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);