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Date:	Sat, 18 Jan 2014 00:26:54 +0000
From:	Dominic Curran <dominic.curran@...rix.com>
To:	<netdev@...r.kernel.org>
CC:	Dominic Curran <dominic.curran@...rix.com>,
	Jason Wang <jasowang@...hat.com>,
	Maxim Krasnyansky <maxk@...lcomm.com>
Subject: [RFC PATCH] tuntap: Fix for a race in accessing numqueue

A patch for fixing a race between queue selection and changing queues
was introduced in commit 92bb73ea2c434618a68a5.

The fix was to prevent the driver from re-reading the tun->numqueues
more than once within tun_select_queue().

We have been experiancing  'Divide-by-zero' errors in 
tun_net_xmit() since we moved from 3.6 to 3.10, and believe that they 
come from a simular source where the value of tun->numqueues changes 
to zero between the first and second read of tun->numqueues.

Signed-off-by: Dominic Curran <dominic.curran@...rix.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: Maxim Krasnyansky <maxk@...lcomm.com>

---
 drivers/net/tun.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: net-next/drivers/net/tun.c
===================================================================
--- net-next.orig/drivers/net/tun.c	2014-01-17 23:41:54.000000000 +0000
+++ net-next/drivers/net/tun.c	2014-01-17 23:51:55.000000000 +0000
@@ -738,12 +738,14 @@ static netdev_tx_t tun_net_xmit(struct s
 	struct tun_struct *tun = netdev_priv(dev);
 	int txq = skb->queue_mapping;
 	struct tun_file *tfile;
+	u32 numqueues = 0;
 
 	rcu_read_lock();
 	tfile = rcu_dereference(tun->tfiles[txq]);
+	numqueues = ACCESS_ONCE(tun->numqueues);
 
 	/* Drop packet if interface is not attached */
-	if (txq >= tun->numqueues)
+	if (txq >= numqueues)
 		goto drop;
 
 	if (tun->numqueues == 1) {
@@ -780,7 +782,7 @@ static netdev_tx_t tun_net_xmit(struct s
 	 * number of queues.
 	 */
 	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
-			  >= dev->tx_queue_len / tun->numqueues)
+			  >= dev->tx_queue_len / numqueues)
 		goto drop;
 
 	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
--
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