btrfs: make inodes hold a ref on their roots

If we make sure all the inodes have refs on their root we don't have to
worry about the root disappearing while we have open inodes.
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/disk-io.c

@@ -2096,7 +2096,7 @@ static void btrfs_init_btree_inode(struct btrfs_fs_info *fs_info)
 
 	BTRFS_I(inode)->io_tree.ops = &btree_extent_io_ops;
 
-	BTRFS_I(inode)->root = fs_info->tree_root;
+	BTRFS_I(inode)->root = btrfs_grab_root(fs_info->tree_root);
 	memset(&BTRFS_I(inode)->location, 0, sizeof(struct btrfs_key));
 	set_bit(BTRFS_INODE_DUMMY, &BTRFS_I(inode)->runtime_flags);
 	btrfs_insert_inode_hash(inode);

fs/btrfs/inode.c

@@ -5235,7 +5235,8 @@ static int btrfs_init_locked_inode(struct inode *inode, void *p)
 	inode->i_ino = args->location->objectid;
 	memcpy(&BTRFS_I(inode)->location, args->location,
 	       sizeof(*args->location));
-	BTRFS_I(inode)->root = args->root;
+	BTRFS_I(inode)->root = btrfs_grab_root(args->root);
+	BUG_ON(args->root && !BTRFS_I(inode)->root);
 	return 0;
 }
 
@@ -5316,7 +5317,7 @@ static struct inode *new_simple_dir(struct super_block *s,
 	if (!inode)
 		return ERR_PTR(-ENOMEM);
 
-	BTRFS_I(inode)->root = root;
+	BTRFS_I(inode)->root = btrfs_grab_root(root);
 	memcpy(&BTRFS_I(inode)->location, key, sizeof(*key));
 	set_bit(BTRFS_INODE_DUMMY, &BTRFS_I(inode)->runtime_flags);
 
@@ -5884,7 +5885,7 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
 	 */
 	BTRFS_I(inode)->index_cnt = 2;
 	BTRFS_I(inode)->dir_index = *index;
-	BTRFS_I(inode)->root = root;
+	BTRFS_I(inode)->root = btrfs_grab_root(root);
 	BTRFS_I(inode)->generation = trans->transid;
 	inode->i_generation = BTRFS_I(inode)->generation;
 
@@ -8926,6 +8927,7 @@ void btrfs_destroy_inode(struct inode *inode)
 	inode_tree_del(inode);
 	btrfs_drop_extent_cache(BTRFS_I(inode), 0, (u64)-1, 0);
 	btrfs_inode_clear_file_extent_range(BTRFS_I(inode), 0, (u64)-1);
+	btrfs_put_root(BTRFS_I(inode)->root);
 }
 
 int btrfs_drop_inode(struct inode *inode)