> These are OS primitives I'm talking about - I haven't checked out the standard library version but the parking_lot version uses a spinlock with thread sleep when the wait times get too high - it has no way of getting notified when the mutex gets unblocked nor does it support priority inversion.

Uhh no, everyone in Linux userspace uses futexes these days to wait on a contended lock.

https://github.com/Amanieu/parking_lot/blob/03d36d62fecbd85c...