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

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Message-ID: <CAG53R5WYHQ+w3R621KqDMLjUo9VBBjDhnJzwwJtfmo3oUWCxUg@mail.gmail.com>
Date:	Fri, 15 Jan 2016 19:24:00 +0530
From:	Parav Pandit <pandit.parav@...il.com>
To:	Christoph Hellwig <hch@....de>
Cc:	linux-rdma@...r.kernel.org, sagig@....mellanox.co.il,
	Bart Van Assche <bart.vanassche@...disk.com>, axboe@...com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/13] IB: add a proper completion queue abstraction

Hi Christoph, Sagi,

On Tue, Dec 8, 2015 at 2:21 AM, Christoph Hellwig <hch@....de> 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).

IB spec says: Any CQ entries that existed before the notify is enabled
will not result in a call to the handler.

I did try to follow this thread on similar notes.
http://www.spinics.net/lists/linux-nfs/msg54266.html

I believe above code possibly needs fix based above two comments? Or I
must be missing something basic here.