Re: [PATCH 07/13] IB: add a proper completion queue abstraction

From: Sagi Grimberg
Date: Sun Jan 17 2016 - 04:26:15 EST

Hi Christoph, Sagi,

On Tue, Dec 8, 2015 at 2:21 AM, Christoph Hellwig <hch@xxxxxx> wrote:

+static void ib_cq_poll_work(struct work_struct *work)
+ struct ib_cq *cq = container_of(work, struct ib_cq, work);
+ int completed;
+ completed = __ib_process_cq(cq, IB_POLL_BUDGET_WORKQUEUE);
+ if (completed >= IB_POLL_BUDGET_WORKQUEUE ||
+ ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0)
+ queue_work(ib_comp_wq, &cq->work);

In above code, Let says completion is added in a time window where
ib_process_cq is completed (CQ is diarmed in hw at that point) and
ib_req_notify_cq is yet to be called.
Provider vendor driver say mlx4 or mlx5 as specific case always
returns ib_req_notify_cq = 0.
Will it result into a missed notification? (so queue_work is not done).

If we have not drained the CQ (consumed budget or more) the second
condition (ib_req_notify_cq) will not be invoked. We are only rearming
the CQ when we drained it completely. So I don't see how we can end up
with missed notifications.