On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
<[email protected]> wrote:
>
> 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/[email protected]/
>
> Co-developed-by: Tim Gebauer <[email protected]>
> Signed-off-by: Tim Gebauer <[email protected]>
> Signed-off-by: Simon Schippers <[email protected]>
> ---
> 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);
I'm not sure I will get here, but I think those should be moved to the
following if(ret) check. If __ptr_ring_produce() succeed, there's no
need to bother with those queue stop/wake logic?
> + }
> + 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
>
Thanks
