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

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Date: Wed, 20 Dec 2023 15:53:04 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Lukasz Luba <lukasz.luba@....com>
Cc: linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org, rafael@...nel.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 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."

> 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()."

>
> 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.

>         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;
>  }
> --