> Reason: locks that have the ability to put the thread to sleep on a queue must do compare-and-swap (or at least an atomic RMW) on `unlock`. But spinlocks can get away with just doing a store-release (or just a store with a compiler fence on X86) to `unlock`.
This is something I've thinking about a lot over time, that the CAS is only there to atomically determine if there are any sleeping waiters on unlock and you have to do a futex_wake. I would really want some way to get away with non-fenced operations (at least on x86), but I don't know if it's just that nobody has figured out why, or there is a fundamental reason why that's not possible.
This is something I've thinking about a lot over time, that the CAS is only there to atomically determine if there are any sleeping waiters on unlock and you have to do a futex_wake. I would really want some way to get away with non-fenced operations (at least on x86), but I don't know if it's just that nobody has figured out why, or there is a fundamental reason why that's not possible.