diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 9eb2595..e2cefab 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -2,6 +2,7 @@
  * Copyright (C) 2001 Momchil Velikov
  * Portions Copyright (C) 2001 Christoph Hellwig
  * Copyright (C) 2005 SGI, Christoph Lameter <clameter@sgi.com>
+ * Copyright (C) 2006 Nick Piggin
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -30,6 +31,7 @@
 #include <linux/gfp.h>
 #include <linux/string.h>
 #include <linux/bitops.h>
+#include <linux/rcupdate.h>
 
 
 #ifdef __KERNEL__
@@ -45,7 +47,9 @@
 	((RADIX_TREE_MAP_SIZE + BITS_PER_LONG - 1) / BITS_PER_LONG)
 
 struct radix_tree_node {
+	unsigned int	height;		/* Height from the bottom */
 	unsigned int	count;
+	struct rcu_head	rcu_head;
 	void		*slots[RADIX_TREE_MAP_SIZE];
 	unsigned long	tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
 };
@@ -100,13 +104,21 @@
 			rtp->nr--;
 		}
 	}
+	BUG_ON(radix_tree_is_direct_ptr(ret));
 	return ret;
 }
 
+static void radix_tree_node_rcu_free(struct rcu_head *head)
+{
+	struct radix_tree_node *node =
+			container_of(head, struct radix_tree_node, rcu_head);
+	kmem_cache_free(radix_tree_node_cachep, node);
+}
+
 static inline void
 radix_tree_node_free(struct radix_tree_node *node)
 {
-	kmem_cache_free(radix_tree_node_cachep, node);
+	call_rcu(&node->rcu_head, radix_tree_node_rcu_free);
 }
 
 /*
@@ -222,11 +234,12 @@
 	}
 
 	do {
+		unsigned int newheight;
 		if (!(node = radix_tree_node_alloc(root)))
 			return -ENOMEM;
 
 		/* Increase the height.  */
-		node->slots[0] = root->rnode;
+		node->slots[0] = radix_tree_direct_to_ptr(root->rnode);
 
 		/* Propagate the aggregated tag info into the new root */
 		for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
@@ -234,9 +247,11 @@
 				tag_set(node, tag, 0);
 		}
 
+		newheight = root->height+1;
+		node->height = newheight;
 		node->count = 1;
-		root->rnode = node;
-		root->height++;
+		rcu_assign_pointer(root->rnode, node);
+		root->height = newheight;
 	} while (height > root->height);
 out:
 	return 0;
@@ -258,6 +273,8 @@
 	int offset;
 	int error;
 
+	BUG_ON(radix_tree_is_direct_ptr(item));
+
 	/* Make sure the tree is high enough.  */
 	if (index > radix_tree_maxindex(root->height)) {
 		error = radix_tree_extend(root, index);
@@ -275,11 +292,12 @@
 			/* Have to add a child node.  */
 			if (!(slot = radix_tree_node_alloc(root)))
 				return -ENOMEM;
+			slot->height = height;
 			if (node) {
-				node->slots[offset] = slot;
+				rcu_assign_pointer(node->slots[offset], slot);
 				node->count++;
 			} else
-				root->rnode = slot;
+				rcu_assign_pointer(root->rnode, slot);
 		}
 
 		/* Go a level down */
@@ -295,11 +313,11 @@
 
 	if (node) {
 		node->count++;
-		node->slots[offset] = item;
+		rcu_assign_pointer(node->slots[offset], item);
 		BUG_ON(tag_get(node, 0, offset));
 		BUG_ON(tag_get(node, 1, offset));
 	} else {
-		root->rnode = item;
+		rcu_assign_pointer(root->rnode, radix_tree_ptr_to_direct(item));
 		BUG_ON(root_tag_get(root, 0));
 		BUG_ON(root_tag_get(root, 1));
 	}
@@ -308,48 +326,53 @@
 }
 EXPORT_SYMBOL(radix_tree_insert);
 
