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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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