Ubuntu Servers :: Process Hangs While Cleaning Cache?

I have a NFS server and I'm periodically cleaning the cache by using the following lines:

Code:

/bin/sync; /bin/echo 1 > /proc/sys/vm/drop_caches
/bin/sync; /bin/echo 2 > /proc/sys/vm/drop_caches

during the last days the cleaning process gets stucked at '/bin/echo 2', here's the process data:

Code:

27426 root 20 0 5488 704 576 R 100 0.0 896:06.34 /bin/echo 2

the process keeps running, so I find this more strange since it's not in zombie or D state.