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If L1 cache has a cache miss it will request data from L2 cache, creating one outgoing read request to fill up the buffer. Now, there might be a cache coherence transaction and the actual data in the L1 cache is dirty and L2 requests L1 to flush it. Suppose another third transaction comes in and if L2 is busy with it and L2 needs to tell L1 about this transaction. This situation is deadlock because L2->L1 queue is full and L1 can't drain that queue because it's waiting for the request in L1->L2 queue to be completed.


I had a misunderstanding here that queues are channels and both L1 and L2 caches have their own buffers. There is no deadlock in such case.

But it is wrong, because queues are buffers here and caches cannot do anything until buffer is ready/empty.


Armed with the knowledge from the following 2 slides, I believe the problem here is that the queues are used for both requests and responses. Both asking for data and responding take a slot in the queue. When both processors are asking for things from one an-otter, there is nowhere to put the response; the queues are full. So processor 2 is waiting for processor 1 to finish servicing the request its working on and clear out the queue, so 2 can put its response to 1's request onto the queue. But processor 1 is waiting for processor 2 to do the same thing. Hence, deadlock.