Why Isn’t REISUB Working on Debian?

debiankernelreboot

I'm using Debian 'Jessie'.
Sometimes my computer freezes, and then I can't use Ctrl+Alt+Del to reboot, Ctrl+Alt+Backspace to kill the X Window System nor Ctrl+Alt+F1 to open a new shell. I've read in several sites that in a computer freeze you can use the basic kernel commands that are used pressing Alt+Sysreq (holding Alt+Sysreq and pressing REISUB one key)

But in my computer that 'trick' isn't working when it's frozen. Has the kernel frozen as well? I heard that one of the best things of Linux was that you never had to turn off the computer by holding the power button, but It's not being true for me :/

Best Answer

Magic keys tend to be disabled in Debian these days, so you can't just hard-reboot your machine or kill all your X processes by pressing a few keys accidentally.

The X Ctrl+Alt+Backspace key sequence is controlled by the "DontZap" option in /etc/X11/xorg.conf -- man xorg.conf for more details. I think you want this, though:

Section "ServerFlags"
    Option "DontZap" "false"
EndSection

The sysreq keys are controlled by the kernel options during kernel compile time, boot time, and also sysctl options. To enable it on Debian, put 

kernel.sysrq=1

into /etc/sysctl.conf, and either reload that file (sysctl -p /etc/sysctl.conf; man sysctl for more), or just edit the file and reboot.

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