leds: trigger: use RCU to protect the led_cdevs list

Even with the previous commit 27af8e2c90fb
("leds: trigger: fix potential deadlock with libata")
to this file, we still get lockdep unhappy, and Boqun
explained the report here:
https://lore.kernel.org/r/YNA+d1X4UkoQ7g8a@boqun-archlinux

Effectively, this means that the read_lock_irqsave() isn't
enough here because another CPU might be trying to do a
write lock, and thus block the readers.

This is all pretty messy, but it doesn't seem right that
the LEDs framework imposes some locking requirements on
users, in particular we'd have to make the spinlock in the
iwlwifi driver always disable IRQs, even if we don't need
that for any other reason, just to avoid this deadlock.

Since writes to the led_cdevs list are rare (and are done
by userspace), just switch the list to RCU. This costs a
synchronize_rcu() at removal time so we can ensure things
are correct, but that seems like a small price to pay for
getting lock-free iterations and no deadlocks (nor any
locking requirements imposed on users.)

Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: Pavel Machek <[e-mail]>

drivers/leds/led-triggers.c

···
 /* Caller must ensure led_cdev->trigger_lock held */
 int led_trigger_set(struct led_classdev *led_cdev, struct led_trigger *trig)
 {
-	unsigned long flags;
 	char *event = NULL;
 	char *envp[2];
 	const char *name;
···
 
 	/* Remove any existing trigger */
 	if (led_cdev->trigger) {
-		write_lock_irqsave(&led_cdev->trigger->leddev_list_lock, flags);
-		list_del(&led_cdev->trig_list);
-		write_unlock_irqrestore(&led_cdev->trigger->leddev_list_lock,
-			flags);
+		spin_lock(&led_cdev->trigger->leddev_list_lock);
+		list_del_rcu(&led_cdev->trig_list);
+		spin_unlock(&led_cdev->trigger->leddev_list_lock);
+
+		/* ensure it's no longer visible on the led_cdevs list */
+		synchronize_rcu();
+
 		cancel_work_sync(&led_cdev->set_brightness_work);
 		led_stop_software_blink(led_cdev);
 		if (led_cdev->trigger->deactivate)
···
 		led_set_brightness(led_cdev, LED_OFF);
 	}
 	if (trig) {
-		write_lock_irqsave(&trig->leddev_list_lock, flags);
-		list_add_tail(&led_cdev->trig_list, &trig->led_cdevs);
-		write_unlock_irqrestore(&trig->leddev_list_lock, flags);
+		spin_lock(&trig->leddev_list_lock);
+		list_add_tail_rcu(&led_cdev->trig_list, &trig->led_cdevs);
+		spin_unlock(&trig->leddev_list_lock);
 		led_cdev->trigger = trig;
 
 		if (trig->activate)
···
 		trig->deactivate(led_cdev);
 err_activate:
 
-	write_lock_irqsave(&led_cdev->trigger->leddev_list_lock, flags);
-	list_del(&led_cdev->trig_list);
-	write_unlock_irqrestore(&led_cdev->trigger->leddev_list_lock, flags);
+	spin_lock(&led_cdev->trigger->leddev_list_lock);
+	list_del_rcu(&led_cdev->trig_list);
+	spin_unlock(&led_cdev->trigger->leddev_list_lock);
+	synchronize_rcu();
 	led_cdev->trigger = NULL;
 	led_cdev->trigger_data = NULL;
 	led_set_brightness(led_cdev, LED_OFF);
···
 	struct led_classdev *led_cdev;
 	struct led_trigger *_trig;
 
-	rwlock_init(&trig->leddev_list_lock);
+	spin_lock_init(&trig->leddev_list_lock);
 	INIT_LIST_HEAD(&trig->led_cdevs);
 
 	down_write(&triggers_list_lock);
···
 			enum led_brightness brightness)
 {
 	struct led_classdev *led_cdev;
-	unsigned long flags;
 
 	if (!trig)
 		return;
 
-	read_lock_irqsave(&trig->leddev_list_lock, flags);
-	list_for_each_entry(led_cdev, &trig->led_cdevs, trig_list)
+	rcu_read_lock();
+	list_for_each_entry_rcu(led_cdev, &trig->led_cdevs, trig_list)
 		led_set_brightness(led_cdev, brightness);
-	read_unlock_irqrestore(&trig->leddev_list_lock, flags);
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(led_trigger_event);
 
···
 			     int invert)
 {
 	struct led_classdev *led_cdev;
-	unsigned long flags;
 
 	if (!trig)
 		return;
 
-	read_lock_irqsave(&trig->leddev_list_lock, flags);
-	list_for_each_entry(led_cdev, &trig->led_cdevs, trig_list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(led_cdev, &trig->led_cdevs, trig_list) {
 		if (oneshot)
 			led_blink_set_oneshot(led_cdev, delay_on, delay_off,
 					      invert);
 		else
 			led_blink_set(led_cdev, delay_on, delay_off);
 	}
-	read_unlock_irqrestore(&trig->leddev_list_lock, flags);
+	rcu_read_unlock();
 }
 
 void led_trigger_blink(struct led_trigger *trig,

include/linux/leds.h

···
 	struct led_hw_trigger_type *trigger_type;
 
 	/* LEDs under control by this trigger (for simple triggers) */
-	rwlock_t	  leddev_list_lock;
+	spinlock_t	  leddev_list_lock;
 	struct list_head  led_cdevs;
 
 	/* Link to next registered trigger */