Importantly, prior to this (and, hell, even since) state of the art was to try atomically, then yield (or sleep(0)) then try usleep. The kernel had no idea wha

Importantly, prior to this (and, hell, even since) state of the art was to try atomically, then yield (or sleep(0)) then try usleep. The kernel had no idea what was going on, so had no idea how to schedule such a thing. It particularly didn't know to wake you when the lock (which it has no idea about) became available.