[PATCH] reiser4: add radix_tree_lookup_slot()

Reiser4 uses radix trees to solve a trouble reiser4_readdir has serving nfs
requests.

Unfortunately, radix tree api lacks an operation suitable for modifying
existing entry.  This patch adds radix_tree_lookup_slot which returns pointer
to found item within the tree.  That location can be then updated.

Both Nick and Christoph Lameter have patches which need this as well.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 39 insertions, 13 deletions

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 9f0f9281f42a..36e5d269612f 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -46,6 +46,7 @@ do {									\
 
 int radix_tree_insert(struct radix_tree_root *, unsigned long, void *);
 void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
+void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
 void *radix_tree_delete(struct radix_tree_root *, unsigned long);
 unsigned int
 radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index d1c057e71b68..88511c3805ad 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -281,35 +281,60 @@ int radix_tree_insert(struct radix_tree_root *root,
 }
 EXPORT_SYMBOL(radix_tree_insert);
 
-/**
- *      radix_tree_lookup    -    perform lookup operation on a radix tree
- *      @root:          radix tree root
- *      @index:         index key
- *
- *      Lookup the item at the position @index in the radix tree @root.
- */
-void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
+static inline void **__lookup_slot(struct radix_tree_root *root,
+                                   unsigned long index)
 {
         unsigned int height, shift;
-        struct radix_tree_node *slot;
+        struct radix_tree_node **slot;
 
         height = root->height;
         if (index > radix_tree_maxindex(height))
                 return NULL;
 
         shift = (height-1) * RADIX_TREE_MAP_SHIFT;
-        slot = root->rnode;
+        slot = &root->rnode;
 
         while (height > 0) {
-                if (slot == NULL)
+                if (*slot == NULL)
                         return NULL;
 
-                slot = slot->slots[(index >> shift) & RADIX_TREE_MAP_MASK];
+                slot = (struct radix_tree_node **)
+                        ((*slot)->slots +
+                                ((index >> shift) & RADIX_TREE_MAP_MASK));
                 shift -= RADIX_TREE_MAP_SHIFT;
                 height--;
         }
 
-        return slot;
+        return (void **)slot;
+}
+
+/**
+ *      radix_tree_lookup_slot    -    lookup a slot in a radix tree
+ *      @root:          radix tree root
+ *      @index:         index key
+ *
+ *      Lookup the slot corresponding to the position @index in the radix tree
+ *      @root. This is useful for update-if-exists operations.
+ */
+void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index)
+{
+        return __lookup_slot(root, index);
+}
+EXPORT_SYMBOL(radix_tree_lookup_slot);
+
+/**
+ *      radix_tree_lookup    -    perform lookup operation on a radix tree
+ *      @root:          radix tree root
+ *      @index:         index key
+ *
+ *      Lookup the item at the position @index in the radix tree @root.
+ */
+void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
+{
+        void **slot;
+
+        slot = __lookup_slot(root, index);
+        return slot != NULL ? *slot : NULL;
 }
 EXPORT_SYMBOL(radix_tree_lookup);