How to kill a process which can't be killed without rebooting?

There are 5 processes which can't be killed by kill -9 $PID and executing cat /proc/$PID/cmdline will hang the current session. Maybe they're zombie processes. Executing ps -ef or htop will also hang the current session. But top and ps -e are working fine. So it seems that there are two problems the filesystem not responding. This is a production machine running virtual machines, so rebooting isn't an option. The following processes ids aren't working: 16181 16765 5985 7427 7547 The parent of these processes is init ├─collectd(16765)─┬─{collectd}(16776) │ ├─{collectd}(16777) │ ├─{collectd}(16778) │ ├─{collectd}(16779) │ ├─{collectd}(16780) │ └─{collectd}(16781) ├─collectd(28642)───{collectd}(28650) ├─collectd(29868)─┬─{collectd}(29873) │ ├─{collectd}(29874) │ ├─{collectd}(29875) │ └─{collectd}(29876) And one of the qemu processes not working |-qemu-system-x86(16181)-+-{qemu-system-x86}(16232) | |-{qemu-system-x86}(16238) | |-{qemu-system-x86}(16803) | |-{qemu-system-x86}(17990) | |-{qemu-system-x86}(17991) | |-{qemu-system-x86}(17992) | |-{qemu-system-x86}(18062) | |-{qemu-system-x86}(18066) | |-{qemu-system-x86}(18072) | |-{qemu-system-x86}(18073) | |-{qemu-system-x86}(18074) | |-{qemu-system-x86}(18078) | |-{qemu-system-x86}(18079) | |-{qemu-system-x86}(18086) | |-{qemu-system-x86}(18088) | |-{qemu-system-x86}(18092) | |-{qemu-system-x86}(18107) | |-{qemu-system-x86}(18108) | |-{qemu-system-x86}(18111) | |-{qemu-system-x86}(18113) | |-{qemu-system-x86}(18114) | |-{qemu-system-x86}(18119) | |-{qemu-system-x86}(23147) | `-{qemu-system-x86}(27051)