net: decreasing real_num_tx_queues needs to flush qdisc

Reducing real_num_queues needs to flush the qdisc otherwise
skbs with queue_mappings greater then real_num_tx_queues can
be sent to the underlying driver.

The flow for this is,

dev_queue_xmit()
	dev_pick_tx()
		skb_tx_hash()  => hash using real_num_tx_queues
		skb_set_queue_mapping()
	...
	qdisc_enqueue_root() => enqueue skb on txq from hash
...
dev->real_num_tx_queues -= n
...
sch_direct_xmit()
	dev_hard_start_xmit()
		ndo_start_xmit(skb,dev) => skb queue set with old hash

skbs are enqueued on the qdisc with skb->queue_mapping set
0 < queue_mappings < real_num_tx_queues.  When the driver
decreases real_num_tx_queues skb's may be dequeued from the
qdisc with a queue_mapping greater then real_num_tx_queues.

This fixes a case in ixgbe where this was occurring with DCB
and FCoE. Because the driver is using queue_mapping to map
skbs to tx descriptor rings we can potentially map skbs to
rings that no longer exist.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbe/ixgbe_main.c

@@ -4001,7 +4001,7 @@ static void ixgbe_set_num_queues(struct ixgbe_adapter *adapter)
 
 done:
 	/* Notify the stack of the (possibly) reduced Tx Queue count. */
-	adapter->netdev->real_num_tx_queues = adapter->num_tx_queues;
+	netif_set_real_num_tx_queues(adapter->netdev, adapter->num_tx_queues);
 }
 
 static void ixgbe_acquire_msix_vectors(struct ixgbe_adapter *adapter,

include/linux/netdevice.h

@@ -1656,6 +1656,9 @@ static inline int netif_is_multiqueue(const struct net_device *dev)
 	return (dev->num_tx_queues > 1);
 }
 
+extern void netif_set_real_num_tx_queues(struct net_device *dev,
+					 unsigned int txq);
+
 /* Use this variant when it is known for sure that it
  * is executing from hardware interrupt context or with hardware interrupts
  * disabled.

include/net/sch_generic.h

@@ -313,13 +313,12 @@ extern void qdisc_calculate_pkt_len(struct sk_buff *skb,
 extern void tcf_destroy(struct tcf_proto *tp);
 extern void tcf_destroy_chain(struct tcf_proto **fl);
 
-/* Reset all TX qdiscs of a device.  */
-static inline void qdisc_reset_all_tx(struct net_device *dev)
+/* Reset all TX qdiscs greater then index of a device.  */
+static inline void qdisc_reset_all_tx_gt(struct net_device *dev, unsigned int i)
 {
-	unsigned int i;
 	struct Qdisc *qdisc;
 
-	for (i = 0; i < dev->num_tx_queues; i++) {
+	for (; i < dev->num_tx_queues; i++) {
 		qdisc = netdev_get_tx_queue(dev, i)->qdisc;
 		if (qdisc) {
 			spin_lock_bh(qdisc_lock(qdisc));
@@ -329,6 +328,11 @@ static inline void qdisc_reset_all_tx(struct net_device *dev)
 	}
 }
 
+static inline void qdisc_reset_all_tx(struct net_device *dev)
+{
+	qdisc_reset_all_tx_gt(dev, 0);
+}
+
 /* Are all TX queues of the device empty?  */
 static inline bool qdisc_all_tx_empty(const struct net_device *dev)
 {

net/core/dev.c

@@ -1553,6 +1553,24 @@ static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
 	rcu_read_unlock();
 }
 
+/*
+ * Routine to help set real_num_tx_queues. To avoid skbs mapped to queues
+ * greater then real_num_tx_queues stale skbs on the qdisc must be flushed.
+ */
+void netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
+{
+	unsigned int real_num = dev->real_num_tx_queues;
+
+	if (unlikely(txq > dev->num_tx_queues))
+		;
+	else if (txq > real_num)
+		dev->real_num_tx_queues = txq;
+	else if (txq < real_num) {
+		dev->real_num_tx_queues = txq;
+		qdisc_reset_all_tx_gt(dev, txq);
+	}
+}
+EXPORT_SYMBOL(netif_set_real_num_tx_queues);
 
 static inline void __netif_reschedule(struct Qdisc *q)
 {