A weird KASAN problem that Zygo reported could have been easily caught
if we checked for basic things in our backref freeing code.  We have two
methods of freeing a backref node

- btrfs_backref_free_node: this just is kfree() essentially.
- btrfs_backref_drop_node: this actually unlinks the node and cleans up
  everything and then calls btrfs_backref_free_node().

We should mostly be using btrfs_backref_drop_node(), to make sure the
node is properly unlinked from the backref cache, and only use
btrfs_backref_free_node() when we know the node isn't actually linked to
the backref cache.  We made a mistake here and thus got the KASAN splat.
Make this style of issue easier to find by adding some ASSERT()'s to
btrfs_backref_free_node() and adjusting our deletion stuff to properly
init the list so we can rely on list_empty() checks working properly.

Link: https://lore.kernel.org/linux-btrfs/20201208194607.GI31381@hungrycats.org/ (local)
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/backref.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index ff705cc564a9..17abde7f794c 100644
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h

@@ -296,6 +296,9 @@ static inline void btrfs_backref_free_node(struct btrfs_backref_cache *cache,
 					   struct btrfs_backref_node *node)
 {
 	if (node) {
+		ASSERT(list_empty(&node->list));
+		ASSERT(list_empty(&node->lower));
+		ASSERT(node->eb == NULL);
 		cache->nr_nodes--;
 		btrfs_put_root(node->root);
 		kfree(node);

@@ -340,11 +343,11 @@ static inline void btrfs_backref_drop_node_buffer(
 static inline void btrfs_backref_drop_node(struct btrfs_backref_cache *tree,
 					   struct btrfs_backref_node *node)
 {
-	BUG_ON(!list_empty(&node->upper));
+	ASSERT(list_empty(&node->upper));
 
 	btrfs_backref_drop_node_buffer(node);
-	list_del(&node->list);
-	list_del(&node->lower);
+	list_del_init(&node->list);
+	list_del_init(&node->lower);
 	if (!RB_EMPTY_NODE(&node->rb_node))
 		rb_erase(&node->rb_node, &tree->rb_root);
 	btrfs_backref_free_node(tree, node);

-- 
2.26.2