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[Xen-devel] [PATCH v2 06/20] rbtree: empty nodes have no color

Empty nodes have no color.  We can make use of this property to simplify
the code emitted by the RB_EMPTY_NODE and RB_CLEAR_NODE macros.  Also,
we can get rid of the rb_init_node function which had been introduced by
commit 88d19cf37952 ("timers: Add rb_init_node() to allow for stack
allocated rb nodes") to avoid some issue with the empty node's color not
being initialized.

I'm not sure what the RB_EMPTY_NODE checks in rb_prev() / rb_next() are
doing there, though.  axboe introduced them in commit 10fd48f2376d
("rbtree: fixed reversed RB_EMPTY_NODE and rb_next/prev").  The way I
see it, the 'empty node' abstraction is only used by rbtree users to
flag nodes that they haven't inserted in any rbtree, so asking the
predecessor or successor of such nodes doesn't make any sense.

One final rb_init_node() caller was recently added in sysctl code to
implement faster sysctl name lookups.  This code doesn't make use of
RB_EMPTY_NODE at all, and from what I could see it only called
rb_init_node() under the mistaken assumption that such initialization was
required before node insertion.

[sfr@xxxxxxxxxxxxxxxx: fix net/ceph/osd_client.c build]
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Acked-by: David Woodhouse <David.Woodhouse@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Daniel Santos <daniel.santos@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[Linux commit 4c199a93a2d36b277a9fd209a0f2793f8460a215]

Ported rbtree.h and rbtree.c changes which are relevant to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@xxxxxxxxx>
 xen/common/rbtree.c      | 4 ++--
 xen/include/xen/rbtree.h | 9 ++++++---
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/xen/common/rbtree.c b/xen/common/rbtree.c
index 9d3c5fab95..2561dbc7d9 100644
--- a/xen/common/rbtree.c
+++ b/xen/common/rbtree.c
@@ -317,7 +317,7 @@ struct rb_node *rb_next(const struct rb_node *node)
     struct rb_node *parent;
-    if (rb_parent(node) == node)
+    if (RB_EMPTY_NODE(node))
         return NULL;
     /* If we have a right-hand child, go down and then left as far
@@ -346,7 +346,7 @@ struct rb_node *rb_prev(const struct rb_node *node)
     struct rb_node *parent;
-    if (rb_parent(node) == node)
+    if (RB_EMPTY_NODE(node))
         return NULL;
     /* If we have a left-hand child, go down and then right as far
diff --git a/xen/include/xen/rbtree.h b/xen/include/xen/rbtree.h
index 3eb527eb37..f74b68ce62 100644
--- a/xen/include/xen/rbtree.h
+++ b/xen/include/xen/rbtree.h
@@ -52,9 +52,12 @@ static inline void rb_set_color(struct rb_node *rb, int 
 #define RB_ROOT (struct rb_root) { NULL, }
 #define rb_entry(ptr, type, member) container_of(ptr, type, member)
-#define RB_EMPTY_ROOT(root) ((root)->rb_node == NULL)
-#define RB_EMPTY_NODE(node) (rb_parent(node) == node)
-#define RB_CLEAR_NODE(node) (rb_set_parent(node, node))
+#define RB_EMPTY_ROOT(root)  ((root)->rb_node == NULL)
+/* 'empty' nodes are nodes that are known not to be inserted in an rbree */
+#define RB_EMPTY_NODE(node)  ((node)->rb_parent_color == (unsigned long)(node))
+#define RB_CLEAR_NODE(node)  ((node)->rb_parent_color = (unsigned long)(node))
 extern void rb_insert_color(struct rb_node *, struct rb_root *);
 extern void rb_erase(struct rb_node *, struct rb_root *);

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