cs460-pa1

freelist.mru.c

/*
 * CS460
 *
 * This will put a buffer into MRU list. This function should be only called by 
 * unpinning a buffer. Since it is MRU, we always put at head position. This
 * is consistent with StrategyGetBuffer where we pick also at head position.
 * This is not the same as InvalidateBuffer where a buffer is surely not useful
 * any more and should be put into free buffer instead.
 * 
 * MRU是将buf插入头部，LRU是将buf插入尾部
 */
void StrategyMRUEnqueue(volatile BufferDesc *buf) 
{
    	// 如果buf已经存在于list，则返回
	if (buf->mruNext != MRU_NOT_IN_LIST) 
	{
		return;
	}
	
	// link to prev
	buf->mruPrev = MRUPREV_HEAD_OF_LIST;
	
	// link to next
	buf->mruNext = StrategyControl->mruHead;
	StrategyControl->mruHead = buf->buf_id;
	
	// 若list不为空
	if (buf->mruNext != MRUNEXT_END_OF_LIST) 
	{
	    	// 确认list head指向的buf的mruPrev指向list head
		Assert((BufferDescriptors + buf->mruNext)->mruPrev == MRUPREV_HEAD_OF_LIST);
		// 将list head的mruPrev指向需要enque的buf
		(BufferDescriptors + buf->mruNext)->mruPrev = buf->buf_id;
	}
}


/*
 * CS460
 */
void StrategyMRUDequeue(volatile BufferDesc *buf)
{
	/* if it is already dequeued. this may happen if buf comes from free list */
	// 所有被deque过的buffer都会指向MRU_NOT_IN_LIST (e.g., line 68, 69)
	if (buf->mruNext == MRU_NOT_IN_LIST) 
	{
		return;
	}
	
	// StrategyControl的mruHead已经mruTail分别指向buffer list的头部和尾部
	// Assert用来debug，如果mruHead指向MRUNEXT_END_OF_LIST，表示这个list里面已经没有buf可以deque
	Assert(StrategyControl->mruHead != MRUNEXT_END_OF_LIST);
		
	/* unlink from prev */
	if (buf->mruPrev == MRUPREV_HEAD_OF_LIST)
		StrategyControl->mruHead = buf->mruNext;
	else  
	    	// 这行代码我在piazza上问过，你可以去看
		(BufferDescriptors + buf->mruPrev)->mruNext = buf->mruNext;
		
	/* unlink from next */	
	if (buf->mruNext == MRUNEXT_END_OF_LIST)
		StrategyControl->mruTail = buf->mruPrev;
	else
		(BufferDescriptors + buf->mruNext)->mruPrev = buf->mruPrev;
	
	// buf已经被deque，所以放入MRU_NOT_IN_LIST
	buf->mruPrev = MRU_NOT_IN_LIST;
	buf->mruNext = MRU_NOT_IN_LIST;
}