struct rwlock {
	int write;
	int read;
};

static inline void
rwlock_init(struct rwlock *lock) {
	lock->write = 0;
	lock->read = 0;
}

static inline void
rwlock_rlock(struct rwlock *lock) {
	for (;;) {
		// isuued a full memory barrier. This typically means that operations issued 
		// prior to the barrier are guaranteed to be performed before operations issued after the barrier.
		while(lock->write) {
			__sync_synchronize();
		}
		__sync_add_and_fetch(&lock->read,1);
		// 在给nreaders + 1 之后再次检查是否有写入者，有的话此次读锁请求失败
		if (lock->write) {
			__sync_sub_and_fetch(&lock->read,1);
		} else {
			break;
		}
	}
}

static inline void
rwlock_wlock(struct rwlock *lock) {
	// 如果没有写者，__sync_lock_test_and_set会返回0，表示此次请求写锁成功；
	// 否则表示有其它写者，则空转
	while (__sync_lock_test_and_set(&lock->write,1)) {}
	// 在开始写入之前发现有读者进入，则要等到前面的操作完成
	while(lock->read) {
		__sync_synchronize();
	}
}

static inline void
rwlock_wunlock(struct rwlock *lock) {
	__sync_lock_release(&lock->write);
}

static inline void
rwlock_runlock(struct rwlock *lock) {
	__sync_sub_and_fetch(&lock->read,1);
}