Re: [Xen-devel] [PATCH v4 12/18] xen/pvcalls: implement poll command

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 20 Jun 2017, Boris Ostrovsky wrote:
> > @@ -499,6 +521,55 @@ static int pvcalls_back_accept(struct xenbus_device 
> > *dev,
> >  static int pvcalls_back_poll(struct xenbus_device *dev,
> >                          struct xen_pvcalls_request *req)
> >  {
> > +   struct pvcalls_fedata *fedata;
> > +   struct sockpass_mapping *mappass;
> > +   struct xen_pvcalls_response *rsp;
> > +   struct inet_connection_sock *icsk;
> > +   struct request_sock_queue *queue;
> > +   unsigned long flags;
> > +   int ret;
> > +   bool data;
> > +
> > +   fedata = dev_get_drvdata(&dev->dev);
> > +
> > +   mappass = radix_tree_lookup(&fedata->socketpass_mappings, 
> > req->u.poll.id);
> > +   if (mappass == NULL)
> > +           return -EINVAL;
> > +
> > +   /*
> > +    * Limitation of the current implementation: only support one
> > +    * concurrent accept or poll call on one socket.
> > +    */
> > +   spin_lock_irqsave(&mappass->copy_lock, flags);
> > +   if (mappass->reqcopy.cmd != 0) {
> > +           ret = -EINTR;
> > +           goto out;
> > +   }
> > +
> > +   mappass->reqcopy = *req;
> > +   icsk = inet_csk(mappass->sock->sk);
> > +   queue = &icsk->icsk_accept_queue;
> > +   spin_lock(&queue->rskq_lock);
> > +   data = queue->rskq_accept_head != NULL;
> > +   spin_unlock(&queue->rskq_lock);
> 
> What is the purpose of the queue lock here?

It is only there to protect accesses to rskq_accept_head. Functions that
change rskq_accept_head take this lock, see for example
net/ipv4/inet_connection_sock.c:inet_csk_reqsk_queue_add. I'll add an
in-code comment.


> > +   if (data) {
> > +           mappass->reqcopy.cmd = 0;
> > +           ret = 0;
> > +           goto out;
> > +   }
> > +   spin_unlock_irqrestore(&mappass->copy_lock, flags);
> > +
> > +   /* Tell the caller we don't need to send back a notification yet */
> > +   return -1;

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