[PATCH] led: core: Fix race on software blink cancellation

[Jacek Anaszewski]

Commit d23a22a74fde ("leds: delay led_set_brightness if stopping soft-blink")
made a modifications to the LED core allowing for led_set_brightness() to be
called from hard-irq context when soft blink is being handled in soft-irq.

Since that time LED core has undergone modifications related to addition of
generic support for delegating brightness setting to a workqueue as well as
subsequent fixes for blink setting use cases.

After that the LED core code became hard to maintain and analyze, especially
due to the imposed hard-irq context compatibility. It also turned out that
in some cases a LED remained off after executing following sequence of commands:

1. echo timer > trigger
2. echo 0 > brightness
3. echo 100 > brightness

The reason was LED_BLINK_DISABLE operation delegated to a set_brightness_work,
triggered in 2., which was handled after 3. took effect.

In order to serialize above operations and in the same time avoid
code overcomplication the hard-irq context compatibility is being removed,
which allows to use spin_lock_bh() for LED blink setting serialization.

>From now, if in hard-irq context, users need to delegate led_set_brightness()
to a workqueue in the place of use.

Reported-by: Craig McQueen <craig.mcqueen@xxxxxxxxxxxxxxxxx>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>
---
Craig,

It would be great if you could confirm if this fixes your use case.

 drivers/leds/led-core.c                  | 79 +++++++++++++++++++-------------
 drivers/leds/trigger/ledtrig-activity.c  |  3 --
 drivers/leds/trigger/ledtrig-heartbeat.c |  3 --
 include/linux/leds.h                     |  5 +-
 4 files changed, 49 insertions(+), 41 deletions(-)

diff --git a/drivers/leds/led-core.c b/drivers/leds/led-core.c
index ede4fa0..25d711b 100644
--- a/drivers/leds/led-core.c
+++ b/drivers/leds/led-core.c
@@ -22,6 +22,9 @@
 DECLARE_RWSEM(leds_list_lock);
 EXPORT_SYMBOL_GPL(leds_list_lock);
 
+DEFINE_SPINLOCK(leds_soft_blink_lock);
+EXPORT_SYMBOL_GPL(leds_soft_blink_lock);
+
 LIST_HEAD(leds_list);
 EXPORT_SYMBOL_GPL(leds_list);
 
@@ -51,26 +54,31 @@ static void led_timer_function(struct timer_list *t)
 	unsigned long brightness;
 	unsigned long delay;
 
+	spin_lock_bh(&leds_soft_blink_lock);
+
+	/*
+	 * Check if soft blinking wasn't disabled via led_set_brightness()
+	 * in the meantime.
+	 */
+	if (!test_bit(LED_BLINK_SW, &led_cdev->work_flags))
+		goto unlock;
+
 	if (!led_cdev->blink_delay_on || !led_cdev->blink_delay_off) {
 		led_set_brightness_nosleep(led_cdev, LED_OFF);
 		clear_bit(LED_BLINK_SW, &led_cdev->work_flags);
-		return;
+		goto unlock;
 	}
 
 	if (test_and_clear_bit(LED_BLINK_ONESHOT_STOP,
 			       &led_cdev->work_flags)) {
 		clear_bit(LED_BLINK_SW, &led_cdev->work_flags);
-		return;
+		goto unlock;
 	}
 
 	brightness = led_get_brightness(led_cdev);
 	if (!brightness) {
 		/* Time to switch the LED on. */
-		if (test_and_clear_bit(LED_BLINK_BRIGHTNESS_CHANGE,
-					&led_cdev->work_flags))
-			brightness = led_cdev->new_blink_brightness;
-		else
-			brightness = led_cdev->blink_brightness;
+		brightness = led_cdev->blink_brightness;
 		delay = led_cdev->blink_delay_on;
 	} else {
 		/* Store the current brightness value to be able
@@ -100,6 +108,9 @@ static void led_timer_function(struct timer_list *t)
 	}
 
 	mod_timer(&led_cdev->blink_timer, jiffies + msecs_to_jiffies(delay));
+
+unlock:
+	spin_unlock_bh(&leds_soft_blink_lock);
 }
 
 static void set_brightness_delayed(struct work_struct *ws)
@@ -108,11 +119,6 @@ static void set_brightness_delayed(struct work_struct *ws)
 		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);
 	if (ret == -ENOTSUPP)
 		ret = __led_set_brightness_blocking(led_cdev,
@@ -131,6 +137,8 @@ static void led_set_software_blink(struct led_classdev *led_cdev,
 {
 	int current_brightness;
 
+	spin_lock_bh(&leds_soft_blink_lock);
+
 	current_brightness = led_get_brightness(led_cdev);
 	if (current_brightness)
 		led_cdev->blink_brightness = current_brightness;
@@ -143,18 +151,21 @@ static void led_set_software_blink(struct led_classdev *led_cdev,
 	/* never on - just set to off */
 	if (!delay_on) {
 		led_set_brightness_nosleep(led_cdev, LED_OFF);
-		return;
+		goto unlock;
 	}
 
 	/* never off - just set to brightness */
 	if (!delay_off) {
 		led_set_brightness_nosleep(led_cdev,
 					   led_cdev->blink_brightness);
-		return;
+		goto unlock;
 	}
 
 	set_bit(LED_BLINK_SW, &led_cdev->work_flags);
 	mod_timer(&led_cdev->blink_timer, jiffies + 1);
+
+unlock:
+	spin_unlock_bh(&leds_soft_blink_lock);
 }
 
