struct rwlock {
	int reader;
	struct lock read_lock;
	struct lock write_lock;
};

void rlock(struct rwlock *lock) {
	lock(&lock->read_lock);
	lock->reader += 1;
	if(lock->reader == 1) 		// 第一个Reader
		lock(&lock->write_lock);
	unlock(&lock->read_lock);
}
void unrlock(struct rwlock *lock) {
	lock(&lock->read_lock);
	lock->reader -= 1;
	if(lock->reader == 0) 		// 没有Reader了
		unlock(&lock->write_lock);
	unlock(&lock->read_lock);
}
void wlock(struct rwlock *lock) {
	lock(&lock->write_lock);
}
void wunlock(struct rwlock *lock) {
	unlock(&lock->write_lock);
}