6.9. Locked System
When a computer "locks", also called "hung", the keyboard and mouse become completely unresponsive. This is a direct consequence of a bug in the Linux kernel. While Linux is known for stability, these things do happen, especiallly for gaming which entails highly synchronized hardware events which occur very fast, even to a computer. When a computer locks, it can be a "hard lock", meaning the kernel has completely stopped functioning. This often indicates misbehaving or faulty hardware. There's no recovery from this kind of lock other than pressing the reset or power button. The lock can also be a "soft lock", meaning that the kernel is still functioning in some capacity. It's possible to recover from this gracefully.
The first thing you should try is to hit control-alt-backspace which kills X. If you gain control of your system, the kernel wasn't really locked in the first place. If this didn't work after a few seconds, you'll definitely want to reboot the system using the following instructions.
Use control-alt-delete to reboot the system. You'll know this worked if you hear the computer beep after a few seconds (this is BIOS saying "I'm OK" during a power on cycle).
Log into another system and ssh into the hung system. If you can ssh in, reboot or halt the system.
If you can't ssh into the system, you'll need to use the "magic SysRq key" which is documented in /usr/src/linux/Documentation/sysrq.txt . Here's a summary for the x86 architecture (see the documentation for other architectures). Note if your keyboard doesn't have a SysRq key, use the PrintScreen key:
Hit alt-SysRq-s . This will attempt to sync your mounted filesystems so that changes to files get flushed to disk. You may hear disk activity. If you're looking at a console, the system should print the devices which were flushed.
A few seconds later, hit alt-SysRq-u . This will attempt to remount all your mounted filesystems as read-only). You should hear disk activity. If you're looking at a console, the system will print the devices which were remounted.
A few seconds later, use alt-SysRq-b to reboot the system.
You can hit alt-SysRq-h for a very terse help screen.
To use the magic SysRq key, your kernel needs to have been compiled with magic SysRq support. You'll find this option under "Kernel Hacking | Kernel Debugging | Magic SysRq key " in whatever kernel config menu you like to use. If the magic SysRq key sequence doesn't shut your system down gracefully, your kernel has crashed hard and you'll need to use the reset or power button to recover.
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