[[PATCH stable <3.12]] xen-netback: fix refcnt unbalance for 3.11 and earlier versions

[Wei Liu <wei.liu2@citrix.com>]

With the introduction of "xen-netback: Don't destroy the netdev until
the vif is shut down" (upstream commit id 279f438e36), vif disconnect
and free are separated. However in the backported verion reference
counting code was not correctly modified, and the reset of vif->tx_irq
was lost. If frontend goes through vif life cycle more than once the
reference counting is skewed.

This patch adds back the missing tx_irq reset line. It also moves
several lines of the reference counting code to vif_free, so the moved
code corresponds to the counterpart in vif_alloc, thus the reference
counting is balanced.

3.12 and onward versions are not affected by this bug, because reference
counting code was removed due to the introduction of 1:1 model.

This pacth should be backported to all stable verions which are lower
than 3.12 and have 279f438e36.

Reported-and-tested-by: Tomasz Wroblewski <tomasz.wroblewski@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Konrad Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
---
 drivers/net/xen-netback/interface.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index d28324a..342d4e5 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -418,9 +418,6 @@ void xenvif_disconnect(struct xenvif *vif)
 	if (netif_carrier_ok(vif->dev))
 		xenvif_carrier_off(vif);
 
-	atomic_dec(&vif->refcnt);
-	wait_event(vif->waiting_to_free, atomic_read(&vif->refcnt) == 0);
-
 	if (vif->tx_irq) {
 		if (vif->tx_irq == vif->rx_irq)
 			unbind_from_irqhandler(vif->tx_irq, vif);
@@ -428,6 +425,7 @@ void xenvif_disconnect(struct xenvif *vif)
 			unbind_from_irqhandler(vif->tx_irq, vif);
 			unbind_from_irqhandler(vif->rx_irq, vif);
 		}
+		vif->tx_irq = 0;
 	}
 
 	xen_netbk_unmap_frontend_rings(vif);
@@ -435,6 +433,9 @@ void xenvif_disconnect(struct xenvif *vif)
 
 void xenvif_free(struct xenvif *vif)
 {
+	atomic_dec(&vif->refcnt);
+	wait_event(vif->waiting_to_free, atomic_read(&vif->refcnt) == 0);
+
 	unregister_netdev(vif->dev);
 
 	free_netdev(vif->dev);
-- 
1.7.10.4