A small comment after a cursory look: the signaling in CoroTracer.h:162 is susceptible of missed wakeups; you would need a #StoreLoad fence between the slot .seq store and the sleeping flag load (and for the load to be an acquire). The traces probably needs a mirror change.

As the load is relaxed, I assume you are not worried by missed wakeups and it is ok if the tracer is woken up by a latter trace. Of course there might not be a latter trace and you would need a timeout on the sleep.