rumpuser_mutex_tryenter
if (rumpuser_mutex_tryenter(mtx) != 0) {
while (rumpuser_mutex_tryenter(mtx) != 0)
rv = rumpuser_mutex_tryenter(mtx);
int rumpuser_mutex_tryenter(struct rumpuser_mtx *);
if (rumpuser_mutex_tryenter(rump_giantlock) != 0) {
error = rumpuser_mutex_tryenter(RUMPMTX(mtx));