[PATCH 03/10] rbtree: Provide rb_find_rcu() / rb_find_add_rcu()

[Peter Zijlstra]

Much like latch_tree, add two RCU methods for the regular RB-tree,
which can be used in conjunction with a seqcount to provide lockless
lookups.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/rbtree.h |   67 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -245,6 +245,42 @@ rb_find_add(struct rb_node *node, struct
 }
 
 /**
+ * rb_find_add_rcu() - find equivalent @node in @tree, or add @node
+ * @node: node to look-for / insert
+ * @tree: tree to search / modify
+ * @cmp: operator defining the node order
+ *
+ * Adds a Store-Release for link_node.
+ *
+ * Returns the rb_node matching @node, or NULL when no match is found and @node
+ * is inserted.
+ */
+static __always_inline struct rb_node *
+rb_find_add_rcu(struct rb_node *node, struct rb_root *tree,
+		int (*cmp)(struct rb_node *, const struct rb_node *))
+{
+	struct rb_node **link = &tree->rb_node;
+	struct rb_node *parent = NULL;
+	int c;
+
+	while (*link) {
+		parent = *link;
+		c = cmp(node, parent);
+
+		if (c < 0)
+			link = &parent->rb_left;
+		else if (c > 0)
+			link = &parent->rb_right;
+		else
+			return parent;
+	}
+
+	rb_link_node_rcu(node, parent, link);
+	rb_insert_color(node, tree);
+	return NULL;
+}
+
+/**
  * rb_find() - find @key in tree @tree
  * @key: key to match
  * @tree: tree to search
@@ -268,6 +304,37 @@ rb_find(const void *key, const struct rb
 		else
 			return node;
 	}
+
+	return NULL;
+}
+
+/**
+ * rb_find_rcu() - find @key in tree @tree
+ * @key: key to match
+ * @tree: tree to search
+ * @cmp: operator defining the node order
+ *
+ * Notably, tree descent vs concurrent tree rotations is unsound and can result
+ * in false-negatives.
+ *
+ * Returns the rb_node matching @key or NULL.
+ */
+static __always_inline struct rb_node *
+rb_find_rcu(const void *key, const struct rb_root *tree,
+	    int (*cmp)(const void *key, const struct rb_node *))
+{
+	struct rb_node *node = tree->rb_node;
+
+	while (node) {
+		int c = cmp(key, node);
+
+		if (c < 0)
+			node = rcu_dereference_raw(node->rb_left);
+		else if (c > 0)
+			node = rcu_dereference_raw(node->rb_right);
+		else
+			return node;
+	}
 
 	return NULL;
 }