Unable to unmount a filesystem

Issue

• Using the umount command to unmount a filesystem does not work and shows an error:

  # umount /appl/cmsicacc/project_files
  umount: /appl/cmsicacc/project_files: device is busy
  

  # umount /opt
  umount: /opt: target is busy.
          (In some cases useful info about processes that use
           the device is found by lsof(8) or fuser(1))
  

• Autofs does not work for mount point, and attempting to restart autofs gives the following error in /var/log/messages:

  automount[20340]: do_umount_autofs_direct: ask umount returned busy for /mnt/point