[NET]: qdisc_restart - readability changes plus one bug fix.

New changes :

- Incorporated Peter Waskiewicz's comments.
- Re-added back one warning message (on driver returning wrong value).

Previous changes :

- Converted to use switch/case code which looks neater.

- "if (ret == NETDEV_TX_LOCKED && lockless)" is buggy, and the lockless
  check should be removed, since driver will return NETDEV_TX_LOCKED only
  if lockless is true and driver has to do the locking. In the original
  code as well as the latest code, this code can result in a bug where
  if LLTX is not set for a driver (lockless == 0) but the driver is written
  wrongly to do a trylock (despite LLTX being set), the driver returns
  LOCKED. But since lockless is zero, the packet is requeue'd instead of
  calling collision code which will issue warning and free up the skb.
  Instead this skb will be retried with this driver next time, and the same
  result will ensue. Removing this check will catch these driver bugs instead
  of hiding the problem. I am keeping this change to readability section
  since :
  	a. it is confusing to check two things as it is; and
  	b. it is difficult to keep this check in the changed 'switch' code.

- Changed some names, like try_get_tx_pkt to dev_dequeue_skb (as that is
  the work being done and easier to understand) and do_dev_requeue to
  dev_requeue_skb, merged handle_dev_cpu_collision and tx_islocked to
  dev_handle_collision (handle_dev_cpu_collision is a small routine with only
  one caller, so there is no need to have two separate routines which also
  results in getting rid of two macros, etc.

- Removed an XXX comment as it should never fail (I suspect this was related
  to batch skb WIP, Jamal ?). Converted some functions to original coding
  style of having the return values and the function name on same line, eg
  prio2list.
Signed-off-by: NKrishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -34,9 +34,6 @@
 #include <net/sock.h>
 #include <net/pkt_sched.h>
 
-#define SCHED_TX_DROP -2
-#define SCHED_TX_QUEUE -3
-
 /* Main transmission queue. */
 
 /* Modifications to data participating in scheduling must be protected with
@@ -68,41 +65,24 @@ static inline int qdisc_qlen(struct Qdisc *q)
 	return q->q.qlen;
 }
 
-static inline int handle_dev_cpu_collision(struct net_device *dev)
+static inline int dev_requeue_skb(struct sk_buff *skb, struct net_device *dev,
+				  struct Qdisc *q)
 {
-	if (unlikely(dev->xmit_lock_owner == smp_processor_id())) {
-		if (net_ratelimit())
-			printk(KERN_WARNING
-			       "Dead loop on netdevice %s, fix it urgently!\n",
-			       dev->name);
-		return SCHED_TX_DROP;
-	}
-	__get_cpu_var(netdev_rx_stat).cpu_collision++;
-	return SCHED_TX_QUEUE;
-}
-
-static inline int
-do_dev_requeue(struct sk_buff *skb, struct net_device *dev, struct Qdisc *q)
-{
-
 	if (unlikely(skb->next))
 		dev->gso_skb = skb;
 	else
 		q->ops->requeue(skb, q);
-	/* XXX: Could netif_schedule fail? Or is the fact we are
-	 * requeueing imply the hardware path is closed
-	 * and even if we fail, some interupt will wake us
-	 */
+
 	netif_schedule(dev);
 	return 0;
 }
 
-static inline struct sk_buff *
-try_get_tx_pkt(struct net_device *dev, struct Qdisc *q)
+static inline struct sk_buff *dev_dequeue_skb(struct net_device *dev,
+					      struct Qdisc *q)
 {
-	struct sk_buff *skb = dev->gso_skb;
+	struct sk_buff *skb;
 
-	if (skb)
+	if ((skb = dev->gso_skb))
 		dev->gso_skb = NULL;
 	else
 		skb = q->dequeue(q);
@@ -110,91 +90,116 @@ try_get_tx_pkt(struct net_device *dev, struct Qdisc *q)
 	return skb;
 }
 
-static inline int
-tx_islocked(struct sk_buff *skb, struct net_device *dev, struct Qdisc *q)
+static inline int handle_dev_cpu_collision(struct sk_buff *skb,
+					   struct net_device *dev,
+					   struct Qdisc *q)
 {
-	int ret = handle_dev_cpu_collision(dev);
+	int ret;
 
-	if (ret == SCHED_TX_DROP) {
+	if (unlikely(dev->xmit_lock_owner == smp_processor_id())) {
+		/*
+		 * Same CPU holding the lock. It may be a transient
+		 * configuration error, when hard_start_xmit() recurses. We
+		 * detect it by checking xmit owner and drop the packet when
+		 * deadloop is detected. Return OK to try the next skb.
+		 */
 		kfree_skb(skb);
-		return qdisc_qlen(q);
+		if (net_ratelimit())
+			printk(KERN_WARNING "Dead loop on netdevice %s, "
+			       "fix it urgently!\n", dev->name);
+		ret = qdisc_qlen(q);
+	} else {
+		/*
+		 * Another cpu is holding lock, requeue & delay xmits for
+		 * some time.
+		 */
+		__get_cpu_var(netdev_rx_stat).cpu_collision++;
+		ret = dev_requeue_skb(skb, dev, q);
 	}
 
