This is one of the buglets in the current code - this is what happens
when you do a "int $0x2" - the code thinks it's a valid (external) NMI.
Without the fix it would have generated a #GP fault (which linux turns
into a segmentation fault), because the IDT entry for #2 would have been
marked as privileged - while the new page fault handler has no
reasonable way of finding out what's up...
Perfecting the patch is going to take some time - the current one
essentially just fixes the lock-up.
Linus