Namely, it caused nodes be adding to the pool multiple times. Returned spin back, but use it only in cases node valency is zero. So now valency is decreasing without any locks, then if it's zero spin lock happens, node color (which indicates whether node is scheduled or not) happens. Actual new task creation happens outside of locks. This might sound a bit complicated, but it's straightforward code which is free from any thread synchronization latency.