I fail to reconnect the machine via ssh as usual after I reboot it. Then I check the machine physically, and find the reason:

LNetError: 131-3: Received notification of device removal
Please shutdown LNET to allow this to proceed
INFO: task reboot: 30396 blocked for more than 120 seconds.

Please shutdown LNET to allow this to proceed

It seems that we need shutdown LNET(Lustre service) before we execute the reboot command. So I run the below commands to shutdown/stop Lustre service.

# umount /lustre
# lustre_rmmod

Now, everything is OK.