+/**
+ * radix_tree_lookup_slot - lookup a slot in a radix tree
+ * @root: radix tree root
+ * @index: index key
+ *
+ * Returns: the slot corresponding to the position @index in the
+ * radix tree @root. This is useful for update-if-exists operations.
+ *
+ * This function cannot be called under rcu_read_lock, it must be
+ * excluded from writers, as must the returned slot for subsequent
+ * use by radix_tree_deref_slot() and radix_tree_replace slot.
+ * Caller must hold tree write locked across slot lookup and
+ * replace.
+ */
+void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index)
+{
+ unsigned int height, shift;
+ struct radix_tree_node *node, **slot;
+
+ node = root->rnode;
+ if (node == NULL)
+ return NULL;
+
+ if (radix_tree_is_direct_ptr(node)) {
+ if (index > 0)
+ return NULL;
+ return (void **)&root->rnode;
+ }
+
+ height = node->height;
+ if (index > radix_tree_maxindex(height))
+ return NULL;
+
+ shift = (height-1) * RADIX_TREE_MAP_SHIFT;
+
+ do {
+ slot = (struct radix_tree_node **)
+ (node->slots + ((index>>shift) & RADIX_TREE_MAP_MASK));
+ node = *slot;
+ if (node == NULL)
+ return NULL;
+
+ shift -= RADIX_TREE_MAP_SHIFT;
+ height--;
+ } while (height > 0);
+
+ return (void **)slot;
+}
+EXPORT_SYMBOL(radix_tree_lookup_slot);
+