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rbtree: handle 1-child recoloring in rb_erase() instead of rb_erase_color()

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An interesting observation for rb_erase() is that when a node has
exactly one child, the node must be black and the child must be red.
An interesting consequence is that removing such a node can be done by
simply replacing it with its child and making the child black,
which we can do efficiently in rb_erase(). __rb_erase_color() then
only needs to handle the no-childs case and can be modified accordingly.

Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michel Lespinasse 2012-10-08 16:31:11 -07:00 committed by Linus Torvalds
parent 60670b8034
commit 46b6135a74
1 changed files with 62 additions and 43 deletions
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105
lib/rbtree.c
View File

@ -2,7 +2,8 @@
Red Black Trees
(C) 1999 Andrea Arcangeli <andrea@suse.de>
(C) 2002 David Woodhouse <dwmw2@infradead.org>
(C) 2012 Michel Lespinasse <walken@google.com>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
@ -50,6 +51,11 @@
#define rb_is_red(r) (!rb_color(r))
#define rb_is_black(r) rb_color(r)
static inline void rb_set_black(struct rb_node *rb)
{
rb->__rb_parent_color |= RB_BLACK;
}
static inline void rb_set_parent(struct rb_node *rb, struct rb_node *p)
{
rb->__rb_parent_color = rb_color(rb) | (unsigned long)p;
@ -214,27 +220,18 @@ void rb_insert_color(struct rb_node *node, struct rb_root *root)
}
EXPORT_SYMBOL(rb_insert_color);
static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
struct rb_root *root)
static void __rb_erase_color(struct rb_node *parent, struct rb_root *root)
{
struct rb_node *sibling, *tmp1, *tmp2;
struct rb_node *node = NULL, *sibling, *tmp1, *tmp2;
while (true) {
/*
* Loop invariant: all leaf paths going through node have a
* black node count that is 1 lower than other leaf paths.
*
* If node is red, we can flip it to black to adjust.
* If node is the root, all leaf paths go through it.
* Otherwise, we need to adjust the tree through color flips
* and tree rotations as per one of the 4 cases below.
* Loop invariants:
* - node is black (or NULL on first iteration)
* - node is not the root (parent is not NULL)
* - All leaf paths going through parent and node have a
* black node count that is 1 lower than other leaf paths.
*/
if (node && rb_is_red(node)) {
rb_set_parent_color(node, parent, RB_BLACK);
break;
} else if (!parent) {
break;
}
sibling = parent->rb_right;
if (node != sibling) { /* node == parent->rb_left */
if (rb_is_red(sibling)) {
@ -268,17 +265,22 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
* / \ / \
* Sl Sr Sl Sr
*
* This leaves us violating 5), so
* recurse at p. If p is red, the
* recursion will just flip it to black
* and exit. If coming from Case 1,
* p is known to be red.
* This leaves us violating 5) which
* can be fixed by flipping p to black
* if it was red, or by recursing at p.
* p is red when coming from Case 1.
*/
rb_set_parent_color(sibling, parent,
RB_RED);
node = parent;
parent = rb_parent(node);
continue;
if (rb_is_red(parent))
rb_set_black(parent);
else {
node = parent;
parent = rb_parent(node);
if (parent)
continue;
}
break;
}
/*
* Case 3 - right rotate at sibling
@ -339,9 +341,15 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
/* Case 2 - sibling color flip */
rb_set_parent_color(sibling, parent,
RB_RED);
node = parent;
parent = rb_parent(node);
continue;
if (rb_is_red(parent))
rb_set_black(parent);
else {
node = parent;
parent = rb_parent(node);
if (parent)
continue;
}
break;
}
/* Case 3 - right rotate at sibling */
sibling->rb_right = tmp1 = tmp2->rb_left;
@ -369,23 +377,31 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
void rb_erase(struct rb_node *node, struct rb_root *root)
{
struct rb_node *child = node->rb_right, *tmp = node->rb_left;
struct rb_node *parent;
int color;
struct rb_node *parent, *rebalance;
if (!tmp) {
case1:
/* Case 1: node to erase has no more than 1 child (easy!) */
/*
* Case 1: node to erase has no more than 1 child (easy!)
*
* Note that if there is one child it must be red due to 5)
* and node must be black due to 4). We adjust colors locally
* so as to bypass __rb_erase_color() later on.
*/
parent = rb_parent(node);
color = rb_color(node);
if (child)
rb_set_parent(child, parent);
__rb_change_child(node, child, parent, root);
if (child) {
rb_set_parent_color(child, parent, RB_BLACK);
rebalance = NULL;
} else {
rebalance = rb_is_black(node) ? parent : NULL;
}
} else if (!child) {
/* Still case 1, but this time the child is node->rb_left */
child = tmp;
goto case1;
parent = rb_parent(node);
__rb_change_child(node, tmp, parent, root);
rb_set_parent_color(tmp, parent, RB_BLACK);
rebalance = NULL;
} else {
struct rb_node *old = node, *left;
@ -397,26 +413,29 @@ void rb_erase(struct rb_node *node, struct rb_root *root)
child = node->rb_right;
parent = rb_parent(node);
color = rb_color(node);
if (parent == old) {
parent = node;
} else {
if (child)
rb_set_parent(child, parent);
parent->rb_left = child;
node->rb_right = old->rb_right;
rb_set_parent(old->rb_right, node);
}
if (child) {
rb_set_parent_color(child, parent, RB_BLACK);
rebalance = NULL;
} else {
rebalance = rb_is_black(node) ? parent : NULL;
}
node->__rb_parent_color = old->__rb_parent_color;
node->rb_left = old->rb_left;
rb_set_parent(old->rb_left, node);
}
if (color == RB_BLACK)
__rb_erase_color(child, parent, root);
if (rebalance)
__rb_erase_color(rebalance, root);
}
EXPORT_SYMBOL(rb_erase);