umount device failed

excute command umount device failed.

#umount /home/betty/nfs

Umount: /home/betty/nfs: device is busy. 
solve way:
This is because the device is used by other program now, so you must kill this procdure at first
.How to find this program
use command 
#fuser -m /home/betty/nfs
会显示使用目录的进程
#kill pid
now it is ok!