netdev: Add netdev->select_queue() method.

Devices or device layers can set this to control the queue selection
performed by dev_pick_tx().

This function runs under RCU protection, which allows overriding
functions to have some way of synchronizing with things like dynamic
->real_num_tx_queues adjustments.

This makes the spinlock prefetch in dev_queue_xmit() a little bit
less effective, but that's the price right now for correctness.

Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 9 insertions, 3 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index fdac1159253e..9464e6452967 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -724,6 +724,9 @@ struct net_device
         void                    (*poll_controller)(struct net_device *dev);
 #endif
 
+        u16                     (*select_queue)(struct net_device *dev,
+                                                struct sk_buff *skb);
+
 #ifdef CONFIG_NET_NS
         /* Network namespace this network device is inside */
         struct net              *nd_net;
diff --git a/net/core/dev.c b/net/core/dev.c
index f027a1ac4fbb..7ca9564d2f44 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1670,6 +1670,9 @@ static struct netdev_queue *dev_pick_tx(struct net_device *dev,
 {
         u16 queue_index = 0;
 
+        if (dev->select_queue)
+                queue_index = dev->select_queue(dev, skb);
+
         skb_set_queue_mapping(skb, queue_index);
         return netdev_get_tx_queue(dev, queue_index);
 }
@@ -1710,14 +1713,14 @@ int dev_queue_xmit(struct sk_buff *skb)
         }
 
 gso:
-        txq = dev_pick_tx(dev, skb);
-        spin_lock_prefetch(&txq->lock);
-
         /* Disable soft irqs for various locks below. Also
          * stops preemption for RCU.
          */
         rcu_read_lock_bh();
 
+        txq = dev_pick_tx(dev, skb);
+        spin_lock_prefetch(&txq->lock);
+
         /* Updates of qdisc are serialized by queue->lock.
          * The struct Qdisc which is pointed to by qdisc is now a
          * rcu structure - it may be accessed without acquiring