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

[Lukasz Luba]

On 12/20/23 14:53, Rafael J. Wysocki wrote:
> On Tue, Dec 12, 2023 at 2:48 PM Lukasz Luba <lukasz.luba@xxxxxxx> 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@xxxxxxx>
> > ---
> >   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;
> >   }
> > --