rbtree: add some additional comments for rebalancing cases

While overall the code is very nicely commented, it might not be
immediately obvious from the diagrams what is going on.  Add a very
brief summary of each case.  Opposite cases where the node is the left
child are left untouched.

Link: http://lkml.kernel.org/r/20170719014603.19029-4-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/lib/rbtree.c b/lib/rbtree.c
index e7cce12f404f..ba4a9d165f1b 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -132,7 +132,7 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
                 if (parent != tmp) {    /* parent == gparent->rb_left */
                         if (tmp && rb_is_red(tmp)) {
                                 /*
-                                 * Case 1 - color flips
+                                 * Case 1 - node's uncle is red (color flips).
                                  *
                                  *       G            g
                                  *      / \          / \
@@ -155,7 +155,8 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
                         tmp = parent->rb_right;
                         if (node == tmp) {
                                 /*
-                                 * Case 2 - left rotate at parent
+                                 * Case 2 - node's uncle is black and node is
+                                 * the parent's right child (left rotate at parent).
                                  *
                                  *      G             G
                                  *     / \           / \
@@ -179,7 +180,8 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
                         }
 
                         /*
-                         * Case 3 - right rotate at gparent
+                         * Case 3 - node's uncle is black and node is
+                         * the parent's left child (right rotate at gparent).
                          *
                          *        G           P
                          *       / \         / \