From 74ed07944ad082dc54d5bcead9faae35ee83099f Mon Sep 17 00:00:00 2001 From: Michel Lespinasse Date: Wed, 20 Dec 2017 18:02:31 +0100 Subject: [PATCH] rbtree: adjust root color in rb_insert_color() only when necessary The root node of an rbtree must always be black. However, rb_insert_color() only needs to maintain this invariant when it has been broken - that is, when it exits the loop due to the current (red) node being the root. In all other cases (exiting after tree rotations, or exiting due to an existing black parent) the invariant is already satisfied, so there is no need to adjust the root node color. Signed-off-by: Michel Lespinasse Acked-by: David Woodhouse Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [Linux commit 6d58452dc066db61acdff7b84671db1b11a3de1c] Ported to Xen. Signed-off-by: Praveen Kumar Acked-by: Jan Beulich --- xen/common/rbtree.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c index 9dc296e0d8..244f1d8818 100644 --- a/xen/common/rbtree.c +++ b/xen/common/rbtree.c @@ -91,8 +91,21 @@ void rb_insert_color(struct rb_node *node, struct rb_root *root) { struct rb_node *parent, *gparent; - while ((parent = rb_parent(node)) && rb_is_red(parent)) - { + while (true) { + /* + * Loop invariant: node is red + * + * If there is a black parent, we are done. + * Otherwise, take some corrective action as we don't + * want a red root or two consecutive red nodes. + */ + parent = rb_parent(node); + if (!parent) { + rb_set_black(node); + break; + } else if (rb_is_black(parent)) + break; + gparent = rb_parent(parent); if (parent == gparent->rb_left) @@ -142,8 +155,6 @@ void rb_insert_color(struct rb_node *node, struct rb_root *root) break; } } - - rb_set_black(root->rb_node); } EXPORT_SYMBOL(rb_insert_color); -- 2.39.5