Willem de Bruijn wrote:
> Simon Schippers wrote:
>> The netdev queue is stopped in tun_net_xmit after inserting an SKB into
>> the ring buffer if the ring buffer became full because of that. If the
>> insertion into the ptr_ring fails, the netdev queue is also stopped and
>> the SKB is dropped. However, this never happened in my testing.
>
> Indeed, since the last successful insertion will always pause the
> queue before this can happen. Since this cannot be reached, no need
> to add the code defensively. If in doubt, maybe add a
> NET_DEBUG_WARN_ON_ONCE.
>
>> To ensure
>> that the ptr_ring change is available to the consumer before the netdev
>> queue stop, an smp_wmb() is used.
>>
>> Then in tun_ring_recv, the new helper wake_netdev_queue is called in the
>> blocking wait queue and after consuming an SKB from the ptr_ring. This
>> helper first checks if the netdev queue has stopped. Then with the paired
>> smp_rmb() it is known that tun_net_xmit will not produce SKBs anymore.
>> With that knowledge, the helper can then wake the netdev queue if there is
>> at least a single spare slot in the ptr_ring by calling ptr_ring_spare
>> with cnt=1.
>>
>> Co-developed-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
>> Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
>> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
>> ---
>> drivers/net/tun.c | 33 ++++++++++++++++++++++++++++++---
>> 1 file changed, 30 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index cc6c50180663..735498e221d8 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1060,13 +1060,21 @@ 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);
>> + if (unlikely(ptr_ring_produce(&tfile->tx_ring, skb))) {
>> + /* Paired with smp_rmb() in wake_netdev_queue. */
>> + smp_wmb();
>> + netif_tx_stop_queue(queue);
>> drop_reason = SKB_DROP_REASON_FULL_RING;
>> goto drop;
>> }
>> + if (ptr_ring_full(&tfile->tx_ring)) {
>> + /* Paired with smp_rmb() in wake_netdev_queue. */
>> + smp_wmb();
>> + netif_tx_stop_queue(queue);
>> + }
>>
>> /* 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 */
>> @@ -2110,6 +2118,24 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>> return total;
>> }
>>
>> +static inline void wake_netdev_queue(struct tun_file *tfile)
>> +{
>> + struct netdev_queue *txq;
>> + struct net_device *dev;
>> +
>> + rcu_read_lock();
>> + dev = rcu_dereference(tfile->tun)->dev;
>> + txq = netdev_get_tx_queue(dev, tfile->queue_index);
>> +
>> + if (netif_tx_queue_stopped(txq)) {
>> + /* Paired with smp_wmb() in tun_net_xmit. */
>> + smp_rmb();
>> + if (ptr_ring_spare(&tfile->tx_ring, 1))
>> + netif_tx_wake_queue(txq);
>> + }
>> + rcu_read_unlock();
>> +}
>> +
>> static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>> {
>> DECLARE_WAITQUEUE(wait, current);
>> @@ -2139,7 +2165,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>> error = -EFAULT;
>> break;
>> }
>> -
>> + wake_netdev_queue(tfile);
>
> Why wake when no entry was consumed?
I do it because the queue may not have been woken the last time after
consuming an SKB. However, I am not sure if it is still absolutely
necessary after all the changes in the code. Still, I think it is wise to
do it to avoid being stuck in the wait queue under any circumstances.
>
> Also keep the empty line.
>
Okay :)
>> schedule();
>> }
>>
>> @@ -2147,6 +2173,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>> remove_wait_queue(&tfile->socket.wq.wait, &wait);
>>
>> out:
>> + wake_netdev_queue(tfile);
>> *err = error;
>> return ptr;
>> }
>> --
>> 2.43.0
>>
>
>
