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 <walken@google.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[Linux commit 6d58452dc066db61acdff7b84671db1b11a3de1c]

Ported to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c
index 9dc296e0d88216eee540bb51fd824ef41359a553..244f1d8818a35e99913b716dab54ea2432744c01 100644 (file)
--- 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);