btrfs: record new subvolume in parent dir earlier to avoid dir logging races

Instead of recording that a new subvolume was created in a directory after
we add the entry do the directory, record it before adding the entry. This
is to avoid races where after creating the entry and before recording the
new subvolume in the directory (the call to btrfs_record_new_subvolume()),
another task logs the directory, so we end up with a log tree where we
logged a directory that has an entry pointing to a root that was not yet
committed, resulting in an invalid entry if the log is persisted and
replayed later due to a power failure or crash.

Also state this requirement in the function comment for
btrfs_record_new_subvolume(), similar to what we do for the
btrfs_record_unlink_dir() and btrfs_record_snapshot_destroy().

Fixes: 45c4102 ("btrfs: avoid transaction commit on any fsync after subvolume creation")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ioctl.c

@@ -666,14 +666,14 @@ static noinline int create_subvol(struct mnt_idmap *idmap,
 		goto out;
 	}
 
+	btrfs_record_new_subvolume(trans, BTRFS_I(dir));
+
 	ret = btrfs_create_new_inode(trans, &new_inode_args);
 	if (ret) {
 		btrfs_abort_transaction(trans, ret);
 		goto out;
 	}
 
-	btrfs_record_new_subvolume(trans, BTRFS_I(dir));
-
 	d_instantiate_new(dentry, new_inode_args.inode);
 	new_inode_args.inode = NULL;
 

fs/btrfs/tree-log.c

@@ -7454,6 +7454,8 @@ void btrfs_record_snapshot_destroy(struct btrfs_trans_handle *trans,
  * full log sync.
  * Also we don't need to worry with renames, since btrfs_rename() marks the log
  * for full commit when renaming a subvolume.
+ *
+ * Must be called before creating the subvolume entry in its parent directory.
  */
 void btrfs_record_new_subvolume(const struct btrfs_trans_handle *trans,
 				struct btrfs_inode *dir)