Re: [PATCH net-next v6 3/8] tun/tap: add synchronized ring produce/consume with queue management

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

On Wed, Nov 26, 2025 at 05:04:25PM +0100, Simon Schippers wrote:
> On 11/26/25 16:25, Michael S. Tsirkin wrote:
> > On Wed, Nov 26, 2025 at 10:23:50AM +0100, Simon Schippers wrote:
> >> On 11/25/25 17:54, Michael S. Tsirkin wrote:
> >>> On Thu, Nov 20, 2025 at 04:29:08PM +0100, Simon Schippers wrote:
> >>>> Implement new ring buffer produce and consume functions for tun and tap
> >>>> drivers that provide lockless producer-consumer synchronization and
> >>>> netdev queue management to prevent ptr_ring tail drop and permanent
> >>>> starvation.
> >>>>
> >>>> - tun_ring_produce(): Produces packets to the ptr_ring with proper memory
> >>>>   barriers and proactively stops the netdev queue when the ring is about
> >>>>   to become full.
> >>>>
> >>>> - __tun_ring_consume() / __tap_ring_consume(): Internal consume functions
> >>>>   that check if the netdev queue was stopped due to a full ring, and wake
> >>>>   it when space becomes available. Uses memory barriers to ensure proper
> >>>>   ordering between producer and consumer.
> >>>>
> >>>> - tun_ring_consume() / tap_ring_consume(): Wrapper functions that acquire
> >>>>   the consumer lock before calling the internal consume functions.
> >>>>
> >>>> Key features:
> >>>> - Proactive queue stopping using __ptr_ring_full_next() to stop the queue
> >>>>   before it becomes completely full.
> >>>> - Not stopping the queue when the ptr_ring is full already, because if
> >>>>   the consumer empties all entries in the meantime, stopping the queue
> >>>>   would cause permanent starvation.
> >>>
> >>> what is permanent starvation? this comment seems to answer this
> >>> question:
> >>>
> >>>
> >>> 	/* Do not stop the netdev queue if the ptr_ring is full already.
> >>> 	 * The consumer could empty out the ptr_ring in the meantime
> >>> 	 * without noticing the stopped netdev queue, resulting in a
> >>> 	 * stopped netdev queue and an empty ptr_ring. In this case the
> >>> 	 * netdev queue would stay stopped forever.
> >>> 	 */
> >>>
> >>>
> >>> why having a single entry in
> >>> the ring we never use helpful to address this?
> >>>
> >>>
> >>>
> >>>
> >>> In fact, all your patch does to solve it, is check
> >>> netif_tx_queue_stopped on every consumed packet.
> >>>
> >>>
> >>> I already proposed:
> >>>
> >>> static inline int __ptr_ring_peek_producer(struct ptr_ring *r)
> >>> {
> >>>         if (unlikely(!r->size) || r->queue[r->producer])
> >>>                 return -ENOSPC;
> >>>         return 0;
> >>> }
> >>>
> >>> And with that, why isn't avoiding the race as simple as
> >>> just rechecking after stopping the queue?
> >>  
> >> I think you are right and that is quite similar to what veth [1] does.
> >> However, there are two differences:
> >>
> >> - Your approach avoids returning NETDEV_TX_BUSY by already stopping
> >>   when the ring becomes full (and not when the ring is full already)
> >> - ...and the recheck of the producer wakes on !full instead of empty.
> >>
> >> I like both aspects better than the veth implementation.
> > 
> > Right.
> > 
> > Though frankly, someone should just fix NETDEV_TX_BUSY already
> > at least with the most popular qdiscs.
> > 
> > It is a common situation and it is just annoying that every driver has
> > to come up with its own scheme.
> 
> I can not judge it, but yes, it would have made this patchset way
> simpler.
> 
> > 
> > 
> > 
> > 
> > 
> >> Just one thing: like the veth implementation, we probably need a
> >> smp_mb__after_atomic() after netif_tx_stop_queue() as they also discussed
> >> in their v6 [2].
> > 
> > yea makes sense.
> > 
> >>
> >> On the consumer side, I would then just do:
> >>
> >> __ptr_ring_consume();
> >> if (unlikely(__ptr_ring_consume_created_space()))
> >>     netif_tx_wake_queue(txq);
> >>
> >> Right?
> >>
> >> And for the batched consume method, I would just call this in a loop.
> > 
> > Well tun does not use batched consume does it?
> 
> tun does not but vhost-net does.
> 
> Since vhost-net also uses tun_net_xmit() as its ndo_start_xmit in a
> tap+vhost-net setup, its consumer must also be changed. Else
> tun_net_xmit() would stop the queue, but it would never be woken again.


Ah, ok.



> > 
> > 
> >> Thank you!
> >>
> >> [1] Link: https://lore.kernel.org/netdev/174559288731.827981.8748257839971869213.stgit@firesoul/T/#m2582fcc48901e2e845b20b89e0e7196951484e5f
> >> [2] Link: https://lore.kernel.org/all/174549933665.608169.392044991754158047.stgit@firesoul/T/#m63f2deb86ffbd9ff3a27e1232077a3775606c14d
> >>
> >>>
> >>> __ptr_ring_produce();
> >>> if (__ptr_ring_peek_producer())
> >>> 	netif_tx_stop_queue
> >>
> >> smp_mb__after_atomic(); // Right here
> >>
> >>> 	if (!__ptr_ring_peek_producer())
> >>> 		netif_tx_wake_queue(txq);
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >