I was coding while a background update ran in parallel. The past few days I'd been pulling all-nighters, so I hadn't noticed anything off. Today I rebooted the machine and realized something was wrong. Shortly after the kernel booted, the console kept spewing debug messages — something that had never happened before. After booting, whenever I worked at the console, debug messages would pop up now and then. That much was still tolerable. What was worse, whether I ran shutdown now or reboot, debug messages would flash by saying kill all process .. unmount ..., and finally it would freeze at the line

KVM:exiting hardware virtualization

and go no further.

Thinking back over the changes I'd made, they were basically user-level tweaks that should have nothing to do with this kind of failure; the only exception was running yum update. Checking the update history, I found a major change: the kernel had been upgraded to 3.9.2.

I usually enjoy wrestling with these kinds of stubborn issues head-on, but I didn't have the time, so I had to take the easy way out. I modified GRUB and switched the boot kernel back to 3.8. Problem solved.