xen-netback: only deinitialized hash if it was initialized

A domain with a frontend that does not implement a control ring has been
seen to cause a crash during domain save. This was apparently because
the call to xenvif_deinit_hash() in xenvif_disconnect_ctrl() is made
regardless of whether a control ring was connected, and hence
xenvif_hash_init() was called.

This patch brings the call to xenvif_deinit_hash() in
xenvif_disconnect_ctrl() inside the if clause that checks whether the
control ring event channel was connected. This is sufficient to ensure
it is only called if xenvif_init_hash() was called previously.
Signed-off-by: NPaul Durrant <paul.durrant@citrix.com>
Reported-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -780,9 +780,8 @@ void xenvif_disconnect_ctrl(struct xenvif *vif)
 		vif->ctrl_task = NULL;
 	}
 
-	xenvif_deinit_hash(vif);
-
 	if (vif->ctrl_irq) {
+		xenvif_deinit_hash(vif);
 		unbind_from_irqhandler(vif->ctrl_irq, vif);
 		vif->ctrl_irq = 0;
 	}