Re: [PATCH net-next V7 2/2] veth: apply qdisc backpressure on full ptr_ring to reduce TX drops

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 6/10/25 2:40 PM, Jesper Dangaard Brouer wrote:
> 
> 
> On 10/06/2025 20.26, Ihor Solodrai wrote:
>> On 6/10/25 8:56 AM, Jesper Dangaard Brouer wrote:
>>>
>>>
>>> On 10/06/2025 13.43, Jesper Dangaard Brouer wrote:
>>>>
>>>> On 10/06/2025 00.09, Ihor Solodrai wrote:
>>> [...]
>>>>
>>>> Can you give me the output from below command (on your compiled 
>>>> kernel):
>>>>
>>>>   ./scripts/faddr2line drivers/net/veth.o veth_xdp_rcv.constprop.0+0x6b
>>>>
>>>
>>> Still need above data/info please.
>>
>> root@devvm7589:/ci/workspace# ./scripts/faddr2line ./kout.gcc/drivers/ 
>> net/veth.o veth_xdp_rcv.constprop.0+0x6b
>> veth_xdp_rcv.constprop.0+0x6b/0x390:
>> netdev_get_tx_queue at /ci/workspace/kout.gcc/../include/linux/ 
>> netdevice.h:2637
>> (inlined by) veth_xdp_rcv at /ci/workspace/kout.gcc/../drivers/net/ 
>> veth.c:912
>>
>> Which is:
>>
>> veth.c:912
>>      struct veth_priv *priv = netdev_priv(rq->dev);
>>      int queue_idx = rq->xdp_rxq.queue_index;
>>      struct netdev_queue *peer_txq;
>>      struct net_device *peer_dev;
>>      int i, done = 0, n_xdpf = 0;
>>      void *xdpf[VETH_XDP_BATCH];
>>
>>      /* NAPI functions as RCU section */
>>      peer_dev = rcu_dereference_check(priv->peer, 
>> rcu_read_lock_bh_held());
>>   --->    peer_txq = netdev_get_tx_queue(peer_dev, queue_idx);
>>
>> netdevice.h:2637
>>      static inline
>>      struct netdev_queue *netdev_get_tx_queue(const struct net_device 
>> *dev,
>>                       unsigned int index)
>>      {
>>          DEBUG_NET_WARN_ON_ONCE(index >= dev->num_tx_queues);
>>   --->        return &dev->_tx[index];
>>      }
>>
>> So the suspect is peer_dev (priv->peer)?
> 
> Yes, this is the problem!
> 
> So, it seems that peer_dev (priv->peer) can become a NULL pointer.
> 
> Managed to reproduce - via manually deleting the peer device:
>   - ip link delete dev veth42
>   - while overloading veth41 via XDP redirecting packets into it.
> 
> Managed to trigger concurrent crashes on two CPUs (C0 + C3)
>   - so below output gets interlaced a bit:
> 
> [...]
> 
> A fix could look like this:
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index e58a0f1b5c5b..a3046142cb8e 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -909,7 +909,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
> 
>          /* NAPI functions as RCU section */
>          peer_dev = rcu_dereference_check(priv->peer, 
> rcu_read_lock_bh_held());
> -       peer_txq = netdev_get_tx_queue(peer_dev, queue_idx);
> +       peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : 
> NULL;
> 
>          for (i = 0; i < budget; i++) {
>                  void *ptr = __ptr_ring_consume(&rq->xdp_ring);
> @@ -959,7 +959,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
>          rq->stats.vs.xdp_packets += done;
>          u64_stats_update_end(&rq->stats.syncp);
> 
> -       if (unlikely(netif_tx_queue_stopped(peer_txq)))
> +       if (peer_txq && unlikely(netif_tx_queue_stopped(peer_txq)))
>                  netif_tx_wake_queue(peer_txq);
> 

Great! I presume you will send a patch separately?

> 
> 
> 
> --Jesper
> 
>