code: func (c *portworxVolumeUnmounter) TearDownAt(dir string) error {
  	klog.Infof("Portworx Volume TearDown of %s", dir)
  
  	if err := c.manager.UnmountVolume(c, dir); err != nil {
  		return err
  	}
  
  	// Call Portworx Detach Volume.
  	if err := c.manager.DetachVolume(c); err != nil {
  		return 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.