This is a userspace prototype. This will behave similarly to a userspace
spinlock in that case, which is not great in terms of CPU usage, but
should eventually unblock the waiter, unless it has a RT priority that
really prevents any progress from the emergency slot owner.
On my TODO list, I have a bullet about integrating with sys_futex to
block on wait, wake up on slot release. I would then use the wait/wakeup
code based on sys_futex already present in liburcu.