xen-netback: bookkeep number of active queues in our own module

The original code uses netdev->real_num_tx_queues to bookkeep number of
queues and invokes netif_set_real_num_tx_queues to set the number of
queues. However, netif_set_real_num_tx_queues doesn't allow
real_num_tx_queues to be smaller than 1, which means setting the number
to 0 will not work and real_num_tx_queues is untouched.

This is bogus when xenvif_free is invoked before any number of queues is
allocated. That function needs to iterate through all queues to free
resources. Using the wrong number of queues results in NULL pointer
dereference.

So we bookkeep the number of queues in xen-netback to solve this
problem. This fixes a regression introduced by multiqueue patchset in
3.16-rc1.

There's another bug in original code that the real number of RX queues
is never set. In current Xen multiqueue design, the number of TX queues
and RX queues are in fact the same. We need to set the numbers of TX and
RX queues to the same value.

Also remove xenvif_select_queue and leave queue selection to core
driver, as suggested by David Miller.
Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/common.h

@@ -222,6 +222,7 @@ struct xenvif {
 
 	/* Queues */
 	struct xenvif_queue *queues;
+	unsigned int num_queues; /* active queues, resource allocated */
 
 	/* Miscellaneous private stuff. */
 	struct net_device *dev;

drivers/net/xen-netback/interface.c

@@ -137,32 +137,11 @@ static void xenvif_wake_queue_callback(unsigned long data)
 	}
 }
 
-static u16 xenvif_select_queue(struct net_device *dev, struct sk_buff *skb,
-			       void *accel_priv, select_queue_fallback_t fallback)
-{
-	unsigned int num_queues = dev->real_num_tx_queues;
-	u32 hash;
-	u16 queue_index;
-
-	/* First, check if there is only one queue to optimise the
-	 * single-queue or old frontend scenario.
-	 */
-	if (num_queues == 1) {
-		queue_index = 0;
-	} else {
-		/* Use skb_get_hash to obtain an L4 hash if available */
-		hash = skb_get_hash(skb);
-		queue_index = hash % num_queues;
-	}
-
-	return queue_index;
-}
-
 static int xenvif_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct xenvif *vif = netdev_priv(dev);
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	u16 index;
 	int min_slots_needed;
 
@@ -225,7 +204,7 @@ static struct net_device_stats *xenvif_get_stats(struct net_device *dev)
 {
 	struct xenvif *vif = netdev_priv(dev);
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	unsigned long rx_bytes = 0;
 	unsigned long rx_packets = 0;
 	unsigned long tx_bytes = 0;
@@ -256,7 +235,7 @@ static struct net_device_stats *xenvif_get_stats(struct net_device *dev)
 static void xenvif_up(struct xenvif *vif)
 {
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = vif->dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	unsigned int queue_index;
 
 	for (queue_index = 0; queue_index < num_queues; ++queue_index) {
@@ -272,7 +251,7 @@ static void xenvif_up(struct xenvif *vif)
 static void xenvif_down(struct xenvif *vif)
 {
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = vif->dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	unsigned int queue_index;
 
 	for (queue_index = 0; queue_index < num_queues; ++queue_index) {
@@ -379,7 +358,7 @@ static void xenvif_get_ethtool_stats(struct net_device *dev,
 				     struct ethtool_stats *stats, u64 * data)
 {
 	struct xenvif *vif = netdev_priv(dev);
-	unsigned int num_queues = dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	int i;
 	unsigned int queue_index;
 	struct xenvif_stats *vif_stats;
@@ -424,7 +403,6 @@ static const struct net_device_ops xenvif_netdev_ops = {
 	.ndo_fix_features = xenvif_fix_features,
 	.ndo_set_mac_address = eth_mac_addr,
 	.ndo_validate_addr   = eth_validate_addr,
-	.ndo_select_queue = xenvif_select_queue,
 };
 
 struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
@@ -438,7 +416,7 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
 	snprintf(name, IFNAMSIZ - 1, "vif%u.%u", domid, handle);
 	/* Allocate a netdev with the max. supported number of queues.
 	 * When the guest selects the desired number, it will be updated
-	 * via netif_set_real_num_tx_queues().
+	 * via netif_set_real_num_*_queues().
 	 */
 	dev = alloc_netdev_mq(sizeof(struct xenvif), name, ether_setup,
 			      xenvif_max_queues);
@@ -458,11 +436,9 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
 	vif->dev = dev;
 	vif->disabled = false;
 
