int get_evtchn_to_irq(evtchn_port_t evtchn)
{
	if (evtchn >= xen_evtchn_max_channels())
		return -1;
	if (evtchn_to_irq[EVTCHN_ROW(evtchn)] == NULL)
		return -1;
	return evtchn_to_irq[EVTCHN_ROW(evtchn)][EVTCHN_COL(evtchn)];
}
The program contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.