Btrfs: eliminate the exceptional root_tree refs=0

The fact that btrfs_root_refs() returned 0 for the tree_root caused
bugs in the past, therefore it is set to 1 with this patch and
(hopefully) all affected code is adapted to this change.

I verified this change by temporarily adding WARN_ON() checks
everywhere where btrfs_root_refs() is used, checking whether the
logic of the code is changed by btrfs_root_refs() returning 1
instead of 0 for root->root_key.objectid == BTRFS_ROOT_TREE_OBJECTID.
With these added checks, I ran the xfstests './check -g auto'.

The two roots chunk_root and log_root_tree that are only referenced
by the superblock and the log_roots below the log_root_tree still
have btrfs_root_refs() == 0, only the tree_root is changed.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/disk-io.c

@@ -2670,6 +2670,7 @@ int open_ctree(struct super_block *sb,
 
 	btrfs_set_root_node(&tree_root->root_item, tree_root->node);
 	tree_root->commit_root = btrfs_root_node(tree_root);
+	btrfs_set_root_refs(&tree_root->root_item, 1);
 
 	location.objectid = BTRFS_EXTENT_TREE_OBJECTID;
 	location.type = BTRFS_ROOT_ITEM_KEY;

fs/btrfs/inode-map.c

@@ -412,8 +412,7 @@ int btrfs_save_ino_cache(struct btrfs_root *root,
 		return 0;
 
 	/* Don't save inode cache if we are deleting this root */
-	if (btrfs_root_refs(&root->root_item) == 0 &&
-	    root != root->fs_info->tree_root)
+	if (btrfs_root_refs(&root->root_item) == 0)
 		return 0;
 
 	if (!btrfs_test_opt(root, INODE_MAP_CACHE))

fs/btrfs/inode.c

@@ -4472,7 +4472,9 @@ void btrfs_evict_inode(struct inode *inode)
 	trace_btrfs_inode_evict(inode);
 
 	truncate_inode_pages(&inode->i_data, 0);
-	if (inode->i_nlink && (btrfs_root_refs(&root->root_item) != 0 ||
+	if (inode->i_nlink &&
+	    ((btrfs_root_refs(&root->root_item) != 0 &&
+	      root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID) ||
 	     btrfs_is_free_space_inode(inode)))
 		goto no_delete;
 
@@ -4490,7 +4492,8 @@ void btrfs_evict_inode(struct inode *inode)
 	}
 
 	if (inode->i_nlink > 0) {
-		BUG_ON(btrfs_root_refs(&root->root_item) != 0);
+		BUG_ON(btrfs_root_refs(&root->root_item) != 0 &&
+		       root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID);
 		goto no_delete;
 	}
 
@@ -4731,14 +4734,7 @@ static void inode_tree_del(struct inode *inode)
 	}
 	spin_unlock(&root->inode_lock);
 
-	/*
-	 * Free space cache has inodes in the tree root, but the tree root has a
-	 * root_refs of 0, so this could end up dropping the tree root as a
-	 * snapshot, so we need the extra !root->fs_info->tree_root check to
-	 * make sure we don't drop it.
-	 */
-	if (empty && btrfs_root_refs(&root->root_item) == 0 &&
-	    root != root->fs_info->tree_root) {
+	if (empty && btrfs_root_refs(&root->root_item) == 0) {
 		synchronize_srcu(&root->fs_info->subvol_srcu);
 		spin_lock(&root->inode_lock);
 		empty = RB_EMPTY_ROOT(&root->inode_tree);
@@ -7857,8 +7853,7 @@ int btrfs_drop_inode(struct inode *inode)
 		return 1;
 
 	/* the snap/subvol tree is on deleting */
-	if (btrfs_root_refs(&root->root_item) == 0 &&
-	    root != root->fs_info->tree_root)
+	if (btrfs_root_refs(&root->root_item) == 0)
 		return 1;
 	else
 		return generic_drop_inode(inode);