-	/* Start out with no queues. The call below does not require
-	 * rtnl_lock() as it happens before register_netdev().
-	 */
+	/* Start out with no queues. */
 	vif->queues = NULL;
-	netif_set_real_num_tx_queues(dev, 0);
+	vif->num_queues = 0;
 
 	dev->netdev_ops	= &xenvif_netdev_ops;
 	dev->hw_features = NETIF_F_SG |
@@ -677,7 +653,7 @@ static void xenvif_wait_unmap_timeout(struct xenvif_queue *queue,
 void xenvif_disconnect(struct xenvif *vif)
 {
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = vif->dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	unsigned int queue_index;
 
 	if (netif_carrier_ok(vif->dev))
@@ -724,7 +700,7 @@ void xenvif_deinit_queue(struct xenvif_queue *queue)
 void xenvif_free(struct xenvif *vif)
 {
 	struct xenvif_queue *queue = NULL;
-	unsigned int num_queues = vif->dev->real_num_tx_queues;
+	unsigned int num_queues = vif->num_queues;
 	unsigned int queue_index;
 	/* Here we want to avoid timeout messages if an skb can be legitimately
 	 * stuck somewhere else. Realistically this could be an another vif's
@@ -748,12 +724,9 @@ void xenvif_free(struct xenvif *vif)
 		xenvif_deinit_queue(queue);
 	}
 
-	/* Free the array of queues. The call below does not require
-	 * rtnl_lock() because it happens after unregister_netdev().
-	 */
-	netif_set_real_num_tx_queues(vif->dev, 0);
 	vfree(vif->queues);
 	vif->queues = NULL;
+	vif->num_queues = 0;
 
 	free_netdev(vif->dev);
 

drivers/net/xen-netback/xenbus.c

@@ -527,9 +527,7 @@ static void connect(struct backend_info *be)
 	/* Use the number of queues requested by the frontend */
 	be->vif->queues = vzalloc(requested_num_queues *
 				  sizeof(struct xenvif_queue));
-	rtnl_lock();
-	netif_set_real_num_tx_queues(be->vif->dev, requested_num_queues);
-	rtnl_unlock();
+	be->vif->num_queues = requested_num_queues;
 
 	for (queue_index = 0; queue_index < requested_num_queues; ++queue_index) {
 		queue = &be->vif->queues[queue_index];
@@ -546,9 +544,7 @@ static void connect(struct backend_info *be)
 			 * earlier queues can be destroyed using the regular
 			 * disconnect logic.
 			 */
-			rtnl_lock();
-			netif_set_real_num_tx_queues(be->vif->dev, queue_index);
-			rtnl_unlock();
+			be->vif->num_queues = queue_index;
 			goto err;
 		}
 
@@ -561,13 +557,19 @@ static void connect(struct backend_info *be)
 			 * and also clean up any previously initialised queues.
 			 */
 			xenvif_deinit_queue(queue);
-			rtnl_lock();
-			netif_set_real_num_tx_queues(be->vif->dev, queue_index);
-			rtnl_unlock();
+			be->vif->num_queues = queue_index;
 			goto err;
 		}
 	}
 
+	/* Initialisation completed, tell core driver the number of
+	 * active queues.
+	 */
+	rtnl_lock();
+	netif_set_real_num_tx_queues(be->vif->dev, requested_num_queues);
+	netif_set_real_num_rx_queues(be->vif->dev, requested_num_queues);
+	rtnl_unlock();
+
 	xenvif_carrier_on(be->vif);
 
 	unregister_hotplug_status_watch(be);
@@ -582,13 +584,11 @@ static void connect(struct backend_info *be)
 	return;
 
 err:
-	if (be->vif->dev->real_num_tx_queues > 0)
+	if (be->vif->num_queues > 0)
 		xenvif_disconnect(be->vif); /* Clean up existing queues */
 	vfree(be->vif->queues);
 	be->vif->queues = NULL;
-	rtnl_lock();
-	netif_set_real_num_tx_queues(be->vif->dev, 0);
-	rtnl_unlock();
+	be->vif->num_queues = 0;
 	return;
 }
 
@@ -596,7 +596,7 @@ static void connect(struct backend_info *be)
 static int connect_rings(struct backend_info *be, struct xenvif_queue *queue)
 {
 	struct xenbus_device *dev = be->dev;
-	unsigned int num_queues = queue->vif->dev->real_num_tx_queues;
+	unsigned int num_queues = queue->vif->num_queues;
 	unsigned long tx_ring_ref, rx_ring_ref;
 	unsigned int tx_evtchn, rx_evtchn;
 	int err;