Re: [PATCH v3 11/18] xen/pvcalls: implement accept command

[Stefano Stabellini]

On Tue, 13 Jun 2017, Juergen Gross wrote:
> On 02/06/17 21:31, Stefano Stabellini wrote:
> > Implement the accept command by calling inet_accept. To avoid blocking
> > in the kernel, call inet_accept(O_NONBLOCK) from a workqueue, which get
> > scheduled on sk_data_ready (for a passive socket, it means that there
> > are connections to accept).
> > 
> > Use the reqcopy field to store the request. Accept the new socket from
> > the delayed work function, create a new sock_mapping for it, map
> > the indexes page and data ring, and reply to the other end. Allocate an
> > ioworker for the socket.
> > 
> > Only support one outstanding blocking accept request for every socket at
> > any time.
> > 
> > Add a field to sock_mapping to remember the passive socket from which an
> > active socket was created.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> > CC: boris.ostrovsky@xxxxxxxxxx
> > CC: jgross@xxxxxxxx
> > ---
> >  drivers/xen/pvcalls-back.c | 109 ++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 108 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> > index a75586e..f1173f4 100644
> > --- a/drivers/xen/pvcalls-back.c
> > +++ b/drivers/xen/pvcalls-back.c
> > @@ -65,6 +65,7 @@ struct pvcalls_ioworker {
> >  struct sock_mapping {
> >  	struct list_head list;
> >  	struct pvcalls_fedata *priv;
> > +	struct sockpass_mapping *sockpass;
> >  	struct socket *sock;
> >  	uint64_t id;
> >  	grant_ref_t ref;
> > @@ -275,10 +276,79 @@ static int pvcalls_back_release(struct xenbus_device *dev,
> >  
> >  static void __pvcalls_back_accept(struct work_struct *work)
> >  {
> > +	struct sockpass_mapping *mappass = container_of(
> > +		work, struct sockpass_mapping, register_work);
> > +	struct sock_mapping *map;
> > +	struct pvcalls_ioworker *iow;
> > +	struct pvcalls_fedata *priv;
> > +	struct socket *sock;
> > +	struct xen_pvcalls_response *rsp;
> > +	struct xen_pvcalls_request *req;
> > +	int notify;
> > +	int ret = -EINVAL;
> > +	unsigned long flags;
> > +
> > +	priv = mappass->priv;
> > +	/* We only need to check the value of "cmd" atomically on read. */
> > +	spin_lock_irqsave(&mappass->copy_lock, flags);
> > +	req = &mappass->reqcopy;
> > +	if (req->cmd != PVCALLS_ACCEPT) {
> > +		spin_unlock_irqrestore(&mappass->copy_lock, flags);
> > +		return;
> > +	}
> > +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> 
> What about:
> 	req = &mappass->reqcopy;
> 	if (ACCESS_ONCE(req->cmd) != PVCALLS_ACCEPT)
> 		return;
> 
> I can't see the need for taking a lock here.

Sure, good idea


> > +
> > +	sock = sock_alloc();
> > +	if (sock == NULL)
> > +		goto out_error;
> > +	sock->type = mappass->sock->type;
> > +	sock->ops = mappass->sock->ops;
> > +
> > +	ret = inet_accept(mappass->sock, sock, O_NONBLOCK, true);
> > +	if (ret == -EAGAIN) {
> > +		sock_release(sock);
> > +		goto out_error;
> > +	}
> > +
> > +	map = pvcalls_new_active_socket(priv,
> > +					req->u.accept.id_new,
> > +					req->u.accept.ref,
> > +					req->u.accept.evtchn,
> > +					sock);
> > +	if (!map) {
> > +		sock_release(sock);
> > +		goto out_error;
> > +	}
> > +
> > +	map->sockpass = mappass;
> > +	iow = &map->ioworker;
> > +	atomic_inc(&map->read);
> > +	atomic_inc(&map->io);
> > +	queue_work_on(iow->cpu, iow->wq, &iow->register_work);
> > +
> > +out_error:
> > +	rsp = RING_GET_RESPONSE(&priv->ring, priv->ring.rsp_prod_pvt++);
> > +	rsp->req_id = req->req_id;
> > +	rsp->cmd = req->cmd;
> > +	rsp->u.accept.id = req->u.accept.id;
> > +	rsp->ret = ret;
> > +	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&priv->ring, notify);
> > +	if (notify)
> > +		notify_remote_via_irq(priv->irq);
> > +
> > +	spin_lock_irqsave(&mappass->copy_lock, flags);
> > +	mappass->reqcopy.cmd = 0;
> > +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> 
> ACCESS_ONCE(mappass->reqcopy.cmd) = 0;

OK


> >  }
> >  
> >  static void pvcalls_pass_sk_data_ready(struct sock *sock)
> >  {
> > +	struct sockpass_mapping *mappass = sock->sk_user_data;
> > +
> > +	if (mappass == NULL)
> > +		return;
> > +
> > +	queue_work(mappass->wq, &mappass->register_work);
> >  }
> >  
> >  static int pvcalls_back_bind(struct xenbus_device *dev,
> > @@ -380,7 +450,44 @@ static int pvcalls_back_listen(struct xenbus_device *dev,
> >  static int pvcalls_back_accept(struct xenbus_device *dev,
> >  			       struct xen_pvcalls_request *req)
> >  {
> > -	return 0;
> > +	struct pvcalls_fedata *priv;
> > +	struct sockpass_mapping *mappass;
> > +	int ret = -EINVAL;
> > +	struct xen_pvcalls_response *rsp;
> > +	unsigned long flags;
> > +
> > +	priv = dev_get_drvdata(&dev->dev);
> > +
> > +	mappass = radix_tree_lookup(&priv->socketpass_mappings,
> > +		req->u.accept.id);
> > +	if (mappass == NULL)
> > +		goto out_error;
> > +
> > +	/* 
> > +	 * 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) {
> > +		spin_unlock_irqrestore(&mappass->copy_lock, flags);
> > +		ret = -EINTR;
> > +		goto out_error;
> > +	}
> > +
> > +	mappass->reqcopy = *req;
> 
> This time you need the lock, however you should use:
> 
> ACCESS_ONCE(mappass->reqcopy) = *req;

I don't think that guarantees atomic accesses to the cmd field of the
struct. Shouldn't this be:

  ACCESS_ONCE(mappass->reqcopy.cmd) = req->cmd;
  mappass->reqcopy = *req;


> > +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> > +	queue_work(mappass->wq, &mappass->register_work);
> > +
> > +	/* Tell the caller we don't need to send back a notification yet */
> > +	return -1;
> > +
> > +out_error:
> > +	rsp = RING_GET_RESPONSE(&priv->ring, priv->ring.rsp_prod_pvt++);
> > +	rsp->req_id = req->req_id;
> > +	rsp->cmd = req->cmd;
> > +	rsp->u.accept.id = req->u.accept.id;
> > +	rsp->ret = ret;
> > +	return ret;
> 
> return 0?

Yes, thanks