Great that you had the time to be curious and dig into what was going on. QEMU is quite an amazing tool.

I'm kind of surprised there isn't a fairly robust kernel test around this issue, since it locks the machine down and I think the fix was to prevent a stuck CPU last time as well?

It's also vaguely surprising that this hasn't been encountered more often, particularly by the https://news.ycombinator.com/user?id=everlier talking in links to this HN post about "20-30 containers" running simultaneously and occasionally locking up the machine.

If you're still thinking about the bug a little, you could look over how other kernel tests work and implement a failing test around it....?

I imagine the tests have some way of detecting a locked up kernel... I don't know exactly how they'd do it, but they probably have a technique. Most likely since the kernel is literally in a loop it won't respond to anything.. so starting any process, something as simple as creating any process, even one as simple as printing "Hello World!!" would fail and indicate the machine is locked.

Perhaps this is one of those cases where something like UserModeLinux would allow a test to be easily put together, rather than spawning complete VMs via some kind of VM software. Again, would be interesting to know what Linux does with this kind of test.