesn't help much.  You may as well use
> instance->tz->lock directly below.

OK, I'll change that.

> 
>>          int ret, weight;
>>
>>          ret = kstrtoint(buf, 0, &weight);
>> @@ -968,7 +969,12 @@ ssize_t weight_store(struct device *dev, struct device_attribute *attr,
>>                  return ret;
>>
>>          instance = container_of(attr, struct thermal_instance, weight_attr);
>> +       tz = instance->tz;
>> +
>> +       /* Don't race with governors using the 'weight' value */
>> +       mutex_lock(&tz->lock);
>>          instance->weight = weight;
>> +       mutex_unlock(&tz->lock);
>>
>>          return count;
>>   }
>> --