rbtree: provide rb_find_rcu() / rb_find_add_rcu()

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@infradead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240903174603.3554182-7-andrii@kernel.org

diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
index f7edca369edaddb6adf4b7ba8a9ad745fea52356..7c173aa64e1e3f26a38f94ffa5c5031d9aa99c53 100644 (file)
--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -244,6 +244,42 @@ rb_find_add(struct rb_node *node, struct rb_root *tree,
        return NULL;
 }
 
+/**
+ * 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
@@ -272,6 +308,37 @@ rb_find(const void *key, const struct rb_root *tree,
        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;
+}
+
 /**
  * rb_find_first() - find the first @key in @tree
  * @key: key to match