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

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Date:   Wed, 14 Jun 2017 14:03:08 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        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 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.