linux-kernel - Re: [PATCH v4 12/18] xen/pvcalls: implement poll command

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Date:   Wed, 21 Jun 2017 13:39:40 -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 v4 12/18] xen/pvcalls: implement poll command

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;