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

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  Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-06-13 00:10 +0200
    Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the  frontend Stefano Stabellini <sstabellini@kernel.org> - 2017-06-14 23:10 +0200
      Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-06-15 16:40 +0200
        Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the  frontend Stefano Stabellini <sstabellini@kernel.org> - 2017-06-15 21:00 +0200

#1664262 — Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend

> +
>  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()?

-boris

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#1666229 — Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend

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.

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#1666759

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

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#1667154 — Re: [PATCH v3 06/18] xen/pvcalls: handle commands from the frontend

On Thu, 15 Jun 2017, Boris Ostrovsky wrote:
> 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.

I guess one way to work around the issue would be to use multiple work
items, and queue a new (different) work item at each pvcalls_back_event.
But that approach would use more memory and would need a new lock
in pvcalls_back_work.

Given that the race is only theoretical (I am running nginx inside a
VM and hitting it with as many multiple requests as I can and still I
cannot reproduce it), I am tempted to leave it as-is with a comment. We
can revisit it in the future if we find any real issues.

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