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>

1 files changed, 18 insertions, 0 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 2b3bf53bc687..723a34710ad4 100644
--- a/net/core/dev.c
+++ b/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)
 {