Hand-waving about a phased sleep

Actually, what Peter Zijlstra has done (and we do in the -rt patch) is not to spin on any atomic operations. We simply spin and check a variable. For instance, when we fail to get the lock, we go into a loop, checking the lock owner, and if that owner is still running. No atomic operations involved. There are a few races in different implementations, but none of those races are detrimental. A missed race just means we may sleep when we could have gotten the lock (current behavior). When the lock owner changes, or the current owner goes to sleep, we try to retake the lock. If the owner is asleep, we then schedule.

In the -rt patch, we are a bit more conservative and avoid these races, but we still do not spin on any atomics. Thus, we do not slow down the bus traffic of other CPUS.