RDF_OPENING
while ((sc->sc_flags & (RDF_OPENING | RDF_CLOSING)) != 0)
sc->sc_flags |= RDF_OPENING;
sc->sc_flags &= ~RDF_OPENING;
while (sc->sc_flags & (RDF_OPENING | RDF_CLOSING))
sc->sc_flags |= RDF_OPENING;
sc->sc_flags &= ~RDF_OPENING;