On Tue, Sep 23, 2025 at 12:15:48AM +0200, Simon Schippers wrote: > Stop the netdev queue ahead of __ptr_ring_produce when > __ptr_ring_full_next signals the ring is about to fill. Due to the > smp_wmb() of __ptr_ring_produce the consumer is guaranteed to be able to > notice the stopped netdev queue after seeing the new ptr_ring entry. As > both __ptr_ring_full_next and __ptr_ring_produce need the producer_lock, > the lock is held during the execution of both methods. > > dev->lltx is disabled to ensure that tun_net_xmit is not called even > though the netdev queue is stopped (which happened in my testing, > resulting in rare packet drops). Consequently, the update of trans_start > in tun_net_xmit is also removed. > > 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 | 16 ++++++++++------ > 1 file changed, 10 insertions(+), 6 deletions(-) > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c > index 86a9e927d0ff..c6b22af9bae8 100644 > --- a/drivers/net/tun.c > +++ b/drivers/net/tun.c > @@ -931,7 +931,7 @@ static int tun_net_init(struct net_device *dev) > dev->vlan_features = dev->features & > ~(NETIF_F_HW_VLAN_CTAG_TX | > NETIF_F_HW_VLAN_STAG_TX); > - dev->lltx = true; > + dev->lltx = false; > > tun->flags = (tun->flags & ~TUN_FEATURES) | > (ifr->ifr_flags & TUN_FEATURES); > @@ -1060,14 +1060,18 @@ 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); > + > + spin_lock(&tfile->tx_ring.producer_lock); > + if (__ptr_ring_full_next(&tfile->tx_ring)) > + netif_tx_stop_queue(queue); > + > + if (unlikely(__ptr_ring_produce(&tfile->tx_ring, skb))) { > + spin_unlock(&tfile->tx_ring.producer_lock); > drop_reason = SKB_DROP_REASON_FULL_RING; > goto drop; > }
The comment makes it sound like you always keep one slot free in the queue but that is not the case - you just check before calling __ptr_ring_produce. But it is racy isn't it? So first of all I suspect you are missing an mb before netif_tx_stop_queue. Second it's racy because more entries can get freed afterwards. Which maybe is ok in this instance? But it really should be explained in more detail, if so. Now - why not just check ring full *after* __ptr_ring_produce? Why do we need all these new APIs, and we can use existing ones which at least are not so hard to understand. > - > - /* dev->lltx requires to do our own update of trans_start */ > - queue = netdev_get_tx_queue(dev, txq); > - txq_trans_cond_update(queue); > + spin_unlock(&tfile->tx_ring.producer_lock); > > /* Notify and wake up reader process */ > if (tfile->flags & TUN_FASYNC) > -- > 2.43.0
