identify the race condition in this code and win the respect of linux-rdma developers!

I have a ping-pong test application that loops doing: send, wait for send completion, wait for receive completion.  The test occasionally hangs in the following code at ibv_get_cq_event() (error handling removed):

Case 1:
	ret = ibv_poll_cq(id->recv_cq, 1, wc);
	ret = ibv_req_notify_cq(id->recv_cq, 0);

	while (!(ret = ibv_poll_cq(id->recv_cq, 1, wc))) {
		ret = ibv_get_cq_event(id->recv_cq_channel, &cq, &context);
		ibv_ack_cq_events(id->recv_cq, 1);
	}

The test does NOT hang if the code is restructured as either of the below: 

Case 2:
	ret = ibv_poll_cq(id->recv_cq, 1, wc);
	ret = ibv_req_notify_cq(id->recv_cq, 0);

	do {
		ret = ibv_get_cq_event(id->recv_cq_channel, &cq, &context);
		ibv_ack_cq_events(id->recv_cq, 1);
	} while (!(ret = ibv_poll_cq(id->recv_cq, 1, wc)));

or
Case 3:
	ret = ibv_poll_cq(id->recv_cq, 1, wc);
	while (1)
		ret = ibv_req_notify_cq(id->recv_cq, 0);

		ret = ibv_poll_cq(id->recv_cq, 1, wc);
		if (ret)
			break;
		ret = ibv_get_cq_event(id->recv_cq_channel, &cq, &context);
		ibv_ack_cq_events(id->recv_cq, 1);
	}

Personally, I would have expected case 2 to hang and cases 1 and 3 to work.  Does anyone see why case 1 should hang?  This is with mlx4.

- Sean
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