I recently had some issues with a linux box that had a few processes in a disk wait state. I couldn't do a "reboot" or even a "reboot -f" the system would just stop waiting for the processes to be killed. But this worked...

echo 1 > /proc/sys/kernel/sysrq
echo b > /proc/sysrq-trigger

It was on a CentOS 5.x based system.