linux-kernel - [PATCH v1 4/6] thermal: trip: Use READ_ONCE() for lockless access to trip properties

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Date: Fri, 10 May 2024 16:18:19 +0200
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Lukasz Luba <lukasz.luba@....com>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Zhang Rui <rui.zhang@...el.com>
Subject:
 [PATCH v1 4/6] thermal: trip: Use READ_ONCE() for lockless access to trip
 properties

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

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@...el.com>
---
 drivers/thermal/thermal_sysfs.c |    6 +++---
 drivers/thermal/thermal_trip.c  |    2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

Index: linux-pm/drivers/thermal/thermal_sysfs.c
===================================================================
--- linux-pm.orig/drivers/thermal/thermal_sysfs.c
+++ linux-pm/drivers/thermal/thermal_sysfs.c
@@ -141,7 +141,7 @@ trip_point_temp_show(struct device *dev,
 	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
@@ -165,7 +165,7 @@ trip_point_hyst_store(struct device *dev
 	trip = &tz->trips[trip_id].trip;

 	if (hyst != trip->hysteresis) {
-		trip->hysteresis = hyst;
+		WRITE_ONCE(trip->hysteresis, hyst);

 		thermal_zone_trip_updated(tz, trip);
 	}
@@ -185,7 +185,7 @@ trip_point_hyst_show(struct device *dev,
 	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
Index: linux-pm/drivers/thermal/thermal_trip.c
===================================================================
--- linux-pm.orig/drivers/thermal/thermal_trip.c
+++ linux-pm/drivers/thermal/thermal_trip.c
@@ -161,7 +161,7 @@ void thermal_zone_set_trip_temp(struct t
 	if (trip->temperature == temp)
 		return;

-	trip->temperature = temp;
+	WRITE_ONCE(trip->temperature, temp);
 	thermal_notify_tz_trip_change(tz, trip);
 }
 EXPORT_SYMBOL_GPL(thermal_zone_set_trip_temp);