xfce4-session stucks in nfsreq and makes heavy nfs traffic

[Kochiro Iwao]

Hi.
I'm in trouble that xfce4-session sometimes stucks in nfsreq.
I cannot kill the xfce4-session even using SIGKILL. It makes heavy
traffic (85Mbit/s average). On my system, /home is mounted over NFS and
the NFS server is built on CentOS. Why cannot I kill the process even
though soft mounted?  How to avoid stucking?

PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
10635 meta      1  53    0   173M 21920K nfsreq  7 400:08 18.36% xfce4-sess

My fstab:
192.168.2.4:/export/home/staff /home/staff nfs rw,soft,retrycnt=60,retrans=100         0       0
192.168.2.4:/export/home/user /home/user nfs rw,soft,retrycnt=60,retrans=100         0       0

XFCE4.8 is installed in my system:
xfce-4.8            The "meta-port" for the XFce 4 desktop environment
xfce4-session-4.8.1 Session manager for the Xfce 4 desktop environment

-- 
kiwao <meta at club.kyutech.ac.jp>