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.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/llist.h b/include/linux/llist.h
index ca91875286bf91c5e330ca0e65b01f59fc7471f7..27bbdf5ddf82ced8a2f0e2ea33276123de74de8a 100644 (file)
--- a/include/linux/llist.h
+++ b/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;
 }
 
 /**

diff --git a/lib/llist.c b/lib/llist.c
index 6c69f1d14c4bf41f2d993d74812b920d19a64f54..878985c4d19d8eef42440dff4fbd0d439e8536a8 100644 (file)
--- a/lib/llist.c
+++ b/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);