This commit prevents tail-drop when a qdisc is present and the ptr_ring
becomes full. Once an entry is successfully produced and the ptr_ring
reaches capacity, the netdev queue is stopped instead of dropping
subsequent packets.

If producing an entry fails anyways, the tun_net_xmit returns
NETDEV_TX_BUSY, again avoiding a drop. Such failures are expected because
LLTX is enabled and the transmit path operates without the usual locking.
As a result, concurrent calls to tun_net_xmit() are not prevented.

The existing __{tun,tap}_ring_consume functions free space in the
ptr_ring and wake the netdev queue. Races between this wakeup and the
queue-stop logic could leave the queue stopped indefinitely. To prevent
this, a memory barrier is enforced (as discussed in a similar
implementation in [1]), followed by a recheck that wakes the queue if
space is already available.

If no qdisc is present, the previous tail-drop behavior is preserved.

+-------------------------+-----------+---------------+----------------+
| pktgen benchmarks to    | Stock     | Patched with  | Patched with   |
| Debian VM, i5 6300HQ,   |           | noqueue qdisc | fq_codel qdisc |
| 10M packets             |           |               |                |
+-----------+-------------+-----------+---------------+----------------+
| TAP       | Transmitted | 196 Kpps  | 195 Kpps      | 185 Kpps       |
|           +-------------+-----------+---------------+----------------+
|           | Lost        | 1618 Kpps | 1556 Kpps     | 0              |
+-----------+-------------+-----------+---------------+----------------+
| TAP       | Transmitted | 577 Kpps  | 582 Kpps      | 578 Kpps       |
|  +        +-------------+-----------+---------------+----------------+
| vhost-net | Lost        | 1170 Kpps | 1109 Kpps     | 0              |
+-----------+-------------+-----------+---------------+----------------+

[1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/ (local)

Co-developed-by: Tim Gebauer <redacted>
Signed-off-by: Tim Gebauer <redacted>
Signed-off-by: Simon Schippers <redacted>
---
 drivers/net/tun.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 71b6981d07d7..74d7fd09e9ba 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -1008,6 +1008,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct netdev_queue *queue;
 	struct tun_file *tfile;
 	int len = skb->len;
+	bool qdisc_present;
+	int ret;
 
 	rcu_read_lock();
 	tfile = rcu_dereference(tun->tfiles[txq]);

@@ -1060,13 +1062,38 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	nf_reset_ct(skb);
 
-	if (ptr_ring_produce(&tfile->tx_ring, skb)) {
+	queue = netdev_get_tx_queue(dev, txq);
+	qdisc_present = !qdisc_txq_has_no_queue(queue);
+
+	spin_lock(&tfile->tx_ring.producer_lock);
+	ret = __ptr_ring_produce(&tfile->tx_ring, skb);
+	if (__ptr_ring_produce_peek(&tfile->tx_ring) && qdisc_present) {
+		netif_tx_stop_queue(queue);
+		/* Avoid races with queue wake-up in
+		 * __{tun,tap}_ring_consume by waking if space is
+		 * available in a re-check.
+		 * The barrier makes sure that the stop is visible before
+		 * we re-check.
+		 */
+		smp_mb__after_atomic();
+		if (!__ptr_ring_produce_peek(&tfile->tx_ring))
+			netif_tx_wake_queue(queue);
+	}
+	spin_unlock(&tfile->tx_ring.producer_lock);
+
+	if (ret) {
+		/* If a qdisc is attached to our virtual device,
+		 * returning NETDEV_TX_BUSY is allowed.
+		 */
+		if (qdisc_present) {
+			rcu_read_unlock();
+			return NETDEV_TX_BUSY;
+		}
 		drop_reason = SKB_DROP_REASON_FULL_RING;
 		goto drop;
 	}
 
 	/* dev->lltx requires to do our own update of trans_start */
-	queue = netdev_get_tx_queue(dev, txq);
 	txq_trans_cond_update(queue);
 
 	/* Notify and wake up reader process */

-- 
2.43.0