Re: [PATCH net-next v5 3/8] TUN, TAP & vhost_net: Stop netdev queue before reaching a full ptr_ring

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 24, 2025 at 07:41:28AM +0200, Simon Schippers wrote:
> Hi,
> first of all thank you very much for your detailed replies! :)
> 
> On 23.09.25 16:47, Michael S. Tsirkin wrote:
> > 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 <tim.gebauer@tu-dortmund.de>
> >> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> >> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> >> ---
> >>  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.
> > 
> 
> I agree.
> 
> > 
> > But it is racy isn't it? So first of all I suspect you
> > are missing an mb before netif_tx_stop_queue.
> > 
> 
> I don’t really get this point right now.

ring full next is a read. stop queue is a write. if you are
relying on ordering them in some way you need a full mb generally.




> > 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.
> > 
> 
> Will be covered in the next mail.
> 
> > 
> > 
> > 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.
> > 
> > 
> 
> You convinced me about changing my implementation anyway but here my (old) 
> idea:
> I did this in V1-V4. The problem is that vhost_net is only called on 
> EPOLLIN triggered by tun_net_xmit. Then, after consuming a batch from the 
> ptr_ring, it must be able to see if the netdev queue stopped or not. If 
> this is not the case the ptr_ring might get empty and vhost_net is not 
> able to wake the queue again (because it is not stopped from its POV), 
> which happened in my testing in my V4.
> 
> This is the reason why, now in the V5, in tun_net_xmit I stop the netdev 
> queue before producing. With that I exploit the smp_wmb() in 
> __ptr_ring_produce which is paired with the READ_ONCE in __ptr_ring_peek 
> to ensure that the consumer in vhost_net sees that the netdev queue 
> stopped after consuming a batch.

yea you said it somewhere in code, too, and I am not sure I understand it all, but
wmb isn't paired with READ_ONCE generally. barrier pairing
is described in memory-barriers.txt, READ_ONCE is not a barrier
at all.

> > 
> > 
> >> -
> >> -	/* 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
> >