Re: [PATCH net-next v5 3/8] TUN, TAP & vhost_net: Stop netdev queue before reaching a full ptr_ring
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2025-09-23 14:48:00
Also in:
kvm, lkml, virtualization
On Tue, Sep 23, 2025 at 12:15:48AM +0200, Simon Schippers wrote:
quoted hunk ↗ jump to hunk
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 <redacted> Signed-off-by: Tim Gebauer <redacted> Signed-off-by: Simon Schippers <redacted> --- 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