rbtree: optimize case selection logic in __rb_erase_color()

In __rb_erase_color(), we have to select one of 3 cases depending on the
color on the 'other' node children.  If both children are black, we flip a
few node colors and iterate.  Otherwise, we do either one or two tree
rotations, depending on the color of the 'other' child opposite to 'node',
and then we are done.

The corresponding logic had duplicate checks for the color of the 'other'
child opposite to 'node'.  It was checking it first to determine if both
children are black, and then to determine how many tree rotations are
required.  Rearrange the logic to avoid that extra check.

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Daniel Santos <daniel.santos@pobox.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/rbtree.c b/lib/rbtree.c
index baf7c835c57c90515e97151dd8c30ed0142f07c9..eb823a31099c37678768551ed685d2dc0f0a6aca 100644 (file)
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -283,28 +283,24 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
                                __rb_rotate_left(parent, root);
                                other = parent->rb_right;
                        }
-                       if ((!other->rb_left || rb_is_black(other->rb_left)) &&
-                           (!other->rb_right || rb_is_black(other->rb_right)))
-                       {
-                               rb_set_red(other);
-                               node = parent;
-                               parent = rb_parent(node);
-                       }
-                       else
-                       {
-                               if (!other->rb_right || rb_is_black(other->rb_right))
-                               {
-                                       rb_set_black(other->rb_left);
+                       if (!other->rb_right || rb_is_black(other->rb_right)) {
+                               if (!other->rb_left ||
+                                   rb_is_black(other->rb_left)) {
                                        rb_set_red(other);
-                                       __rb_rotate_right(other, root);
-                                       other = parent->rb_right;
+                                       node = parent;
+                                       parent = rb_parent(node);
+                                       continue;
                                }
-                               rb_set_color(other, rb_color(parent));
-                               rb_set_black(parent);
-                               rb_set_black(other->rb_right);
-                               __rb_rotate_left(parent, root);
-                               break;
+                               rb_set_black(other->rb_left);
+                               rb_set_red(other);
+                               __rb_rotate_right(other, root);
+                               other = parent->rb_right;
                        }
+                       rb_set_color(other, rb_color(parent));
+                       rb_set_black(parent);
+                       rb_set_black(other->rb_right);
+                       __rb_rotate_left(parent, root);
+                       break;
                } else {
                        other = parent->rb_left;
                        if (rb_is_red(other))
@@ -314,28 +310,24 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
                                __rb_rotate_right(parent, root);
                                other = parent->rb_left;
                        }
-                       if ((!other->rb_left || rb_is_black(other->rb_left)) &&
-                           (!other->rb_right || rb_is_black(other->rb_right)))
-                       {
-                               rb_set_red(other);
-                               node = parent;
-                               parent = rb_parent(node);
-                       }
-                       else
-                       {
-                               if (!other->rb_left || rb_is_black(other->rb_left))
-                               {
-                                       rb_set_black(other->rb_right);
+                       if (!other->rb_left || rb_is_black(other->rb_left)) {
+                               if (!other->rb_right ||
+                                   rb_is_black(other->rb_right)) {
                                        rb_set_red(other);
-                                       __rb_rotate_left(other, root);
-                                       other = parent->rb_left;
+                                       node = parent;
+                                       parent = rb_parent(node);
+                                       continue;
                                }
-                               rb_set_color(other, rb_color(parent));
-                               rb_set_black(parent);
-                               rb_set_black(other->rb_left);
-                               __rb_rotate_right(parent, root);
-                               break;
+                               rb_set_black(other->rb_right);
+                               rb_set_red(other);
+                               __rb_rotate_left(other, root);
+                               other = parent->rb_left;
                        }
+                       rb_set_color(other, rb_color(parent));
+                       rb_set_black(parent);
+                       rb_set_black(other->rb_left);
+                       __rb_rotate_right(parent, root);
+                       break;
                }
        }
 }