As a last resort on a locked-up linux box you might hold down the power button to restart it. This is not great as the discs will still be mounted.
A safer way is to hold down Atl-SysReq while slowly typing “REISUB”. This will reboot the box safely.
What it does:
- R: Switch the keyboard from raw mode to XLATE mode
- E: Send the SIGTERM signal to all processes except init
- I: Send the SIGKILL signal to all processes except init
- S: Sync all mounted filesystems
- U: Remount all mounted filesystems in read-only mode
- B: Immediately reboot the system, without unmounting partitions or syncing
You can also type “REISUO” to power off, rather than rebooting.