-	return do_dev_requeue(skb, dev, q);
+	return ret;
 }
 
-
 /*
-   NOTE: Called under dev->queue_lock with locally disabled BH.
-
-   __LINK_STATE_QDISC_RUNNING guarantees only one CPU
-   can enter this region at a time.
-
-   dev->queue_lock serializes queue accesses for this device
-   AND dev->qdisc pointer itself.
-
-   netif_tx_lock serializes accesses to device driver.
-
-   dev->queue_lock and netif_tx_lock are mutually exclusive,
-   if one is grabbed, another must be free.
-
-   Multiple CPUs may contend for the two locks.
-
-   Note, that this procedure can be called by a watchdog timer
-
-   Returns to the caller:
-   Returns:  0  - queue is empty or throttled.
-	    >0  - queue is not empty.
-
-*/
-
+ * NOTE: Called under dev->queue_lock with locally disabled BH.
+ *
+ * __LINK_STATE_QDISC_RUNNING guarantees only one CPU can process this
+ * device at a time. dev->queue_lock serializes queue accesses for
+ * this device AND dev->qdisc pointer itself.
+ *
+ *  netif_tx_lock serializes accesses to device driver.
+ *
+ *  dev->queue_lock and netif_tx_lock are mutually exclusive,
+ *  if one is grabbed, another must be free.
+ *
+ * Note, that this procedure can be called by a watchdog timer
+ *
+ * Returns to the caller:
+ *				0  - queue is empty or throttled.
+ *				>0 - queue is not empty.
+ *
+ */
 static inline int qdisc_restart(struct net_device *dev)
 {
 	struct Qdisc *q = dev->qdisc;
-	unsigned lockless = (dev->features & NETIF_F_LLTX);
 	struct sk_buff *skb;
+	unsigned lockless;
 	int ret;
 
-	skb = try_get_tx_pkt(dev, q);
-	if (skb == NULL)
+	/* Dequeue packet */
+	if (unlikely((skb = dev_dequeue_skb(dev, q)) == NULL))
 		return 0;
 
-	/* we have a packet to send */
-	if (!lockless) {
-		if (!netif_tx_trylock(dev))
-			return tx_islocked(skb, dev, q);
+	/*
+	 * When the driver has LLTX set, it does its own locking in
+	 * start_xmit. These checks are worth it because even uncongested
+	 * locks can be quite expensive. The driver can do a trylock, as
+	 * is being done here; in case of lock contention it should return
+	 * NETDEV_TX_LOCKED and the packet will be requeued.
+	 */
+	lockless = (dev->features & NETIF_F_LLTX);
+
+	if (!lockless && !netif_tx_trylock(dev)) {
+		/* Another CPU grabbed the driver tx lock */
+		return handle_dev_cpu_collision(skb, dev, q);
 	}
-	/* all clear .. */
+
+	/* And release queue */
 	spin_unlock(&dev->queue_lock);
 
 	ret = NETDEV_TX_BUSY;
 	if (!netif_queue_stopped(dev))
-		/* churn baby churn .. */
 		ret = dev_hard_start_xmit(skb, dev);
 
 	if (!lockless)
 		netif_tx_unlock(dev);
 
 	spin_lock(&dev->queue_lock);
-
-	/* we need to refresh q because it may be invalid since
-	 * we dropped  dev->queue_lock earlier ...
-	 * So dont try to be clever grasshopper
-	 */
 	q = dev->qdisc;
-	/* most likely result, packet went ok */
-	if (ret == NETDEV_TX_OK)
-		return qdisc_qlen(q);
-	/* only for lockless drivers .. */
-	if (ret == NETDEV_TX_LOCKED && lockless)
-		return tx_islocked(skb, dev, q);
 
+	switch (ret) {
+	case NETDEV_TX_OK:
+		/* Driver sent out skb successfully */
+		ret = qdisc_qlen(q);
+		break;
+
+	case NETDEV_TX_LOCKED:
+		/* Driver try lock failed */
+		ret = handle_dev_cpu_collision(skb, dev, q);
+		break;
+
+	default:
+		/* Driver returned NETDEV_TX_BUSY - requeue skb */
 		if (unlikely (ret != NETDEV_TX_BUSY && net_ratelimit()))
-		printk(KERN_WARNING " BUG %s code %d qlen %d\n",dev->name, ret, q->q.qlen);
+			printk(KERN_WARNING "BUG %s code %d qlen %d\n",
+			       dev->name, ret, q->q.qlen);
 
-	return do_dev_requeue(skb, dev, q);
-}
+		ret = dev_requeue_skb(skb, dev, q);
+		break;
+	}
 
+	return ret;
+}
 
 void __qdisc_run(struct net_device *dev)
 {