while ((r=pthread_rwlock_trywrlock(rw))==EBUSY) {
if (!(r=rw->_rw_lock)) continue;
t = r | 0x80000000;
a_inc(&rw->_rw_waiters);
a_cas(&rw->_rw_lock, r, t);
while ((r=pthread_rwlock_trywrlock(rw))==EBUSY) {
if (!(r=rw->_rw_lock)) continue;
t = r | 0x80000000;
a_inc(&rw->_rw_waiters);
a_cas(&rw->_rw_lock, r, t);