AFAIK there is no spec. MRI is the reference implementation, but many things are experimental or intentionally unspecified.
Given that MRI ships with a GIL, the only core classes that are intentionally aware of multi-threading concerns are Mutex, ConditionVariable, and Queue.
Given that MRI ships with a GIL, the only core classes that are intentionally aware of multi-threading concerns are Mutex, ConditionVariable, and Queue.