Forwarding from http://launchpad.net/bugs/1789922
In short, if you try to resize a partition it fails saying it can't unmount the volume, even though it is not currently mounted. So far I have traced the issue to unuse_data_iterate(). It calls gdu_utils_is_in_use_full(), which incorrectly identifies the mount point of the filesystem as "/". So it looks like either it is finding the wrong mount point for the filesystem and trying to unmount the root ( which you obviously can't do ), or unuse_data_iterate() is actually being called on the root filesystem instead of the one you are trying to resize. Unfortunately I can't tell which since almost all of the data this code is working with gdb tells me has an incomplete type.
This happens with an ext4 filesystem either inside a partitioned loop device, or on a scsi_debug disk.