[patch] radix-tree: fix small lockless radix-tree bug

[Nick Piggin]

Hi guys,

Although this doesn't seem like cause for alarm (as per the analysis),
it may still be a good 2.6.26 candidate as we should have a few more
weeks of testing left.

It should definitely go in -mm with the lockless pagecache patch.

Thanks,
Nick
When shrinking a radix-tree, we do it in a lockless manner by atomically
switching the root pointer away from the redundant node (one that only
has a single entry in the left most slot), and switching it over to its
lone child.

Because a lockless lookup may have got a reference to the parent and be
in the middle of deciding what to do with it while it is being swapped
away for its child. For this reason, we also have to keep it around and
in a valid state for the lookup to proceed and give a valid result, for
at least an RCU grace period. So we need to keep the child in the left
most slot there in case that is requested by the lookup.

This is all pretty standard RCU stuff. It is worth repeating because
in my eagerness to obey the radix tree node constructor scheme, I had
broken this by zeroing the radix tree node before the grace period.

Fix it by clearing those fields in the RCU callback. I would normally
want to rip out the constructor entirely, but radix tree nodes are one
of those places where they make sense (only few cachelines will be
touched soon after allocation).


This was never actually observed in any lockless pagecache testing or
using the test harness, but as a rare problem testing my scalable vmap
rewrite.

Fortunately, it is not a problem anywhere lockless pagecache is used in
mainline kernels (pagecache probe is not a guarantee, and brd does not
have concurrent lookups and deletes).

However, it would eventually pop up for someone using lockless pagecache :P

Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
---
Index: linux-2.6/lib/radix-tree.c
===================================================================
--- linux-2.6.orig/lib/radix-tree.c	2008-06-13 04:26:31.000000000 +1000
+++ linux-2.6/lib/radix-tree.c	2008-06-13 04:31:38.000000000 +1000
@@ -88,6 +88,57 @@ static inline gfp_t root_gfp_mask(struct
 	return root->gfp_mask & __GFP_BITS_MASK;
 }
 
+static inline void tag_set(struct radix_tree_node *node, unsigned int tag,
+		int offset)
+{
+	__set_bit(offset, node->tags[tag]);
+}
+
+static inline void tag_clear(struct radix_tree_node *node, unsigned int tag,
+		int offset)
+{
+	__clear_bit(offset, node->tags[tag]);
+}
+
+static inline int tag_get(struct radix_tree_node *node, unsigned int tag,
+		int offset)
+{
+	return test_bit(offset, node->tags[tag]);
+}
+
+static inline void root_tag_set(struct radix_tree_root *root, unsigned int tag)
+{
+	root->gfp_mask |= (__force gfp_t)(1 << (tag + __GFP_BITS_SHIFT));
+}
+
+static inline void root_tag_clear(struct radix_tree_root *root, unsigned int tag)
+{
+	root->gfp_mask &= (__force gfp_t)~(1 << (tag + __GFP_BITS_SHIFT));
+}
+
+static inline void root_tag_clear_all(struct radix_tree_root *root)
+{
+	root->gfp_mask &= __GFP_BITS_MASK;
+}
+
+static inline int root_tag_get(struct radix_tree_root *root, unsigned int tag)
+{
+	return (__force unsigned)root->gfp_mask & (1 << (tag + __GFP_BITS_SHIFT));
+}
+
+/*
+ * Returns 1 if any slot in the node has this tag set.
+ * Otherwise returns 0.
+ */
+static inline int any_tag_set(struct radix_tree_node *node, unsigned int tag)
+{
+	int idx;
+	for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++) {
+		if (node->tags[tag][idx])
+			return 1;
+	}
+	return 0;
+}
 /*
  * This assumes that the caller has performed appropriate preallocation, and
  * that the caller has pinned this thread of control to the current CPU.
@@ -124,6 +175,17 @@ static void radix_tree_node_rcu_free(str
 {
 	struct radix_tree_node *node =
 			container_of(head, struct radix_tree_node, rcu_head);
+
+	/*
+	 * must only free zeroed nodes into the slab. radix_tree_shrink
+	 * can leave us with a non-NULL entry in the first slot, so clear
+	 * that here to make sure.
+	 */
+	tag_clear(node, 0, 0);
+	tag_clear(node, 1, 0);
+	node->slots[0] = NULL;
+	node->count = 0;
+
 	kmem_cache_free(radix_tree_node_cachep, node);
 }
 
@@ -165,59 +227,6 @@ out:
 }
 EXPORT_SYMBOL(radix_tree_preload);
 
-static inline void tag_set(struct radix_tree_node *node, unsigned int tag,
-		int offset)
-{
-	__set_bit(offset, node->tags[tag]);
-}
-
-static inline void tag_clear(struct radix_tree_node *node, unsigned int tag,
-		int offset)
-{
-	__clear_bit(offset, node->tags[tag]);
-}
-
-static inline int tag_get(struct radix_tree_node *node, unsigned int tag,
-		int offset)
-{
-	return test_bit(offset, node->tags[tag]);
-}
-
-static inline void root_tag_set(struct radix_tree_root *root, unsigned int tag)
-{
-	root->gfp_mask |= (__force gfp_t)(1 << (tag + __GFP_BITS_SHIFT));
-}
-
-
-static inline void root_tag_clear(struct radix_tree_root *root, unsigned int tag)
-{
-	root->gfp_mask &= (__force gfp_t)~(1 << (tag + __GFP_BITS_SHIFT));
-}
-
-static inline void root_tag_clear_all(struct radix_tree_root *root)
-{
-	root->gfp_mask &= __GFP_BITS_MASK;
-}
-
-static inline int root_tag_get(struct radix_tree_root *root, unsigned int tag)
-{
-	return (__force unsigned)root->gfp_mask & (1 << (tag + __GFP_BITS_SHIFT));
-}
-
-/*
- * Returns 1 if any slot in the node has this tag set.
- * Otherwise returns 0.
- */
-static inline int any_tag_set(struct radix_tree_node *node, unsigned int tag)
-{
-	int idx;
-	for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++) {
-		if (node->tags[tag][idx])
-			return 1;
-	}
-	return 0;
-}
-
 /*
  *	Return the maximum key which can be store into a
  *	radix tree with height HEIGHT.
@@ -930,11 +939,6 @@ static inline void radix_tree_shrink(str
 			newptr = radix_tree_ptr_to_indirect(newptr);
 		root->rnode = newptr;
 		root->height--;
-		/* must only free zeroed nodes into the slab */
-		tag_clear(to_free, 0, 0);
-		tag_clear(to_free, 1, 0);
-		to_free->slots[0] = NULL;
-		to_free->count = 0;
 		radix_tree_node_free(to_free);
 	}
 }