code: func (b *storageosUnmounter) TearDownAt(dir string) error {
  	if err := mount.CleanupMountPoint(dir, b.mounter, false); err != nil {
  		klog.V(4).Infof("Unmounted StorageOS volume %s failed with: %v", b.pvName, err)
  	}
  	if err := b.manager.UnmountVolume(b); err != nil {
  		klog.V(4).Infof("Mount reference for volume %s could not be removed from StorageOS: %v", b.pvName, err)
  	}
  	return nil
  }
docs: // Unmounts the bind mount, and detaches the disk only if the PD
  // resource was the last reference to that disk on the kubelet.