func forEachP(reason waitReason, fn func(*p)) {
	systemstack(func() {
		gp := getg().m.curg
		// Mark the user stack as preemptible so that it may be scanned.
		// Otherwise, our attempt to force all P's to a safepoint could
		// result in a deadlock as we attempt to preempt a worker that's
		// trying to preempt us (e.g. for a stack scan).
		//
		// N.B. The execution tracer is not aware of this status