You're referring to a Linux Futex, which is a user-space construct. In kernel-level code, for areas of the kernel that must use real spinlocks, they do not fal

You're referring to a Linux Futex, which is a user-space construct. In kernel-level code, for areas of the kernel that must use real spinlocks, they do not fallback to the equivalent of a Futex because that would mean the current thread of execution would be swapped out and kernel-level spinlocks are designed for critical sections that cannot be swapped out. In critical sections of the kernel that can be swapped out, you'd use a Mutex which does the equivalent of a user-level Futex (and swap out) when the Mutex is contended.