[PATCH] radix-tree: cleanup radix_tree_deref_slot() and _lookup_slot() comments

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

'_deref_slot() function, and adds more explanation of expected/required
locking to the direct slot access functions.  I separated it out,
because it doesn't fix a serious bug, like the previous one.

Paul:  do you agree that we don't need rcu_dereference() in the
_deref_slot() as it can only be used while the tree is held [probably
write] locked?  Do the comments look OK?

Lee

Cleanup radix tree slot dereference and lookup comments - 2.6.18-rc4-mm2

radix_tree_deref_slot() was actually dereferencing the pointer
in the assignment to the local variable 'slot' and then 
rcu_dereference()ing the results of an expression [return value
of an inline function]. 

Because we must hold the tree locked across lookup_slot() and
_deref_slot(), we don't need the rcu_dereference() at all.

Added comments specifying required locking for _lookup_slot()
and _deref_slot().

Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>


 include/linux/radix-tree.h |    9 +++++++--
 lib/radix-tree.c           |    5 ++++-
 2 files changed, 11 insertions(+), 3 deletions(-)

Index: linux-2.6.18-rc4-mm2/lib/radix-tree.c
===================================================================
--- linux-2.6.18-rc4-mm2.orig/lib/radix-tree.c	2006-08-22 14:47:06.000000000 -0400
+++ linux-2.6.18-rc4-mm2/lib/radix-tree.c	2006-08-22 14:48:01.000000000 -0400
@@ -336,7 +336,10 @@ EXPORT_SYMBOL(radix_tree_insert);
  *	@root. This is useful for update-if-exists operations.
  *
  *	This function cannot be called under rcu_read_lock, it must be
- *	excluded from writers, as must the returned slot.
+ *	excluded from writers, as must the returned slot for subsequent
+ *	use by radix_tree_deref_slot() and radix_tree_replace slot.
+ *	Caller must hold tree write locked across slot lookup and
+ *	replace.
  */
 void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index)
 {
Index: linux-2.6.18-rc4-mm2/include/linux/radix-tree.h
===================================================================
--- linux-2.6.18-rc4-mm2.orig/include/linux/radix-tree.h	2006-08-22 14:47:45.000000000 -0400
+++ linux-2.6.18-rc4-mm2/include/linux/radix-tree.h	2006-08-22 14:48:54.000000000 -0400
@@ -122,12 +122,17 @@ do {									\
 /**
  * radix_tree_deref_slot	- dereference a slot
  * @pslot:	pointer to slot, returned by radix_tree_lookup_slot
- * @returns:	item that was stored in that slot.
+ * @returns:	item that was stored in that slot with any direct pointer flag
+ *		removed.
+ *
+ * For use with radix_tree_lookup_slot().  Caller must hold tree at least read
+ * locked across slot lookup and dereference.  More likely, will be used with
+ * radix_tree_replace_slot(), as well, so caller will hold tree write locked.
  */
 static inline void *radix_tree_deref_slot(void *pslot)
 {
 	void *slot = *(void **)pslot;
-	return rcu_dereference(radix_tree_direct_to_ptr(slot));
+	return radix_tree_direct_to_ptr(slot);
 }
 /**
  * radix_tree_replace_slot	- replace item in a slot


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