Re: [PATCH AUTOSEL 6.10 069/121] thermal: trip: Use READ_ONCE() for lockless access to trip properties

Rafael J. Wysocki
2024-08-01T10:24:03Z

On Thu, Aug 1, 2024 at 2:15 AM Sasha Levin <sashal@kernel.org> wrote:
>
> From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
>
> [ Upstream commit a52641bc6293a24f25956a597e7f32148b0e2bb8 ]
>
> When accessing trip temperature and hysteresis without locking, it is
> better to use READ_ONCE() to prevent compiler optimizations possibly
> affecting the read from being applied.
>
> Of course, for the READ_ONCE() to be effective, WRITE_ONCE() needs to
> be used when updating their values.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

This is more of a matter of annotation than practical issue. That's
why I haven't even added a Fixes: tag to it.

Whether or not to take it into "stable" is up to you. It certainly is
low-risk in any case.

> ---
> drivers/thermal/thermal_sysfs.c | 6 +++---
> drivers/thermal/thermal_trip.c | 2 +-
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
> index 88211ccdfbd62..5be6113e7c80f 100644
> --- a/drivers/thermal/thermal_sysfs.c
> +++ b/drivers/thermal/thermal_sysfs.c
> @@ -150,7 +150,7 @@ trip_point_temp_show(struct device *dev, struct device_attribute *attr,
> if (sscanf(attr->attr.name, "trip_point_%d_temp", &trip_id) != 1)
> return -EINVAL;
>
> - return sprintf(buf, "%d\n", tz->trips[trip_id].trip.temperature);
> + return sprintf(buf, "%d\n", READ_ONCE(tz->trips[trip_id].trip.temperature));
> }
>
> static ssize_t
> @@ -174,7 +174,7 @@ trip_point_hyst_store(struct device *dev, struct device_attribute *attr,
> trip = &tz->trips[trip_id].trip;
>
> if (hyst != trip->hysteresis) {
> - trip->hysteresis = hyst;
> + WRITE_ONCE(trip->hysteresis, hyst);
>
> thermal_zone_trip_updated(tz, trip);
> }
> @@ -194,7 +194,7 @@ trip_point_hyst_show(struct device *dev, struct device_attribute *attr,
> if (sscanf(attr->attr.name, "trip_point_%d_hyst", &trip_id) != 1)
> return -EINVAL;
>
> - return sprintf(buf, "%d\n", tz->trips[trip_id].trip.hysteresis);
> + return sprintf(buf, "%d\n", READ_ONCE(tz->trips[trip_id].trip.hysteresis));
> }
>
> static ssize_t
> diff --git a/drivers/thermal/thermal_trip.c b/drivers/thermal/thermal_trip.c
> index 49e63db685172..b4e7411b2fe74 100644
> --- a/drivers/thermal/thermal_trip.c
> +++ b/drivers/thermal/thermal_trip.c
> @@ -152,7 +152,7 @@ void thermal_zone_set_trip_temp(struct thermal_zone_device *tz,
> if (trip->temperature == temp)
> return;
>
> - trip->temperature = temp;
> + WRITE_ONCE(trip->temperature, temp);
> thermal_notify_tz_trip_change(tz, trip);
>
> if (temp == THERMAL_TEMP_INVALID) {
> --
> 2.43.0
>