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: Cody P Schafer <cody@linux.vnet.ibm.com>
Reviewed-by: Seth 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
index 0022c1bb1e26398c9db767a8eebc4fa153bff559..c467151e99509997fa8b9a14ee53c1cd4f9765b8 100644 (file)
--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -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);

diff --git a/lib/rbtree.c b/lib/rbtree.c
index c0e31fe2fabf5b99c160fb01daf618cb7c0488b3..65f4effd117f4350bb5dde964eab219f67caf701 100644 (file)
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -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);