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