[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.*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.centos.org/pipermail/centos/attachments/20101208/ea5f9c03/attachment.html>