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 Post subject: [One][Ice][Two] Regain control of a locked up system
PostPosted: Fri Jul 23, 2010 2:56 pm 
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Joined: Thu May 20, 2010 8:56 am
Posts: 2132
Class: Emergency measures
Difficulty: Straightforward
Time: Thirty seconds
Desirability: Hopefully you'll never need it

Peppermint, like all GNU/Linux distributions, is a complex ecosystem of kernel, low-level subsystems, the X server, and applications. It is inevitable that sometimes things will go wrong and it will crash. A lot of the time the kernel manages to handle this, and you might find an application segfaults or X suddenly restarts. Sometimes, though, everything just seems to lock up.

There is a chance the kernel hasn't panicked, and is still listening to system calls. The following describes how to send those calls to the kernel to safely reboot a locked up system and thereby regain control of it. It is far preferable to use this method rather than performing a "hard reboot" (pushing a reset button) or powering down. With partitions still mounted, the latter methods carry a risk of data loss or corruption, and potentially corrupting initramfs which will effectively make the installed OS not boot again (fixable, but a pain).

Here's the sequence to type:
  1. Hold down alt+sysrq
  2. Type, slowly, in sequence: R E I S U B

There is a very easy way of remembering this sequence - it spells "busier" in reverse. Associate this in mind with a very busy kernel or whatever helps you ;)

This sequence asks the system to do a few things which it normally would (albeit not on an emergency basis) when asked to reboot and power down, including performing an emergency unmount of partitions before the final reboot command. The SysRq key is the one that shares its location with 'Print Screen'. Right-hand alt ("Alt Gr") is fine, and probably easier on the hands. Type the sequence slowly - wait for any disk activity between commands to settle.

It won't work in all cases. Graphics drivers failures, in my experience, cause the kernel to become completely unresponsive and thus the above sequence won't work. Any flashing LEDs on your keyboard or laptop display don't bode well, either. But for those times where it is possible to recover an unresponsive system in this way, this method will safeguard both your data and your installed OS.

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