list.h: add list_cut_position()

This adds list_cut_position() which lets you cut a list into
two lists given a pivot in the list.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --git a/include/linux/list.h b/include/linux/list.h
index a886f27..1d109e2 100644 (file)
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -214,6 +214,46 @@ static inline int list_is_singular(const struct list_head *head)
        return !list_empty(head) && (head->next == head->prev);
 }
 
+static inline void __list_cut_position(struct list_head *list,
+               struct list_head *head, struct list_head *entry)
+{
+       struct list_head *new_first = entry->next;
+       list->next = head->next;
+       list->next->prev = list;
+       list->prev = entry;
+       entry->next = list;
+       head->next = new_first;
+       new_first->prev = head;
+}
+
+/**
+ * list_cut_position - cut a list into two
+ * @list: a new list to add all removed entries
+ * @head: a list with entries
+ * @entry: an entry within head, could be the head itself
+ *     and if so we won't cut the list
+ *
+ * This helper moves the initial part of @head, up to and
+ * including @entry, from @head to @list. You should
+ * pass on @entry an element you know is on @head. @list
+ * should be an empty list or a list you do not care about
+ * losing its data.
+ *
+ */
+static inline void list_cut_position(struct list_head *list,
+               struct list_head *head, struct list_head *entry)
+{
+       if (list_empty(head))
+               return;
+       if (list_is_singular(head) &&
+               (head->next != entry && head != entry))
+               return;
+       if (entry == head)
+               INIT_LIST_HEAD(list);
+       else
+               __list_cut_position(list, head, entry);
+}
+
 static inline void __list_splice(const struct list_head *list,
                                 struct list_head *prev,
                                 struct list_head *next)