Date:	Mon, 04 Jan 2010 10:21:53 +0000
From:	John Fastabend <john.r.fastabend@...el.com>
To:	netdev@...r.kernel.org
Subject: [RFC PATCH] net: consolidate netif_needs_gso() checks

netif_needs_gso() is checked twice in the TX path once,
before submitting the skb to the qdisc and once after
it is dequeued from the qdisc just before calling
ndo_hard_start().  This opens a window for a user to
change the gso/tso or tx checksum settings that can
cause netif_needs_gso to be true in one check and false
in the other.

Specifically, changing TX checksum setting may cause
the warning in skb_gso_segment() to be triggered if
the checksum is calculated earlier.

This consolidates the netif_needs_gso() calls so that
the stack only checks if gso is needed after the skb
is dequeued from the qdisc.  Or if the device has no
queue then before dev_hard_start_xmit() in dev_queue_xmit().

Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
---

 net/core/dev.c |   88 +++++++++++++++++++++++++++++++++++++-------------------
 1 files changed, 58 insertions(+), 30 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index c36a17a..91077df 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1808,12 +1808,39 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 {
 	const struct net_device_ops *ops = dev->netdev_ops;
 	int rc = NETDEV_TX_OK;
+	int need_gso = netif_needs_gso(dev,skb);
+
+	if (!need_gso) {
+		if (skb_has_frags(skb) &&
+		    !(dev->features & NETIF_F_FRAGLIST) &&
+		    __skb_linearize(skb))
+			goto out_kfree_skb;
+
+		/* Fragmented skb is linearized if device does not support SG,
+		 * or if at least one of fragments is in highmem and device
+		 * does not support DMA from it.
+		 */
+		if (skb_shinfo(skb)->nr_frags &&
+		    (!(dev->features & NETIF_F_SG) ||
+		      illegal_highdma(dev, skb)) &&
+		    __skb_linearize(skb))
+			goto out_kfree_skb;
+		/* If packet is not checksummed and device does not support
+		 * checksumming for this protocol, complete checksumming here.
+		 */
+		if (skb->ip_summed == CHECKSUM_PARTIAL) {
+			skb_set_transport_header(skb, skb->csum_start -
+				      skb_headroom(skb));
+			if (!dev_can_checksum(dev, skb) && skb_checksum_help(skb))
+				goto out_kfree_skb;
+		}
+	}
 
 	if (likely(!skb->next)) {
 		if (!list_empty(&ptype_all))
 			dev_queue_xmit_nit(skb, dev);
 
-		if (netif_needs_gso(dev, skb)) {
+		if (need_gso) {
 			if (unlikely(dev_gso_segment(skb)))
 				goto out_kfree_skb;
 			if (skb->next)
@@ -2006,6 +2033,35 @@ int dev_queue_xmit(struct sk_buff *skb)
 	struct Qdisc *q;
 	int rc = -ENOMEM;
 
+	/* Disable soft irqs for various locks below. Also
+	 * stops preemption for RCU.
+	 */
+	rcu_read_lock_bh();
+
+	txq = dev_pick_tx(dev, skb);
+	q = rcu_dereference(txq->qdisc);
+
+#ifdef CONFIG_NET_CLS_ACT
+	skb->tc_verd = SET_TC_AT(skb->tc_verd, AT_EGRESS);
+#endif
+	if (q->enqueue) {
+		rc = __dev_xmit_skb(skb, q, dev, txq);
+		goto out;
+	}
+
+	/* The device has no queue. Common case for software devices:
+	   loopback, all the sorts of tunnels...
+
+	   Really, it is unlikely that netif_tx_lock protection is necessary
+	   here.  (f.e. loopback and IP tunnels are clean ignoring statistics
+	   counters.)
+	   However, it is possible, that they rely on protection
+	   made by us here.
+
+	   Check this and shot the lock. It is not prone from deadlocks.
+	   Either shot noqueue qdisc, it is even simpler 8)
+	 */
+
 	/* GSO will handle the following emulations directly. */
 	if (netif_needs_gso(dev, skb))
 		goto gso;
@@ -2035,34 +2091,6 @@ int dev_queue_xmit(struct sk_buff *skb)
 	}
 
 gso:
-	/* Disable soft irqs for various locks below. Also
-	 * stops preemption for RCU.
-	 */
-	rcu_read_lock_bh();
-
-	txq = dev_pick_tx(dev, skb);
-	q = rcu_dereference(txq->qdisc);
-
-#ifdef CONFIG_NET_CLS_ACT
-	skb->tc_verd = SET_TC_AT(skb->tc_verd, AT_EGRESS);
-#endif
-	if (q->enqueue) {
-		rc = __dev_xmit_skb(skb, q, dev, txq);
-		goto out;
-	}
-
-	/* The device has no queue. Common case for software devices:
-	   loopback, all the sorts of tunnels...
-
-	   Really, it is unlikely that netif_tx_lock protection is necessary
-	   here.  (f.e. loopback and IP tunnels are clean ignoring statistics
-	   counters.)
-	   However, it is possible, that they rely on protection
-	   made by us here.
-
-	   Check this and shot the lock. It is not prone from deadlocks.
-	   Either shot noqueue qdisc, it is even simpler 8)
-	 */
 	if (dev->flags & IFF_UP) {
 		int cpu = smp_processor_id(); /* ok because BHs are off */
 
@@ -4869,7 +4897,7 @@ unsigned long netdev_fix_features(unsigned long features, const char *name)
 			printk(KERN_NOTICE "%s: Dropping NETIF_F_SG since no "
 			       "checksum feature.\n", name);
 		features &= ~NETIF_F_SG;
-	}
+}
 
 	/* TSO requires that SG is present as well. */
 	if ((features & NETIF_F_TSO) && !(features & NETIF_F_SG)) {

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