Revert "tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present"

This reverts commit 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3.

The commit stops the netdev queue when the ptr_ring is full instead of
dropping the packet. My own tests showed no relevant regression, but on
Brett Sheffield's librecast testbed an IPv6 multicast testcase got
slower. With 8 iperf3 TCP threads sending, the throughput dropped from
13.5 Gbit/s to 9.13 Gbit/s.

Reported-by: Brett Sheffield <brett@librecast.net>
Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/
Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260728092240.250257-2-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index ffbe6f13fb1fa2333227a6b085806c82a362c0a4..ec5573f545af6128fe53e51f107e29c362a93923 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1018,7 +1018,6 @@ 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;
-       int ret;
 
        rcu_read_lock();
        tfile = rcu_dereference(tun->tfiles[txq]);
@@ -1073,33 +1072,13 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 
        nf_reset_ct(skb);
 
-       queue = netdev_get_tx_queue(dev, txq);
-
-       spin_lock(&tfile->tx_ring.producer_lock);
-       ret = __ptr_ring_produce(&tfile->tx_ring, skb);
-       if (!qdisc_txq_has_no_queue(queue) &&
-           __ptr_ring_check_produce(&tfile->tx_ring) == -ENOSPC) {
-               netif_tx_stop_queue(queue);
-               /* Paired with smp_mb() in __tun_wake_queue() */
-               smp_mb__after_atomic();
-               if (!__ptr_ring_check_produce(&tfile->tx_ring))
-                       netif_tx_wake_queue(queue);
-       }
-       spin_unlock(&tfile->tx_ring.producer_lock);
-
-       if (ret) {
-               /* This should be a rare case if a qdisc is present, but
-                * can happen due to lltx.
-                * Since skb_tx_timestamp(), skb_orphan(),
-                * run_ebpf_filter() and pskb_trim() could have tinkered
-                * with the SKB, returning NETDEV_TX_BUSY is unsafe and
-                * we must drop instead.
-                */
+       if (ptr_ring_produce(&tfile->tx_ring, skb)) {
                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 */