-static inline void **__lookup_slot(struct radix_tree_root *root,
-				   unsigned long index)
-{
-	unsigned int height, shift;
-	struct radix_tree_node **slot;
-
-	height = root->height;
-
-	if (index > radix_tree_maxindex(height))
-		return NULL;
-
-	if (height == 0 && root->rnode)
-		return (void **)&root->rnode;
-
-	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
-	slot = &root->rnode;
-
-	while (height > 0) {
-		if (*slot == NULL)
-			return NULL;
-
-		slot = (struct radix_tree_node **)
-			((*slot)->slots +
-				((index >> shift) & RADIX_TREE_MAP_MASK));
-		shift -= RADIX_TREE_MAP_SHIFT;
-		height--;
-	}
-
-	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.
+ *	Returns:  the slot corresponding to the position @index in the
+ *	radix tree @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 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)
 {
-	return __lookup_slot(root, index);
+	unsigned int height, shift;
+	struct radix_tree_node *node, **slot;
+
+	node = root->rnode;
+	if (node == NULL)
+		return NULL;
+
+	if (radix_tree_is_direct_ptr(node)) {
+		if (index > 0)
+			return NULL;
+		return (void **)&root->rnode;
+	}
+
+	height = node->height;
+	if (index > radix_tree_maxindex(height))
+		return NULL;
+
+	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
+
+	do {
+		slot = (struct radix_tree_node **)
+			(node->slots + ((index>>shift) & RADIX_TREE_MAP_MASK));
+		node = *slot;
+		if (node == NULL)
+			return NULL;
+
+		shift -= RADIX_TREE_MAP_SHIFT;
+		height--;
+	} while (height > 0);
+
+	return (void **)slot;
 }
 EXPORT_SYMBOL(radix_tree_lookup_slot);
 
@@ -359,13 +382,45 @@
  *	@index:		index key
  *
  *	Lookup the item at the position @index in the radix tree @root.
+ *
+ *	This function can be called under rcu_read_lock, however the caller
+ *	must manage lifetimes of leaf nodes (eg. RCU may also be used to free
+ *	them safely). No RCU barriers are required to access or modify the
+ *	returned item, however.
  */
 void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
 {
-	void **slot;
+	unsigned int height, shift;
+	struct radix_tree_node *node, **slot;
 
-	slot = __lookup_slot(root, index);
-	return slot != NULL ? *slot : NULL;
+	node = rcu_dereference(root->rnode);
+	if (node == NULL)
+		return NULL;
+
+	if (radix_tree_is_direct_ptr(node)) {
+		if (index > 0)
+			return NULL;
+		return radix_tree_direct_to_ptr(node);
+	}
+
+	height = node->height;
+	if (index > radix_tree_maxindex(height))
+		return NULL;
+
+	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
+
+	do {
+		slot = (struct radix_tree_node **)
+			(node->slots + ((index>>shift) & RADIX_TREE_MAP_MASK));
+		node = rcu_dereference(*slot);
+		if (node == NULL)
+			return NULL;
+
+		shift -= RADIX_TREE_MAP_SHIFT;
+		height--;
+	} while (height > 0);
+
+	return node;
 }
 EXPORT_SYMBOL(radix_tree_lookup);
 
@@ -495,27 +550,30 @@
 			unsigned long index, unsigned int tag)
 {
 	unsigned int height, shift;
-	struct radix_tree_node *slot;
+	struct radix_tree_node *node;
 	int saw_unset_tag = 0;
 
-	height = root->height;
-	if (index > radix_tree_maxindex(height))
-		return 0;
-
 	/* check the root's tag bit */
 	if (!root_tag_get(root, tag))
 		return 0;
 
-	if (height == 0)
-		return 1;
+	node = rcu_dereference(root->rnode);
+	if (node == NULL)
+		return 0;
+
+	if (radix_tree_is_direct_ptr(node))
+		return (index == 0);
+
+	height = node->height;
+	if (index > radix_tree_maxindex(height))
+		return 0;
 
