Unable to unmount installation DVD in %post section of kickstart installation in Red Hat Enterprise Linux 7

Issue

• We want to install our software in the %post section of kickstart during Red Hat Enterprise Linux 7 installation.
• Our software is in another disc, and we wanted to unmount the installation DVD in %post section. But it failed with the following error.

umount: /run/install/repo: target is busy.

• This worked well in Red Hat Enterprise Linux 6, but I have found no way to do this properly in Red Hat Enterprise Linux 7.