9p/xen: fix connection sequence

Today the connection sequence of the Xen 9pfs frontend doesn't match
the documented sequence. It can work reliably only for a PV 9pfs device
having been added at boot time already, as the frontend is not waiting
for the backend to have set its state to "XenbusStateInitWait" before
reading the backend properties from Xenstore.

Fix that by following the documented sequence [1] (the documentation
has a bug, so the reference is for the patch fixing that).

[1]: https://lore.kernel.org/xen-devel/20230130090937.31623-1-jgross@suse.com/T/#u

Link: https://lkml.kernel.org/r/20230130113036.7087-3-jgross@suse.com
Fixes: 868eb122 ("xen/9pfs: introduce Xen 9pfs transport driver")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>

net/9p/trans_xen.c

@@ -373,12 +373,11 @@ static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
 	return ret;
 }
 
-static int xen_9pfs_front_probe(struct xenbus_device *dev,
-				const struct xenbus_device_id *id)
+static int xen_9pfs_front_init(struct xenbus_device *dev)
 {
 	int ret, i;
 	struct xenbus_transaction xbt;
-	struct xen_9pfs_front_priv *priv = NULL;
+	struct xen_9pfs_front_priv *priv = dev_get_drvdata(&dev->dev);
 	char *versions, *v;
 	unsigned int max_rings, max_ring_order, len = 0;
 
@@ -406,11 +405,6 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
 	if (p9_xen_trans.maxsize > XEN_FLEX_RING_SIZE(max_ring_order))
 		p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order) / 2;
 
-	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
-	if (!priv)
-		return -ENOMEM;
-
-	priv->dev = dev;
 	priv->num_rings = XEN_9PFS_NUM_RINGS;
 	priv->rings = kcalloc(priv->num_rings, sizeof(*priv->rings),
 			      GFP_KERNEL);
@@ -469,23 +463,35 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
 		goto error;
 	}
 
-	write_lock(&xen_9pfs_lock);
-	list_add_tail(&priv->list, &xen_9pfs_devs);
-	write_unlock(&xen_9pfs_lock);
-	dev_set_drvdata(&dev->dev, priv);
-	xenbus_switch_state(dev, XenbusStateInitialised);
-
 	return 0;
 
  error_xenbus:
 	xenbus_transaction_end(xbt, 1);
 	xenbus_dev_fatal(dev, ret, "writing xenstore");
  error:
-	dev_set_drvdata(&dev->dev, NULL);
 	xen_9pfs_front_free(priv);
 	return ret;
 }
 
+static int xen_9pfs_front_probe(struct xenbus_device *dev,
+				const struct xenbus_device_id *id)
+{
+	struct xen_9pfs_front_priv *priv = NULL;
+
+	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->dev = dev;
+	dev_set_drvdata(&dev->dev, priv);
+
+	write_lock(&xen_9pfs_lock);
+	list_add_tail(&priv->list, &xen_9pfs_devs);
+	write_unlock(&xen_9pfs_lock);
+
+	return 0;
+}
+
 static int xen_9pfs_front_resume(struct xenbus_device *dev)
 {
 	dev_warn(&dev->dev, "suspend/resume unsupported\n");
@@ -504,6 +510,8 @@ static void xen_9pfs_front_changed(struct xenbus_device *dev,
 		break;
 
 	case XenbusStateInitWait:
+		if (!xen_9pfs_front_init(dev))
+			xenbus_switch_state(dev, XenbusStateInitialised);
 		break;
 
 	case XenbusStateConnected: