rumpuser_rw_tryenter
if (rumpuser_rw_tryenter(enum_rumprwlock, rw) != 0) {
while (rumpuser_rw_tryenter(enum_rumprwlock, rw) != 0)
int rumpuser_rw_tryenter(int, struct rumpuser_rw *);
error = rumpuser_rw_tryenter(krw2rumprw(op), RUMPRW(rw));