llist: Return whether list is empty before adding in llist_add()

Extend the llist_add*() functions to return a success indicator, this
allows us in the scheduler code to send an IPI if the queue was empty.

( There's no effect on existing users, because the list_add_xxx() functions
  are inline, thus this will be optimized out by the compiler if not used
  by callers. )
Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1315461646-1379-5-git-send-email-ying.huang@intel.comSigned-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/llist.h

@@ -142,8 +142,10 @@ static inline bool llist_empty(const struct llist_head *head)
  * llist_add - add a new entry
  * @new:	new entry to be added
  * @head:	the head for your lock-less list
+ *
+ * Return whether list is empty before adding.
  */
-static inline void llist_add(struct llist_node *new, struct llist_head *head)
+static inline bool llist_add(struct llist_node *new, struct llist_head *head)
 {
 	struct llist_node *entry, *old_entry;
 
@@ -156,6 +158,8 @@ static inline void llist_add(struct llist_node *new, struct llist_head *head)
 			break;
 		cpu_relax();
 	}
+
+	return old_entry == NULL;
 }
 
 /**

lib/llist.c

@@ -34,8 +34,10 @@
  * @new_first:	first entry in batch to be added
  * @new_last:	last entry in batch to be added
  * @head:	the head for your lock-less list
+ *
+ * Return whether list is empty before adding.
  */
-void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
+bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 		     struct llist_head *head)
 {
 	struct llist_node *entry, *old_entry;
@@ -49,6 +51,8 @@ void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 			break;
 		cpu_relax();
 	}
+
+	return old_entry == NULL;
 }
 EXPORT_SYMBOL_GPL(llist_add_batch);