Gazebo won't shutdown after killing gazebo node

Hi, I've encountered weird problem with gazebo. I used to be able to kill gazebo using `rosnode kill -a` command. But now the gzserver only prints [ WARN] [1498834601.073564298, 17.450000000]: Shutdown request received. [ WARN] [1498834601.073614955, 17.450000000]: Reason given for shutdown: [user request] but won't shutdown anymore. Both gzserver and gzclient are still running. Another weird thing is that I can't even see `gazebo_gui` node with `rosnode list` command, but `gzclient` is running... I'm using gazebo 7.6 with ubuntu 16.04 and ros kinetic.