Received-SPF: pass (google.com: domain of linux-kernel+bounces-192781-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Linux PM <linux-pm@vger.kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
 "Rafael J. Wysocki" <rafael@kernel.org>, Lukasz Luba <lukasz.luba@arm.com>,
 Daniel Lezcano <daniel.lezcano@linaro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
 Zhang Rui <rui.zhang@intel.com>
Subject:
 [PATCH v2 3/5] thermal: trip: Use READ_ONCE() for lockless access to trip
 properties
Date: Tue, 28 May 2024 18:52:13 +0200
Message-ID: <1902136.tdWV9SEqCh@kreacher>
In-Reply-To: <12458899.O9o76ZdvQC@kreacher>
References: <12458899.O9o76ZdvQC@kreacher>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 7Bit
Content-Type: text/plain; charset="UTF-8"

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

v1 -> v2: Rebase.

---
 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
@@ -139,7 +139,7 @@ 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
@@ -163,7 +163,7 @@ 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);
 	}
@@ -183,7 +183,7 @@ 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);
 
 	if (temp == THERMAL_TEMP_INVALID) {