 
@@ -217,40 +228,46 @@ void led_blink_set_oneshot(struct led_classdev *led_cdev,
 }
 EXPORT_SYMBOL_GPL(led_blink_set_oneshot);
 
-void led_stop_software_blink(struct led_classdev *led_cdev)
+void led_stop_software_blink_nolock(struct led_classdev *led_cdev)
 {
-	del_timer_sync(&led_cdev->blink_timer);
 	led_cdev->blink_delay_on = 0;
 	led_cdev->blink_delay_off = 0;
 	clear_bit(LED_BLINK_SW, &led_cdev->work_flags);
 }
+
+void led_stop_software_blink(struct led_classdev *led_cdev)
+{
+	spin_lock_bh(&leds_soft_blink_lock);
+	led_stop_software_blink_nolock(led_cdev);
+	spin_unlock_bh(&leds_soft_blink_lock);
+}
 EXPORT_SYMBOL_GPL(led_stop_software_blink);
 
 void led_set_brightness(struct led_classdev *led_cdev,
 			enum led_brightness brightness)
 {
-	/*
-	 * If software blink is active, delay brightness setting
-	 * until the next timer tick.
-	 */
+	if (in_irq()) {
+		dev_err(led_cdev->dev,
+			"Cannot set LED brightness from hard irq context!");
+		WARN_ON(1);
+		return;
+	}
+
+	spin_lock_bh(&leds_soft_blink_lock);
+
 	if (test_bit(LED_BLINK_SW, &led_cdev->work_flags)) {
-		/*
-		 * If we need to disable soft blinking delegate this to the
-		 * work queue task to avoid problems in case we are called
-		 * from hard irq context.
-		 */
 		if (brightness == LED_OFF) {
-			set_bit(LED_BLINK_DISABLE, &led_cdev->work_flags);
-			schedule_work(&led_cdev->set_brightness_work);
+			led_stop_software_blink_nolock(led_cdev);
 		} else {
-			set_bit(LED_BLINK_BRIGHTNESS_CHANGE,
-				&led_cdev->work_flags);
-			led_cdev->new_blink_brightness = brightness;
+			led_cdev->blink_brightness = brightness;
+			goto unlock;
 		}
-		return;
 	}
 
 	led_set_brightness_nosleep(led_cdev, brightness);
+
+unlock:
+	spin_unlock_bh(&leds_soft_blink_lock);
 }
 EXPORT_SYMBOL_GPL(led_set_brightness);
 
diff --git a/drivers/leds/trigger/ledtrig-activity.c b/drivers/leds/trigger/ledtrig-activity.c
index 5081894..6f62da7 100644
--- a/drivers/leds/trigger/ledtrig-activity.c
+++ b/drivers/leds/trigger/ledtrig-activity.c
@@ -48,9 +48,6 @@ static void led_activity_function(struct timer_list *t)
 	int cpus;
 	int i;
 
-	if (test_and_clear_bit(LED_BLINK_BRIGHTNESS_CHANGE, &led_cdev->work_flags))
-		led_cdev->blink_brightness = led_cdev->new_blink_brightness;
-
 	if (unlikely(panic_detected)) {
 		/* full brightness in case of panic */
 		led_set_brightness_nosleep(led_cdev, led_cdev->blink_brightness);
diff --git a/drivers/leds/trigger/ledtrig-heartbeat.c b/drivers/leds/trigger/ledtrig-heartbeat.c
index f0896de..1453352 100644
--- a/drivers/leds/trigger/ledtrig-heartbeat.c
+++ b/drivers/leds/trigger/ledtrig-heartbeat.c
@@ -47,9 +47,6 @@ static void led_heartbeat_function(struct timer_list *t)
 		return;
 	}
 
-	if (test_and_clear_bit(LED_BLINK_BRIGHTNESS_CHANGE, &led_cdev->work_flags))
-		led_cdev->blink_brightness = led_cdev->new_blink_brightness;
-
 	/* acts like an actual heart beat -- ie thump-thump-pause... */
 	switch (heartbeat_data->phase) {
 	case 0:
diff --git a/include/linux/leds.h b/include/linux/leds.h
index 5579c64..e520503 100644
--- a/include/linux/leds.h
+++ b/include/linux/leds.h
@@ -51,15 +51,13 @@ struct led_classdev {
 #define LED_BRIGHT_HW_CHANGED	BIT(21)
 #define LED_RETAIN_AT_SHUTDOWN	BIT(22)
 
-	/* set_brightness_work / blink_timer flags, atomic, private. */
+	/* blink_timer flags, atomic, private. */
 	unsigned long		work_flags;
 
 #define LED_BLINK_SW			0
 #define LED_BLINK_ONESHOT		1
 #define LED_BLINK_ONESHOT_STOP		2
 #define LED_BLINK_INVERT		3
-#define LED_BLINK_BRIGHTNESS_CHANGE 	4
-#define LED_BLINK_DISABLE		5
 
 	/* Set LED brightness level
 	 * Must not sleep. Use brightness_set_blocking for drivers
@@ -97,7 +95,6 @@ struct led_classdev {
 	unsigned long		 blink_delay_on, blink_delay_off;
 	struct timer_list	 blink_timer;
 	int			 blink_brightness;
-	int			 new_blink_brightness;
 	void			(*flash_resume)(struct led_classdev *led_cdev);
 
 	struct work_struct	set_brightness_work;
-- 
2.1.4