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/ Co-developed-by: Tim Gebauer Signed-off-by: Tim Gebauer Signed-off-by: Simon Schippers --- 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