/*
	 * Keep polling until we either get some packets to read, see
	 * that we got told to break out of the loop, get a fatal error,
	 * or discover that the device went away.
	 *
	 * In non-blocking mode, we must still do one poll() to catch
	 * any pending error indications, but the poll() has a timeout
	 * of 0, so that it doesn't block, and we quit after that one
	 * poll().
	 *
	 * If we've seen an ENETDOWN, it might be the first indication
	 * that the device went away, or it might just be that it was
	 * configured down.  Unfortunately, there's no guarantee that
	 * the device has actually been removed as an interface, because:
	 *
	 * 1) if, as appears to be the case at least some of the time,
	 * the PF_PACKET socket code first gets a NETDEV_DOWN indication
	 * for the device and then gets a NETDEV_UNREGISTER indication
	 * for it, the first indication will cause a wakeup with ENETDOWN
	 * but won't set the packet socket's field for the interface index
	 * to -1, and the second indication won't cause a wakeup (because
	 * the first indication also caused the protocol hook to be
	 * unregistered) but will set the packet socket's field for the
	 * interface index to -1;
	 *
	 * 2) even if just a NETDEV_UNREGISTER indication is registered,
	 * the packet socket's field for the interface index only gets
	 * set to -1 after the wakeup, so there's a small but non-zero
	 * risk that a thread blocked waiting for the wakeup will get
	 * to the "fetch the socket name" code before the interface index
	 * gets set to -1, so it'll get the old interface index.
	 *
	 * Therefore, if we got an ENETDOWN and haven't seen a packet
	 * since then, we assume that we might be waiting for the interface
	 * to disappear, and poll with a timeout to try again in a short
	 * period of time.  If we *do* see a packet, the interface has
	 * come back up again, and is *definitely* still there, so we
	 * don't need to poll.
	 */