 	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
-	slot = root->rnode;
 
 	for ( ; ; ) {
 		int offset;
 
-		if (slot == NULL)
+		if (node == NULL)
 			return 0;
 
 		offset = (index >> shift) & RADIX_TREE_MAP_MASK;
@@ -524,15 +582,15 @@
 		 * This is just a debug check.  Later, we can bale as soon as
 		 * we see an unset tag.
 		 */
-		if (!tag_get(slot, tag, offset))
+		if (!tag_get(node, tag, offset))
 			saw_unset_tag = 1;
 		if (height == 1) {
-			int ret = tag_get(slot, tag, offset);
+			int ret = tag_get(node, tag, offset);
 
 			BUG_ON(ret && saw_unset_tag);
 			return !!ret;
 		}
-		slot = slot->slots[offset];
+		node = rcu_dereference(node->slots[offset]);
 		shift -= RADIX_TREE_MAP_SHIFT;
 		height--;
 	}
@@ -541,47 +599,45 @@
 #endif
 
 static unsigned int
-__lookup(struct radix_tree_root *root, void **results, unsigned long index,
+__lookup(struct radix_tree_node *slot, void **results, unsigned long index,
 	unsigned int max_items, unsigned long *next_index)
 {
 	unsigned int nr_found = 0;
 	unsigned int shift, height;
-	struct radix_tree_node *slot;
 	unsigned long i;
 
-	height = root->height;
-	if (height == 0) {
-		if (root->rnode && index == 0)
-			results[nr_found++] = root->rnode;
+	height = slot->height;
+	if (height == 0)
 		goto out;
-	}
-
 	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
-	slot = root->rnode;
 
 	for ( ; height > 1; height--) {
-
-		for (i = (index >> shift) & RADIX_TREE_MAP_MASK ;
-				i < RADIX_TREE_MAP_SIZE; i++) {
+		i = (index >> shift) & RADIX_TREE_MAP_MASK;
+		for (;;) {
 			if (slot->slots[i] != NULL)
 				break;
 			index &= ~((1UL << shift) - 1);
 			index += 1UL << shift;
 			if (index == 0)
 				goto out;	/* 32-bit wraparound */
+			i++;
+			if (i == RADIX_TREE_MAP_SIZE)
+				goto out;
 		}
-		if (i == RADIX_TREE_MAP_SIZE)
-			goto out;
 
 		shift -= RADIX_TREE_MAP_SHIFT;
-		slot = slot->slots[i];
+		slot = rcu_dereference(slot->slots[i]);
+		if (slot == NULL)
+			goto out;
 	}
 
 	/* Bottom level: grab some items */
 	for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) {
+		struct radix_tree_node *node;
 		index++;
-		if (slot->slots[i]) {
-			results[nr_found++] = slot->slots[i];
+		node = slot->slots[i];
+		if (node) {
+			results[nr_found++] = rcu_dereference(node);
 			if (nr_found == max_items)
 				goto out;
 		}
@@ -603,28 +659,51 @@
  *	*@results.
  *
  *	The implementation is naive.
+ *
+ *	Like radix_tree_lookup, radix_tree_gang_lookup may be called under
+ *	rcu_read_lock. In this case, rather than the returned results being
+ *	an atomic snapshot of the tree at a single point in time, the semantics
+ *	of an RCU protected gang lookup are as though multiple radix_tree_lookups
+ *	have been issued in individual locks, and results stored in 'results'.
  */
 unsigned int
 radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
 			unsigned long first_index, unsigned int max_items)
 {
-	const unsigned long max_index = radix_tree_maxindex(root->height);
+	unsigned long max_index;
+	struct radix_tree_node *node;
 	unsigned long cur_index = first_index;
-	unsigned int ret = 0;
+	unsigned int ret;
 
