[CentOS] Problem with centos nfsroot

I made a diskless node centos. the pxelinux.cfg/default file : default centos label centos kernel vmlinuz-2.6.18-194.el5 append initrd=initrd-2.6.18-194.el5.nfs.img nfsroot=10.10.10.1:/srv/centos ip=dhcp rw

I can successfull*y boot-strap the system with no error.* *But when i shutdown or reboot it . It will show in the screen :* *Unmounting NFS filesystems* *INIT:no more process left in this rulevel* * * * * *and then ,it just hang.* * * *i think it is because it umounted the nfsroot filesystem and can't do the things behind. but I really have no idea about how to resolve that.* *Any help is granted. thanks.*