[hrtimer] Remove listhead from hrtimer struct

The list_head in the hrtimer structure was introduced for easy access
to the first timer with the further extensions of real high resolution
timers in mind, but it turned out in the course of development that
it is not necessary for the standard use case. Remove the list head
and access the first expiry timer by a datafield in the timer base.

Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/hrtimer.h

···
  * struct hrtimer - the basic hrtimer structure
  *
  * @node:	red black tree node for time ordered insertion
- * @list:	list head for easier access to the time ordered list,
- *		without walking the red black tree.
  * @expires:	the absolute expiry time in the hrtimers internal
  *		representation. The time is related to the clock on
  *		which the timer is based.
···
  */
 struct hrtimer {
 	struct rb_node		node;
-	struct list_head	list;
 	ktime_t			expires;
 	enum hrtimer_state	state;
 	int			(*function)(void *);
···
  *		to a base on another cpu.
  * @lock:	lock protecting the base and associated timers
  * @active:	red black tree root node for the active timers
- * @pending:	list of pending timers for simple time ordered access
+ * @first:	pointer to the timer node which expires first
  * @resolution:	the resolution of the clock, in nanoseconds
  * @get_time:	function to retrieve the current time of the clock
  * @curr_timer:	the timer which is executing a callback right now
···
 	clockid_t		index;
 	spinlock_t		lock;
 	struct rb_root		active;
-	struct list_head	pending;
+	struct rb_node		*first;
 	unsigned long		resolution;
 	ktime_t			(*get_time)(void);
 	struct hrtimer		*curr_timer;

kernel/hrtimer.c

···
 static void enqueue_hrtimer(struct hrtimer *timer, struct hrtimer_base *base)
 {
 	struct rb_node **link = &base->active.rb_node;
-	struct list_head *prev = &base->pending;
 	struct rb_node *parent = NULL;
 	struct hrtimer *entry;
 
···
 		 */
 		if (timer->expires.tv64 < entry->expires.tv64)
 			link = &(*link)->rb_left;
-		else {
+		else
 			link = &(*link)->rb_right;
-			prev = &entry->list;
-		}
 	}
 
 	/*
-	 * Insert the timer to the rbtree and to the sorted list:
+	 * Insert the timer to the rbtree and check whether it
+	 * replaces the first pending timer
 	 */
 	rb_link_node(&timer->node, parent, link);
 	rb_insert_color(&timer->node, &base->active);
-	list_add(&timer->list, prev);
 
 	timer->state = HRTIMER_PENDING;
-}
 
+	if (!base->first || timer->expires.tv64 <
+	    rb_entry(base->first, struct hrtimer, node)->expires.tv64)
+		base->first = &timer->node;
+}
 
 /*
  * __remove_hrtimer - internal function to remove a timer
···
 static void __remove_hrtimer(struct hrtimer *timer, struct hrtimer_base *base)
 {
 	/*
-	 * Remove the timer from the sorted list and from the rbtree:
+	 * Remove the timer from the rbtree and replace the
+	 * first entry pointer if necessary.
 	 */
-	list_del(&timer->list);
+	if (base->first == &timer->node)
+		base->first = rb_next(&timer->node);
 	rb_erase(&timer->node, &base->active);
 }
 
···
 static inline void run_hrtimer_queue(struct hrtimer_base *base)
 {
 	ktime_t now = base->get_time();
+	struct rb_node *node;
 
 	spin_lock_irq(&base->lock);
 
-	while (!list_empty(&base->pending)) {
+	while ((node = base->first)) {
 		struct hrtimer *timer;
 		int (*fn)(void *);
 		int restart;
 		void *data;
 
-		timer = list_entry(base->pending.next, struct hrtimer, list);
+		timer = rb_entry(node, struct hrtimer, node);
 		if (now.tv64 <= timer->expires.tv64)
 			break;
 
···
 
 	for (i = 0; i < MAX_HRTIMER_BASES; i++) {
 		spin_lock_init(&base->lock);
-		INIT_LIST_HEAD(&base->pending);
 		base++;
 	}
 }