+	node = rcu_dereference(root->rnode);
+	if (!node)
+		return 0;
+
+	if (radix_tree_is_direct_ptr(node)) {
+		if (first_index > 0)
+			return 0;
+		node = radix_tree_direct_to_ptr(node);
+		results[0] = rcu_dereference(node);
+		return 1;
+	}
+
+	max_index = radix_tree_maxindex(node->height);
+
+	ret = 0;
 	while (ret < max_items) {
 		unsigned int nr_found;
 		unsigned long next_index;	/* Index of next search */
 
 		if (cur_index > max_index)
 			break;
-		nr_found = __lookup(root, results + ret, cur_index,
+		nr_found = __lookup(node, results + ret, cur_index,
 					max_items - ret, &next_index);
 		ret += nr_found;
 		if (next_index == 0)
 			break;
 		cur_index = next_index;
 	}
+
 	return ret;
 }
 EXPORT_SYMBOL(radix_tree_gang_lookup);
@@ -634,55 +713,64 @@
  * open-coding the search.
  */
 static unsigned int
-__lookup_tag(struct radix_tree_root *root, void **results, unsigned long index,
+__lookup_tag(struct radix_tree_node *slot, void **results, unsigned long index,
 	unsigned int max_items, unsigned long *next_index, unsigned int tag)
 {
 	unsigned int nr_found = 0;
-	unsigned int shift;
-	unsigned int height = root->height;
-	struct radix_tree_node *slot;
+	unsigned int shift, height;
 
-	if (height == 0) {
-		if (root->rnode && index == 0)
-			results[nr_found++] = root->rnode;
+	height = slot->height;
+	if (height == 0)
 		goto out;
-	}
+	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
 
-	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
-	slot = root->rnode;
+	while (height > 0) {
+		unsigned long i = (index >> shift) & RADIX_TREE_MAP_MASK ;
 
-	do {
-		unsigned long i = (index >> shift) & RADIX_TREE_MAP_MASK;
-
-		for ( ; i < RADIX_TREE_MAP_SIZE; i++) {
-			if (tag_get(slot, tag, i)) {
-				BUG_ON(slot->slots[i] == NULL);
+		for (;;) {
+			if (tag_get(slot, tag, i))
 				break;
-			}
 			index &= ~((1UL << shift) - 1);
 			index += 1UL << shift;
 			if (index == 0)
 				goto out;	/* 32-bit wraparound */
+			i++;
+			if (i == RADIX_TREE_MAP_SIZE)
+				goto out;
 		}
-		if (i == RADIX_TREE_MAP_SIZE)
-			goto out;
 		height--;
 		if (height == 0) {	/* Bottom level: grab some items */
 			unsigned long j = index & RADIX_TREE_MAP_MASK;
 
 			for ( ; j < RADIX_TREE_MAP_SIZE; j++) {
+				struct radix_tree_node *node;
 				index++;
-				if (tag_get(slot, tag, j)) {
-					BUG_ON(slot->slots[j] == NULL);
-					results[nr_found++] = slot->slots[j];
+				if (!tag_get(slot, tag, j))
+					continue;
+				node = slot->slots[j];
+				/*
+				 * Even though the tag was found set, we need to
+				 * recheck that we have a non-NULL node, because
+				 * if this lookup is lockless, it may have been
+				 * subsequently deleted.
+				 *
+				 * Similar care must be taken in any place that
+				 * lookup ->slots[x] without a lock (ie. can't
+				 * rely on its value remaining the same).
+				 */
+				if (node) {
+					node = rcu_dereference(node);
+					results[nr_found++] = node;
 					if (nr_found == max_items)
 						goto out;
 				}
 			}
 		}
 		shift -= RADIX_TREE_MAP_SHIFT;
-		slot = slot->slots[i];
-	} while (height > 0);
+		slot = rcu_dereference(slot->slots[i]);
+		if (slot == NULL)
+			break;
+	}
 out:
 	*next_index = index;
 	return nr_found;
@@ -706,27 +794,44 @@
 		unsigned long first_index, unsigned int max_items,
 		unsigned int tag)
 {
-	const unsigned long max_index = radix_tree_maxindex(root->height);
+	struct radix_tree_node *node;
+	unsigned long max_index;
 	unsigned long cur_index = first_index;
-	unsigned int ret = 0;
+	unsigned int ret;
 
