leds: Fix set_brightness_delayed() race

When a trigger wants to switch from blinking to LED on it needs to call:
	led_set_brightness(LED_OFF);
	led_set_brightness(LED_FULL);

To first call disables blinking and the second then turns the LED on
(the power-supply charging-blink-full-solid triggers do this).

These calls happen immediately after each other, so it is possible
that set_brightness_delayed() from the first call has not run yet
when the led_set_brightness(LED_FULL) call finishes.

If this race hits then this is causing problems for both
sw- and hw-blinking:

For sw-blinking set_brightness_delayed() clears delayed_set_value
when LED_BLINK_DISABLE is set causing the led_set_brightness(LED_FULL)
call effects to get lost when hitting the race, resulting in the LED
turning off instead of on.

For hw-blinking if the race hits delayed_set_value has been
set to LED_FULL by the time set_brightness_delayed() runs.
So led_cdev->brightness_set_blocking() is never called with
LED_OFF as argument and the hw-blinking is never disabled leaving
the LED blinking instead of on.

Fix both issues by adding LED_SET_BRIGHTNESS and LED_SET_BRIGHTNESS_OFF
work_flags making this 2 separate actions to be run by
set_brightness_delayed().
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Tested-by: Yauhen Kharuzhy <jekhor@gmail.com>
Link: https://lore.kernel.org/r/20230510162234.291439-3-hdegoede@redhat.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/leds/led-core.c

@@ -114,21 +114,14 @@ static void led_timer_function(struct timer_list *t)
 	mod_timer(&led_cdev->blink_timer, jiffies + msecs_to_jiffies(delay));
 }
 
-static void set_brightness_delayed(struct work_struct *ws)
+static void set_brightness_delayed_set_brightness(struct led_classdev *led_cdev,
+						  unsigned int value)
 {
-	struct led_classdev *led_cdev =
-		container_of(ws, struct led_classdev, set_brightness_work);
 	int ret = 0;
 
-	if (test_and_clear_bit(LED_BLINK_DISABLE, &led_cdev->work_flags)) {
-		led_cdev->delayed_set_value = LED_OFF;
-		led_stop_software_blink(led_cdev);
-	}
-
-	ret = __led_set_brightness(led_cdev, led_cdev->delayed_set_value);
+	ret = __led_set_brightness(led_cdev, value);
 	if (ret == -ENOTSUPP)
-		ret = __led_set_brightness_blocking(led_cdev,
-					led_cdev->delayed_set_value);
+		ret = __led_set_brightness_blocking(led_cdev, value);
 	if (ret < 0 &&
 	    /* LED HW might have been unplugged, therefore don't warn */
 	    !(ret == -ENODEV && (led_cdev->flags & LED_UNREGISTERING) &&
@@ -137,6 +130,30 @@ static void set_brightness_delayed(struct work_struct *ws)
 			"Setting an LED's brightness failed (%d)\n", ret);
 }
 
+static void set_brightness_delayed(struct work_struct *ws)
+{
+	struct led_classdev *led_cdev =
+		container_of(ws, struct led_classdev, set_brightness_work);
+
+	if (test_and_clear_bit(LED_BLINK_DISABLE, &led_cdev->work_flags)) {
+		led_stop_software_blink(led_cdev);
+		set_bit(LED_SET_BRIGHTNESS_OFF, &led_cdev->work_flags);
+	}
+
+	/*
+	 * Triggers may call led_set_brightness(LED_OFF),
+	 * led_set_brightness(LED_FULL) in quick succession to disable blinking
+	 * and turn the LED on. Both actions may have been scheduled to run
+	 * before this work item runs once. To make sure this works properly
+	 * handle LED_SET_BRIGHTNESS_OFF first.
+	 */
+	if (test_and_clear_bit(LED_SET_BRIGHTNESS_OFF, &led_cdev->work_flags))
+		set_brightness_delayed_set_brightness(led_cdev, LED_OFF);
+
+	if (test_and_clear_bit(LED_SET_BRIGHTNESS, &led_cdev->work_flags))
+		set_brightness_delayed_set_brightness(led_cdev, led_cdev->delayed_set_value);
+}
+
 static void led_set_software_blink(struct led_classdev *led_cdev,
 				   unsigned long delay_on,
 				   unsigned long delay_off)
@@ -271,8 +288,22 @@ void led_set_brightness_nopm(struct led_classdev *led_cdev, unsigned int value)
 	if (!__led_set_brightness(led_cdev, value))
 		return;
 
-	/* If brightness setting can sleep, delegate it to a work queue task */
+	/*
+	 * Brightness setting can sleep, delegate it to a work queue task.
+	 * value 0 / LED_OFF is special, since it also disables hw-blinking
+	 * (sw-blink disable is handled in led_set_brightness()).
+	 * To avoid a hw-blink-disable getting lost when a second brightness
+	 * change is done immediately afterwards (before the work runs),
+	 * it uses a separate work_flag.
+	 */
+	if (value) {
 		led_cdev->delayed_set_value = value;
+		set_bit(LED_SET_BRIGHTNESS, &led_cdev->work_flags);
+	} else {
+		clear_bit(LED_SET_BRIGHTNESS, &led_cdev->work_flags);
+		set_bit(LED_SET_BRIGHTNESS_OFF, &led_cdev->work_flags);
+	}
+
 	schedule_work(&led_cdev->set_brightness_work);
 }
 EXPORT_SYMBOL_GPL(led_set_brightness_nopm);

include/linux/leds.h

@@ -124,6 +124,9 @@ struct led_classdev {
 #define LED_BLINK_INVERT		3
 #define LED_BLINK_BRIGHTNESS_CHANGE 	4
 #define LED_BLINK_DISABLE		5
+	/* Brightness off also disables hw-blinking so it is a separate action */
+#define LED_SET_BRIGHTNESS_OFF		6
+#define LED_SET_BRIGHTNESS		7
 
 	/* Set LED brightness level
 	 * Must not sleep. Use brightness_set_blocking for drivers