[RFC PATCH 4/6] lib/plist: Provide plist_add_head() for nodes with the same prio

[Xunlei Pang <xlpang@126.com>]

From: Xunlei Pang <pang.xunlei@linaro.org>

If there're multiple nodes with the same prio as @node, currently
plist_add() will add @node behind all of them. Now we need to add
@node before all of these nodes for SMP RT scheduler.

This patch adds a common __plist_add() for adding @node before or
after existing nodes with the same prio, then adds plist_add_head()
and plist_add_tail() inline wrapper functions for convenient uses.

Finally, change plist_add() to an inline wrapper using plist_add_tail()
which has the same behaviour as before.

Reviewed-by: Dan Streetman <ddstreet@ieee.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
---
 include/linux/plist.h | 34 +++++++++++++++++++++++++++++++++-
 lib/plist.c           | 28 +++++++++++++++++++++++-----
 2 files changed, 56 insertions(+), 6 deletions(-)

diff --git a/include/linux/plist.h b/include/linux/plist.h
index 9788360..d060f28 100644
--- a/include/linux/plist.h
+++ b/include/linux/plist.h
@@ -138,7 +138,39 @@ static inline void plist_node_init(struct plist_node *node, int prio)
 	INIT_LIST_HEAD(&node->node_list);
 }
 
-extern void plist_add(struct plist_node *node, struct plist_head *head);
+extern void __plist_add(struct plist_node *node,
+				struct plist_head *head, bool is_head);
+
+/**
+ * plist_add_head - add @node to @head, before all existing same-prio nodes
+ *
+ * @node:	The plist_node to be added to @head
+ * @head:	The plist_head that @node is being added to
+ */
+static inline
+void plist_add_head(struct plist_node *node, struct plist_head *head)
+{
+	__plist_add(node, head, true);
+}
+
+/**
+ * plist_add_tail - add @node to @head, after all existing same-prio nodes
+ *
+ * @node:	The plist_node to be added to @head
+ * @head:	The plist_head that @node is being added to
+ */
+static inline
+void plist_add_tail(struct plist_node *node, struct plist_head *head)
+{
+	__plist_add(node, head, false);
+}
+
+static inline
+void plist_add(struct plist_node *node, struct plist_head *head)
+{
+	plist_add_tail(node, head);
+}
+
 extern void plist_del(struct plist_node *node, struct plist_head *head);
 
 extern void plist_requeue(struct plist_node *node, struct plist_head *head);
diff --git a/lib/plist.c b/lib/plist.c
index 3a30c53..c1ee2b0 100644
--- a/lib/plist.c
+++ b/lib/plist.c
@@ -66,12 +66,18 @@ static void plist_check_head(struct plist_head *head)
 #endif
 
 /**
- * plist_add - add @node to @head
+ * __plist_add - add @node to @head
  *
- * @node:	&struct plist_node pointer
- * @head:	&struct plist_head pointer
+ * @node:    The plist_node to be added to @head
+ * @head:    The plist_head that @node is being added to
+ * @is_head: True if adding to head of prio list, false otherwise
+ *
+ * For nodes of the same prio, @node will be added at the
+ * head of previously added nodes if @is_head is true, or
+ * it will be added at the tail of previously added nodes
+ * if @is_head is false.
  */
-void plist_add(struct plist_node *node, struct plist_head *head)
+void __plist_add(struct plist_node *node, struct plist_head *head, bool is_head)
 {
 	struct plist_node *first, *iter, *prev = NULL;
 	struct list_head *node_next = &head->node_list;
@@ -96,8 +102,20 @@ void plist_add(struct plist_node *node, struct plist_head *head)
 				struct plist_node, prio_list);
 	} while (iter != first);
 
-	if (!prev || prev->prio != node->prio)
+	if (!prev || prev->prio != node->prio) {
 		list_add_tail(&node->prio_list, &iter->prio_list);
+	} else if (is_head) {
+		/*
+		 * prev has the same priority as the node that is being
+		 * added. It is also the first node for this priority,
+		 * but the new node needs to be added ahead of it.
+		 * To accomplish this, replace prev in the prio_list
+		 * with node. Then set node_next to prev->node_list so
+		 * that the new node gets added before prev and not iter.
+		 */
+		list_replace_init(&prev->prio_list, &node->prio_list);
+		node_next = &prev->node_list;
+	}
 ins_node:
 	list_add_tail(&node->node_list, node_next);
 
-- 
1.9.1