Received-SPF: pass (google.com: domain of linux-kernel+bounces-7091-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Precedence: bulk
MIME-Version: 1.0
References: <20231212134844.1213381-1-lukasz.luba@arm.com> <20231212134844.1213381-7-lukasz.luba@arm.com>
In-Reply-To: <20231212134844.1213381-7-lukasz.luba@arm.com>
From: "Rafael J. Wysocki" <rafael@kernel.org>
Date: Wed, 20 Dec 2023 15:53:04 +0100
Message-ID: <CAJZ5v0hes13rmAgcyMYtz1ZXxV72pM3QgRY=LKF9jK=GbAyS=g@mail.gmail.com>
Subject: Re: [PATCH v2 6/8] thermal/sysfs: Update instance->weight under tz lock
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: linux-kernel@vger.kernel.org, daniel.lezcano@linaro.org, rafael@kernel.org, 
	linux-pm@vger.kernel.org, rui.zhang@intel.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Tue, Dec 12, 2023 at 2:48=E2=80=AFPM Lukasz Luba <lukasz.luba@arm.com> w=
rote:
>
> 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@arm.com>
> ---
>  drivers/thermal/thermal_sysfs.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sy=
sfs.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 devic=
e_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 =3D kstrtoint(buf, 0, &weight);
> @@ -968,7 +969,12 @@ ssize_t weight_store(struct device *dev, struct devi=
ce_attribute *attr,
>                 return ret;
>
>         instance =3D container_of(attr, struct thermal_instance, weight_a=
ttr);
> +       tz =3D instance->tz;
> +
> +       /* Don't race with governors using the 'weight' value */
> +       mutex_lock(&tz->lock);
>         instance->weight =3D weight;
> +       mutex_unlock(&tz->lock);
>
>         return count;
>  }
> --