btrfs: introduce BTRFS_NESTING_NEW_ROOT for adding new roots

The way we add new roots is confusing from a locking perspective for
lockdep.  We generally have the rule that we lock things in order from
highest level to lowest, but in the case of adding a new level to the
tree we actually allocate a new block for the root, which makes the
locking go in reverse.  A similar issue exists for snapshotting, we cow
the original root for the root of a new tree, however they're at the
same level.  Address this by using BTRFS_NESTING_NEW_ROOT for these
operations.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -198,7 +198,8 @@ int btrfs_copy_root(struct btrfs_trans_handle *trans,
 		btrfs_node_key(buf, &disk_key, 0);
 
 	cow = btrfs_alloc_tree_block(trans, root, 0, new_root_objectid,
-			&disk_key, level, buf->start, 0, BTRFS_NESTING_NORMAL);
+				     &disk_key, level, buf->start, 0,
+				     BTRFS_NESTING_NEW_ROOT);
 	if (IS_ERR(cow))
 		return PTR_ERR(cow);
 
@@ -3407,7 +3408,7 @@ static noinline int insert_new_root(struct btrfs_trans_handle *trans,
 
 	c = alloc_tree_block_no_bg_flush(trans, root, 0, &lower_key, level,
 					 root->node->start, 0,
-					 BTRFS_NESTING_NORMAL);
+					 BTRFS_NESTING_NEW_ROOT);
 	if (IS_ERR(c))
 		return PTR_ERR(c);
 

fs/btrfs/locking.h

@@ -61,6 +61,15 @@ enum btrfs_lock_nesting {
 	 */
 	BTRFS_NESTING_SPLIT,
 
+	/*
+	 * When promoting a new block to a root we need to have a special
+	 * subclass so we don't confuse lockdep, as it will appear that we are
+	 * locking a higher level node before a lower level one.  Copying also
+	 * has this problem as it appears we're locking the same block again
+	 * when we make a snapshot of an existing root.
+	 */
+	BTRFS_NESTING_NEW_ROOT,
+
 	/*
 	 * We are limited to MAX_LOCKDEP_SUBLCLASSES number of subclasses, so
 	 * add this in here and add a static_assert to keep us from going over