We're currently protecting radix_tree_tag_set/clear with an rwlock in write
mode.  That's silly, because radix_tree_tag_set() only requires that the
overall tree be stabilised.

So change the radix-tree functions to use atomic bitops.  This permits the
user to use a read lock while running radix_tree_tag_set() and
radix_tree_tag_clear().


---

 25-akpm/lib/radix-tree.c |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)

diff -puN lib/radix-tree.c~radix_tree_tag_set-atomic lib/radix-tree.c
--- 25/lib/radix-tree.c~radix_tree_tag_set-atomic	Fri May 21 17:25:03 2004
+++ 25-akpm/lib/radix-tree.c	Fri May 21 17:27:17 2004
@@ -136,13 +136,12 @@ out:
 
 static inline void tag_set(struct radix_tree_node *node, int tag, int offset)
 {
-	if (!test_bit(offset, &node->tags[tag][0]))
-		__set_bit(offset, &node->tags[tag][0]);
+	set_bit(offset, &node->tags[tag][0]);
 }
 
 static inline void tag_clear(struct radix_tree_node *node, int tag, int offset)
 {
-	__clear_bit(offset, &node->tags[tag][0]);
+	clear_bit(offset, &node->tags[tag][0]);
 }
 
 static inline int tag_get(struct radix_tree_node *node, int tag, int offset)
@@ -321,6 +320,11 @@ EXPORT_SYMBOL(radix_tree_lookup);
  *
  *	Returns the address of the tagged item.   Setting a tag on a not-present
  *	item is a bug.
+ *
+ *	radix_tree_tag_set() atomically sets the tag and does not actually
+ *	change the overall tree structure.  Consequently it is sufficient that
+ *	the caller use an rwlock in read-mode while running
+ *	radix_tree_tag_set().
  */
 void *radix_tree_tag_set(struct radix_tree_root *root,
 			unsigned long index, int tag)
@@ -362,6 +366,11 @@ EXPORT_SYMBOL(radix_tree_tag_set);
  *
  *	Returns the address of the tagged item on success, else NULL.  ie:
  *	has the same return value and semantics as radix_tree_lookup().
+ *
+ *	radix_tree_tag_clear() atomically sets the tag and does not actually
+ *	change the overall tree structure.  Consequently it is sufficient that
+ *	the caller use an rwlock in read-mode while running
+ *	radix_tree_tag_clear().
  */
 void *radix_tree_tag_clear(struct radix_tree_root *root,
 			unsigned long index, int tag)

_