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I'm trying Alt + SysRq + REISUB to see how it would be used to restart my system safely in case of emergency.

However, I find that Alt + SysRq + REISUB hangs at "resetting" (without actually resetting/restarting) in Maverick.

All other SysRq combinations appear to work correctly (i.e. Alt + SysRq + REISU).

cat /proc/sys/kernel/sysrq returns 0. But I'm not sure it's relevant because Alt + SysRq certainly works.

What can I do to have "B" actually restart the system?

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Do you see any console messages when you press those keys? – arrange Mar 19 '11 at 23:51
Yes, that's what I was saying, they appear to work correctly: after each key press (beginning with SysRq+E) I'm seeing status updates from the OS: that it synced the file systems and things like that. Even the last keypress SysRq+B appears to work (it says "... resetting") except for the fact that my laptop does not actually restart (as if the hardware does not receive the command required to reboot). – user12681 Mar 20 '11 at 0:13
Maybe you have ACPI issues. Try booting with nolapic and acpi=off and acpi=bios – Janus Troelsen Apr 21 '12 at 13:45
No, SysRq handlers will sometimes shut off at the unmount, and a hard reset is safe. – hexafraction Aug 10 '12 at 15:31
Does SysRq+O (for Off) work? I used to have a system where that worked, but not reBoot, despite all my efforts. – taneli Aug 17 '12 at 8:24

Alt + SysRq +

R - takes keyboard and mouse control from the X server. This can be useful if the X-Server crashed, you can change to a console and kill the X-Server or check the error log. NOTE:- The documentation refers to this key's task as "Turns off keyboard raw mode and sets it to XLATE", but I suppose it's safe enough to assume that it takes back control from X.

E - sends SIGTERM to all processes except init. This means that an attempt is done to end the current processes except init, safely, e.g. saving a document.

I - sends SIGKILL to all processes except init. This means that all the processes except for init are killed, any data in processes that are killed will be lost.

S - writes all data from the disc cache to the hard-discs, it is a sync and is necessary to reduce the chances of data corruption.

U - remounts all mounted filesystems read-only. After using this key, you can reboot the system with Alt+SysRq+B without harming the system.

B - restarts the system without making steps to ensure that the conditions are good for a safe reboot, using this key alone is like doing a cold reboot.

Try without pressing R at begining of your sequence or just do it in another seq.. give a litle time between those keystrokes and do this..

Alt + SysRq + R S E I U B

and better you enable Magic SysRq by entering..

echo 1 > /proc/sys/kernel/sysrq
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