bonding: reset queue mapping prior to transmission to physical device (v5)

The bonding driver is multiqueue enabled, in which each queue represents a slave
to enable optional steering of output frames to given slaves against the default
output policy.  However, it needs to reset the skb->queue_mapping prior to
queuing to the physical device or the physical slave (if it is multiqueue) could
wind up transmitting on an unintended tx queue

Change Notes:
v2) Based on first pass review, updated the patch to restore the origional queue
mapping that was found in bond_select_queue, rather than simply resetting to
zero.  This preserves the value of queue_mapping when it was set on receive in
the forwarding case which is desireable.

v3) Fixed spelling an casting error in skb->cb

v4) fixed to store raw queue_mapping to avoid double decrement

v5) Eric D requested that ->cb access be wrapped in a macro.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -388,6 +388,8 @@ struct vlan_entry *bond_next_vlan(struct bonding *bond, struct vlan_entry *curr)
 	return next;
 }
 
+#define bond_queue_mapping(skb) (*(u16 *)((skb)->cb))
+
 /**
  * bond_dev_queue_xmit - Prepare skb for xmit.
  *
@@ -400,6 +402,9 @@ int bond_dev_queue_xmit(struct bonding *bond, struct sk_buff *skb,
 {
 	skb->dev = slave_dev;
 	skb->priority = 1;
+
+	skb->queue_mapping = bond_queue_mapping(skb);
+
 	if (unlikely(netpoll_tx_running(slave_dev)))
 		bond_netpoll_send_skb(bond_get_slave_by_dev(bond, slave_dev), skb);
 	else
@@ -4206,6 +4211,7 @@ static inline int bond_slave_override(struct bonding *bond,
 	return res;
 }
 
+
 static u16 bond_select_queue(struct net_device *dev, struct sk_buff *skb)
 {
 	/*
@@ -4216,6 +4222,11 @@ static u16 bond_select_queue(struct net_device *dev, struct sk_buff *skb)
 	 */
 	u16 txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) : 0;
 
+	/*
+	 * Save the original txq to restore before passing to the driver
+	 */
+	bond_queue_mapping(skb) = skb->queue_mapping;
+
 	if (unlikely(txq >= dev->real_num_tx_queues)) {
 		do {
 			txq -= dev->real_num_tx_queues;