rbtree: handle 1-child recoloring in rb_erase() instead of rb_erase_color()
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
Linus Torvalds
Родитель
60670b8034
Коммит
46b6135a74
105
lib/rbtree.c
105
lib/rbtree.c
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@ -2,7 +2,8 @@
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Red Black Trees
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(C) 1999 Andrea Arcangeli <andrea@suse.de>
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(C) 2002 David Woodhouse <dwmw2@infradead.org>
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(C) 2012 Michel Lespinasse <walken@google.com>
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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@ -50,6 +51,11 @@
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#define rb_is_red(r) (!rb_color(r))
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#define rb_is_black(r) rb_color(r)
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static inline void rb_set_black(struct rb_node *rb)
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{
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rb->__rb_parent_color |= RB_BLACK;
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}
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static inline void rb_set_parent(struct rb_node *rb, struct rb_node *p)
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{
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rb->__rb_parent_color = rb_color(rb) | (unsigned long)p;
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@ -214,27 +220,18 @@ void rb_insert_color(struct rb_node *node, struct rb_root *root)
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}
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EXPORT_SYMBOL(rb_insert_color);
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static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
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struct rb_root *root)
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static void __rb_erase_color(struct rb_node *parent, struct rb_root *root)
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{
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struct rb_node *sibling, *tmp1, *tmp2;
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struct rb_node *node = NULL, *sibling, *tmp1, *tmp2;
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while (true) {
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/*
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* Loop invariant: all leaf paths going through node have a
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* black node count that is 1 lower than other leaf paths.
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*
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* If node is red, we can flip it to black to adjust.
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* If node is the root, all leaf paths go through it.
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* Otherwise, we need to adjust the tree through color flips
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* and tree rotations as per one of the 4 cases below.
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* Loop invariants:
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* - node is black (or NULL on first iteration)
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* - node is not the root (parent is not NULL)
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* - All leaf paths going through parent and node have a
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* black node count that is 1 lower than other leaf paths.
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*/
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if (node && rb_is_red(node)) {
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rb_set_parent_color(node, parent, RB_BLACK);
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break;
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} else if (!parent) {
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break;
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}
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sibling = parent->rb_right;
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if (node != sibling) { /* node == parent->rb_left */
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if (rb_is_red(sibling)) {
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@ -268,17 +265,22 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
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* / \ / \
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* Sl Sr Sl Sr
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*
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* This leaves us violating 5), so
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* recurse at p. If p is red, the
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* recursion will just flip it to black
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* and exit. If coming from Case 1,
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* p is known to be red.
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* This leaves us violating 5) which
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* can be fixed by flipping p to black
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* if it was red, or by recursing at p.
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* p is red when coming from Case 1.
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*/
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rb_set_parent_color(sibling, parent,
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RB_RED);
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node = parent;
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parent = rb_parent(node);
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continue;
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if (rb_is_red(parent))
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rb_set_black(parent);
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else {
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node = parent;
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parent = rb_parent(node);
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if (parent)
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continue;
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}
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break;
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}
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/*
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* Case 3 - right rotate at sibling
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@ -339,9 +341,15 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
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/* Case 2 - sibling color flip */
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rb_set_parent_color(sibling, parent,
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RB_RED);
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node = parent;
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parent = rb_parent(node);
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continue;
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if (rb_is_red(parent))
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rb_set_black(parent);
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else {
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node = parent;
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parent = rb_parent(node);
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if (parent)
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continue;
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}
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break;
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}
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/* Case 3 - right rotate at sibling */
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sibling->rb_right = tmp1 = tmp2->rb_left;
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@ -369,23 +377,31 @@ static void __rb_erase_color(struct rb_node *node, struct rb_node *parent,
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void rb_erase(struct rb_node *node, struct rb_root *root)
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{
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struct rb_node *child = node->rb_right, *tmp = node->rb_left;
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struct rb_node *parent;
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int color;
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struct rb_node *parent, *rebalance;
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if (!tmp) {
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case1:
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/* Case 1: node to erase has no more than 1 child (easy!) */
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/*
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* Case 1: node to erase has no more than 1 child (easy!)
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*
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* Note that if there is one child it must be red due to 5)
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* and node must be black due to 4). We adjust colors locally
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* so as to bypass __rb_erase_color() later on.
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*/
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parent = rb_parent(node);
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color = rb_color(node);
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if (child)
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rb_set_parent(child, parent);
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__rb_change_child(node, child, parent, root);
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if (child) {
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rb_set_parent_color(child, parent, RB_BLACK);
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rebalance = NULL;
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} else {
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rebalance = rb_is_black(node) ? parent : NULL;
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}
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} else if (!child) {
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/* Still case 1, but this time the child is node->rb_left */
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child = tmp;
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goto case1;
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parent = rb_parent(node);
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__rb_change_child(node, tmp, parent, root);
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rb_set_parent_color(tmp, parent, RB_BLACK);
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rebalance = NULL;
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} else {
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struct rb_node *old = node, *left;
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@ -397,26 +413,29 @@ void rb_erase(struct rb_node *node, struct rb_root *root)
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child = node->rb_right;
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parent = rb_parent(node);
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color = rb_color(node);
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if (parent == old) {
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parent = node;
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} else {
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if (child)
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rb_set_parent(child, parent);
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parent->rb_left = child;
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node->rb_right = old->rb_right;
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rb_set_parent(old->rb_right, node);
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}
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if (child) {
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rb_set_parent_color(child, parent, RB_BLACK);
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rebalance = NULL;
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} else {
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rebalance = rb_is_black(node) ? parent : NULL;
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}
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node->__rb_parent_color = old->__rb_parent_color;
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node->rb_left = old->rb_left;
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rb_set_parent(old->rb_left, node);
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}
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if (color == RB_BLACK)
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__rb_erase_color(child, parent, root);
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if (rebalance)
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__rb_erase_color(rebalance, root);
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}
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EXPORT_SYMBOL(rb_erase);
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