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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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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