 	/* check the root's tag bit */
 	if (!root_tag_get(root, tag))
 		return 0;
 
+	node = rcu_dereference(root->rnode);
+	if (!node)
+		return 0;
+
+	if (radix_tree_is_direct_ptr(node)) {
+		if (first_index > 0)
+			return 0;
+		node = radix_tree_direct_to_ptr(node);
+		results[0] = rcu_dereference(node);
+		return 1;
+	}
+
+	max_index = radix_tree_maxindex(node->height);
+
+	ret = 0;
 	while (ret < max_items) {
 		unsigned int nr_found;
 		unsigned long next_index;	/* Index of next search */
 
 		if (cur_index > max_index)
 			break;
-		nr_found = __lookup_tag(root, results + ret, cur_index,
+		nr_found = __lookup_tag(node, results + ret, cur_index,
 					max_items - ret, &next_index, tag);
 		ret += nr_found;
 		if (next_index == 0)
 			break;
 		cur_index = next_index;
 	}
+
 	return ret;
 }
 EXPORT_SYMBOL(radix_tree_gang_lookup_tag);
@@ -742,8 +847,19 @@
 			root->rnode->count == 1 &&
 			root->rnode->slots[0]) {
 		struct radix_tree_node *to_free = root->rnode;
+		void *newptr;
 
-		root->rnode = to_free->slots[0];
+		/*
+		 * We don't need rcu_assign_pointer(), since we are simply
+		 * moving the node from one part of the tree to another. If
+		 * it was safe to dereference the old pointer to it
+		 * (to_free->slots[0]), it will be safe to dereference the new
+		 * one (root->rnode).
+		 */
+		newptr = to_free->slots[0];
+		if (root->height == 1)
+			newptr = radix_tree_ptr_to_direct(newptr);
+		root->rnode = newptr;
 		root->height--;
 		/* must only free zeroed nodes into the slab */
 		tag_clear(to_free, 0, 0);
@@ -767,6 +883,7 @@
 {
 	struct radix_tree_path path[RADIX_TREE_MAX_PATH], *pathp = path;
 	struct radix_tree_node *slot = NULL;
+	struct radix_tree_node *to_free;
 	unsigned int height, shift;
 	int tag;
 	int offset;
@@ -777,6 +894,7 @@
 
 	slot = root->rnode;
 	if (height == 0 && root->rnode) {
+		slot = radix_tree_direct_to_ptr(slot);
 		root_tag_clear_all(root);
 		root->rnode = NULL;
 		goto out;
@@ -809,10 +927,17 @@
 			radix_tree_tag_clear(root, index, tag);
 	}
 
+	to_free = NULL;
 	/* Now free the nodes we do not need anymore */
 	while (pathp->node) {
 		pathp->node->slots[pathp->offset] = NULL;
 		pathp->node->count--;
+		/*
+		 * Queue the node for deferred freeing after the
+		 * last reference to it disappears (set NULL, above).
+		 */
+		if (to_free)
+			radix_tree_node_free(to_free);
 
 		if (pathp->node->count) {
 			if (pathp->node == root->rnode)
@@ -821,13 +946,15 @@
 		}
 
 		/* Node with zero slots in use so free it */
-		radix_tree_node_free(pathp->node);
-
+		to_free = pathp->node;
 		pathp--;
+
 	}
 	root_tag_clear_all(root);
 	root->height = 0;
 	root->rnode = NULL;
+	if (to_free)
+		radix_tree_node_free(to_free);
 
 out:
 	return slot;