The annoying thing about locks (at least the variant that waits) is not just that you have to enter the kernel and wait when the lock is not available (fair enough), but also that the current holder will have to wake you, which requires another dip into the kernel by the holder.

I have been thinking on and off on how to create a syscall-less wake operation. One way to get almost what you want is to have a polling io_uring. That still requires one kernel thread that busy polls per application. Maybe this is fine in some application architectures but it's not ideal.

It would be nice if there was a way to use Intel's debug registers to write a value to some address, which would then interrupt some kernel task, allowing that kernel task to somehow figure out what futex to wake, without the interrupter having to enter the kernel.