...
BSR internally maintains reference count for backing devices. The reference count is incremented when a device is opened by a particular process and decremented when it is closed to manage the lifecycle for the device and to force a particular process to no longer reference the device volume when it is time to clean up its resources.
Problems arise when the reference count is non-zero, either at the beginning of configuring the BSR resource or during the process of umounting the device. Because some process opened the device and never closed it, BSR detects this and treats it as an error.
...