[PATCH v2] llist: add llist_[add|del_first]_exclusive

[Vitaly Wool]

It sometimes is necessary to be able to be able to use llist in
the following manner:
>	if (node_unlisted(node))
>		llst_add(node, list);
i. e. only add a node to the list if it's not already on a list.

This is not possible without taking locks because otherwise there's
an obvious race between the check and the addition. This patch adds
the routine to add a node only if it is not on any list that is
lockless and race free as long as there's only one consumer.

That is, llist_add_exclusive would only add a node if it's not
already on a list, and llist_del_first_exclusive will delete the
first node off the list and mark it as not being on any list.

Signed-off-by: Vitaly Vul <vitaly.vul@xxxxxxxx>

---
 include/linux/llist.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/include/linux/llist.h b/include/linux/llist.h
index fd4ca0b..7aaab25 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -69,6 +69,15 @@ struct llist_node {
 #define LLIST_HEAD_INIT(name)	{ NULL }
 #define LLIST_HEAD(name)	struct llist_head name = LLIST_HEAD_INIT(name)
 
+#define LLIST_NODE_UNLISTED	((void *)(-1L))
+#define LLIST_NODE_INIT(name)	{ LLIST_NODE_UNLISTED }
+#define LLIST_NODE(name)	struct llist_node name = LLIST_NODE_INIT(name)
+
+static inline void INIT_LLIST_NODE(struct llist_node *node)
+{
+	WRITE_ONCE(node->next, LLIST_NODE_UNLISTED);
+}
+
 /**
  * init_llist_head - initialize lock-less list head
  * @head:	the head for your lock-less list
@@ -197,4 +206,42 @@ extern struct llist_node *llist_del_first(struct llist_head *head);
 
 struct llist_node *llist_reverse_order(struct llist_node *head);
 
+/**
+ * llist_del_first_exclusive - delete the first entry of lock-less list
+ * 			       and make sure it's marked as UNLISTED
+ * @head:	the head for your lock-less list
+ *
+ * If list is empty, return NULL, otherwise, return the first entry
+ * deleted, this is the newest added one.
+ *
+ */
+static inline struct llist_node *llist_del_first_exclusive(
+				struct llist_head *head)
+{
+	struct llist_node *node = llist_del_first(head);
+
+	if (READ_ONCE(node))
+		smp_store_release(&node->next, LLIST_NODE_UNLISTED);
+	return node;
+}
+
+
+/**
+ * llist_add_exclusive - add a node only if it's not on any list
+			 (i. e. marked as UNLISTED)
+ * @node:	the node to be added
+ * @head:	the head for your lock-less list
+ *
+ * Return true if the node was added, or false otherwise.
+ */
+static inline bool llist_add_exclusive(struct llist_node *node,
+					struct llist_head *head)
+{
+	if (cmpxchg(&node->next, LLIST_NODE_UNLISTED, NULL) !=
+				LLIST_NODE_UNLISTED)
+		return false;
+
+	llist_add(node, head);
+	return true;
+}
 #endif /* LLIST_H */
-- 
2.7.4