xen-netback: clear vif->task on disconnect

xenvif_start_xmit() relies on checking vif->task for NULL to determine
whether the vif is ready to accept packets. The task thread is stopped in
xenvif_disconnect() but task is not set to NULL. Thus, on a re-connect the
check will give a false positive.

Also since commit ea732dff5cfa10789007bf4a5b935388a0bb2a8f (Handle backend
state transitions in a more robust way) it should not be possible for
xenvif_connect() to be called if the vif is already connected so change the
check of vif->tx_irq to a BUG_ON() and also add a BUG_ON(vif->task).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -368,11 +368,11 @@ int xenvif_connect(struct xenvif *vif, unsigned long tx_ring_ref,
 		   unsigned long rx_ring_ref, unsigned int tx_evtchn,
 		   unsigned int rx_evtchn)
 {
+	struct task_struct *task;
 	int err = -ENOMEM;
 
-	/* Already connected through? */
-	if (vif->tx_irq)
-		return 0;
+	BUG_ON(vif->tx_irq);
+	BUG_ON(vif->task);
 
 	err = xenvif_map_frontend_rings(vif, tx_ring_ref, rx_ring_ref);
 	if (err < 0)
@@ -411,14 +411,16 @@ int xenvif_connect(struct xenvif *vif, unsigned long tx_ring_ref,
 	}
 
 	init_waitqueue_head(&vif->wq);
-	vif->task = kthread_create(xenvif_kthread,
-				   (void *)vif, "%s", vif->dev->name);
-	if (IS_ERR(vif->task)) {
+	task = kthread_create(xenvif_kthread,
+			      (void *)vif, "%s", vif->dev->name);
+	if (IS_ERR(task)) {
 		pr_warn("Could not allocate kthread for %s\n", vif->dev->name);
-		err = PTR_ERR(vif->task);
+		err = PTR_ERR(task);
 		goto err_rx_unbind;
 	}
 
+	vif->task = task;
+
 	rtnl_lock();
 	if (!vif->can_sg && vif->dev->mtu > ETH_DATA_LEN)
 		dev_set_mtu(vif->dev, ETH_DATA_LEN);
@@ -461,8 +463,10 @@ void xenvif_disconnect(struct xenvif *vif)
 	if (netif_carrier_ok(vif->dev))
 		xenvif_carrier_off(vif);
 
-	if (vif->task)
+	if (vif->task) {
 		kthread_stop(vif->task);
+		vif->task = NULL;
+	}
 
 	if (vif->tx_irq) {
 		if (vif->tx_irq == vif->rx_irq)