linux-kernel - Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend

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Date:   Thu, 15 Jun 2017 10:33:02 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend

On 06/14/2017 05:03 PM, Stefano Stabellini wrote:
> On Mon, 12 Jun 2017, Boris Ostrovsky wrote:
>>> +
>>>  static void pvcalls_back_work(struct work_struct *work)
>>>  {
>>> +	struct pvcalls_fedata *priv = container_of(work,
>>> +		struct pvcalls_fedata, register_work);
>>> +	int notify, notify_all = 0, more = 1;
>>> +	struct xen_pvcalls_request req;
>>> +	struct xenbus_device *dev = priv->dev;
>>> +
>>> +	while (more) {
>>> +		while (RING_HAS_UNCONSUMED_REQUESTS(&priv->ring)) {
>>> +			RING_COPY_REQUEST(&priv->ring,
>>> +					  priv->ring.req_cons++,
>>> +					  &req);
>>> +
>>> +			if (!pvcalls_back_handle_cmd(dev, &req)) {
>>> +				RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(
>>> +					&priv->ring, notify);
>>> +				notify_all += notify;
>>> +			}
>>> +		}
>>> +
>>> +		if (notify_all)
>>> +			notify_remote_via_irq(priv->irq);
>>> +
>>> +		RING_FINAL_CHECK_FOR_REQUESTS(&priv->ring, more);
>>> +	}
>>>  }
>>>  
>>>  static irqreturn_t pvcalls_back_event(int irq, void *dev_id)
>>>  {
>>> +	struct xenbus_device *dev = dev_id;
>>> +	struct pvcalls_fedata *priv = NULL;
>>> +
>>> +	if (dev == NULL)
>>> +		return IRQ_HANDLED;
>>> +
>>> +	priv = dev_get_drvdata(&dev->dev);
>>> +	if (priv == NULL)
>>> +		return IRQ_HANDLED;
>>> +
>>> +	/*
>>> +	 * TODO: a small theoretical race exists if we try to queue work
>>> +	 * after pvcalls_back_work checked for final requests and before
>>> +	 * it returns. The queuing will fail, and pvcalls_back_work
>>> +	 * won't do the work because it is about to return. In that
>>> +	 * case, we lose the notification.
>>> +	 */
>>> +	queue_work(priv->wq, &priv->register_work);
>> Would queuing delayed work (if queue_work() failed) help? And canceling
>> it on next invocation of pvcalls_back_event()?
> Looking at the implementation of queue_delayed_work_on and
> queue_work_on, it looks like that if queue_work fails then also
> queue_delayed_work would fail: they both test on
> WORK_STRUCT_PENDING_BIT.

Right, I should have looked at this myself. And flush_work() I suppose
cannot be used here since it may sleep?

Then I also can't think of anything else.

-boris