// until all Ms are observed to be outside of their seqlock critical section.
	//
	// Note: The seqlock is mutated here and also in traceCPUSample. If you update
	// usage of the seqlock here, make sure to also look at what traceCPUSample is
	// doing.
	seq := mp.trace.seqlock.Add(1)
	if debugTraceReentrancy && seq%2 != 1 {
		throw("bad use of trace.seqlock or tracer is reentrant")
	}