Commit 9dee5c51 authored by Cody P Schafer's avatar Cody P Schafer Committed by Linus Torvalds
Browse files

rbtree: add postorder iteration functions 



Postorder iteration yields all of a node's children prior to yielding the
node itself, and this particular implementation also avoids examining the
leaf links in a node after that node has been yielded.

In what I expect will be its most common usage, postorder iteration allows
the deletion of every node in an rbtree without modifying the rbtree nodes
(no _requirement_ that they be nulled) while avoiding referencing child
nodes after they have been "deleted" (most commonly, freed).

I have only updated zswap to use this functionality at this point, but
numerous bits of code (most notably in the filesystem drivers) use a hand
rolled postorder iteration that NULLs child links as it traverses the
tree.  Each of those instances could be replaced with this common
implementation.

1 & 2 add rbtree postorder iteration functions.
3 adds testing of the iteration to the rbtree runtime tests
4 allows building the rbtree runtime tests as builtins
5 updates zswap.

This patch:

Add postorder iteration functions for rbtree.  These are useful for safely
freeing an entire rbtree without modifying the tree at all.

Signed-off-by: default avatarCody P Schafer <cody@linux.vnet.ibm.com>
Reviewed-by: default avatarSeth Jennings <sjenning@linux.vnet.ibm.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b4bc4a18

include/linux/rbtree.h

+4 −0
Original line number Diff line number Diff line
@@ -68,6 +68,10 @@ extern struct rb_node *rb_prev(const struct rb_node *); 
extern struct rb_node *rb_first(const struct rb_root *);

extern struct rb_node *rb_last(const struct rb_root *);



/* Postorder iteration - always visit the parent after its children */

extern struct rb_node *rb_first_postorder(const struct rb_root *);

extern struct rb_node *rb_next_postorder(const struct rb_node *);



/* Fast replacement of a single node without remove/rebalance/add/rebalance */

extern void rb_replace_node(struct rb_node *victim, struct rb_node *new, 

			    struct rb_root *root);

lib/rbtree.c

+40 −0
Original line number Diff line number Diff line
@@ -518,3 +518,43 @@ void rb_replace_node(struct rb_node *victim, struct rb_node *new, 
	*new = *victim;

}

EXPORT_SYMBOL(rb_replace_node);



static struct rb_node *rb_left_deepest_node(const struct rb_node *node)

{

	for (;;) {

		if (node->rb_left)

			node = node->rb_left;

		else if (node->rb_right)

			node = node->rb_right;

		else

			return (struct rb_node *)node;

	}

}



struct rb_node *rb_next_postorder(const struct rb_node *node)

{

	const struct rb_node *parent;

	if (!node)

		return NULL;

	parent = rb_parent(node);



	/* If we're sitting on node, we've already seen our children */

	if (parent && node == parent->rb_left && parent->rb_right) {

		/* If we are the parent's left node, go to the parent's right

		 * node then all the way down to the left */

		return rb_left_deepest_node(parent->rb_right);

	} else

		/* Otherwise we are the parent's right node, and the parent

		 * should be next */

		return (struct rb_node *)parent;

}

EXPORT_SYMBOL(rb_next_postorder);



struct rb_node *rb_first_postorder(const struct rb_root *root)

{

	if (!root->rb_node)

		return NULL;



	return rb_left_deepest_node(root->rb_node);

}

EXPORT_SYMBOL(rb_first_postorder);