virtio: fix delayed xmit of packet and freeing of old packets.

Because we cache the last failed-to-xmit packet, if there are no
packets queued behind that one we may never send it (reproduced here
as TCP stalls, "cured" by an outgoing ping).

Includes Mark's '[PATCH] virtio_net: Delay dropping tx skbs'

Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/net/virtio_net.c |   37 +++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

diff -r fe931fc6f7fe drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c	Fri May 30 14:39:01 2008 +1000
+++ b/drivers/net/virtio_net.c	Fri May 30 14:39:04 2008 +1000
@@ -47,6 +47,9 @@ struct virtnet_info
 	/* Number of input buffers, and max we've ever had. */
 	unsigned int num, max;
 
+	/* For cleaning up after transmission. */
+	struct tasklet_struct tasklet;
+
 	/* Receive & send queues. */
 	struct sk_buff_head recv;
 	struct sk_buff_head send;
@@ -68,8 +71,13 @@ static void skb_xmit_done(struct virtque
 
 	/* Suppress further interrupts. */
 	svq->vq_ops->disable_cb(svq);
+
 	/* We were waiting for more output buffers. */
 	netif_wake_queue(vi->dev);
+
+	/* Make sure we re-xmit last_xmit_skb: if there are no more packets
+	 * queued, start_xmit won't be called. */
+	tasklet_schedule(&vi->tasklet);
 }
 
 static void receive_skb(struct net_device *dev, struct sk_buff *skb,
@@ -278,6 +286,18 @@ static int xmit_skb(struct virtnet_info 
 	return vi->svq->vq_ops->add_buf(vi->svq, sg, num, 0, skb);
 }
 
+static void xmit_tasklet(unsigned long data)
+{
+	struct virtnet_info *vi = (void *)data;
+
+	netif_tx_lock_bh(vi->dev);
+	if (vi->last_xmit_skb && xmit_skb(vi, vi->last_xmit_skb) == 0) {
+		vi->svq->vq_ops->kick(vi->svq);
+		vi->last_xmit_skb = NULL;
+	}
+	netif_tx_unlock_bh(vi->dev);
+}
+
 static int start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
@@ -288,13 +308,8 @@ again:
 
 	/* If we has a buffer left over from last time, send it now. */
 	if (unlikely(vi->last_xmit_skb)) {
-		if (xmit_skb(vi, vi->last_xmit_skb) != 0) {
-			/* Drop this skb: we only queue one. */
-			vi->dev->stats.tx_dropped++;
-			kfree_skb(skb);
-			skb = NULL;
+		if (xmit_skb(vi, vi->last_xmit_skb) != 0)
 			goto stop_queue;
-		}
 		vi->last_xmit_skb = NULL;
 	}
 
@@ -321,6 +336,11 @@ stop_queue:
 		vi->svq->vq_ops->disable_cb(vi->svq);
 		netif_start_queue(dev);
 		goto again;
+	}
+	if (skb) {
+		/* Drop this skb: we only queue one. */
+		vi->dev->stats.tx_dropped++;
+		kfree_skb(skb);
 	}
 	goto done;
 }
@@ -432,6 +452,8 @@ static int virtnet_probe(struct virtio_d
 	skb_queue_head_init(&vi->recv);
 	skb_queue_head_init(&vi->send);
 
+	tasklet_init(&vi->tasklet, xmit_tasklet, (unsigned long)vi);
+
 	err = register_netdev(dev);
 	if (err) {
 		pr_debug("virtio_net: registering device failed\n");
@@ -465,6 +487,9 @@ static void virtnet_remove(struct virtio
 {
 	struct virtnet_info *vi = vdev->priv;
 	struct sk_buff *skb;
+
+	/* Make sure tasklet isn't still running. */
+	tasklet_kill(&vi->tasklet);
 
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);