Re: [PATCH] can: vcan: use per-CPU device stats

[Yogesh Gaur]

On Wed, Oct 7, 2026 at 10:41 PM Oliver Hartkopp <socketcan@xxxxxxxxxxxx> wrote:
>
>
>
> On 07.10.26 13:36, Yogesh Gaur wrote:
> > vcan_tx() and vcan_rx() update dev->stats with plain increments and
> > rely on the tx queue lock to serialise them.  That only holds for a
> > single tx queue.  vcan does not implement get_num_tx_queues(), so
> > rtnl_create_link() honours IFLA_NUM_TX_QUEUES and a vcan device can be
> > created with several queues
>
> dev_dstats_tx_add() has been introduced in Linux 6.14 so it is probably
> some can-next material for Linux 7.4+.

Agreed.

>
> Btw. why don't we implement get_num_tx_queues() and return 1 as a
> potential stable fix? This would add a correct locking via the existing
> tx queue lock for all vcan statistics right?

The locking argument holds: vcan is not LLTX, and every counter update
(tx, the echo-mode rx in vcan_rx(), and tx_dropped from
can_dropped_invalid_skb()) happens in vcan_tx() under the xmit locak.

But get_num_tx_queues() alone does not stop the multi-queue case, as
rtnl_create_link() only consults it when userspace did not ask.
>
> Best regards,
> Oliver
>
> ps. vxcan should have a similar problem.

Yes - same pattern and the same single-queue argument works there.

Plan for v2: limiting vcan and vxcan to one TX queue, then the dstats
conversion for can-next.

Yogesh
>
> , each with its own xmit lock.  Two CPUs
> > transmitting on different queues then update the same counters
> > concurrently and lose increments:
> >
> >    BUG: KCSAN: data-race in vcan_tx / vcan_tx
> >    read-write to 0xffff88811aad8228 of 8 bytes by interrupt on cpu 1:
> >     vcan_tx+0x325/0x5d0 drivers/net/can/vcan.c:110
> >     dev_hard_start_xmit+0x10c/0x380 net/core/dev.c:3969
> >     __dev_queue_xmit+0xbfd/0x1ec0 net/core/dev.c:4958
> >     can_send+0x584/0x720 net/can/af_can.c:279
> >     bcm_can_tx+0x3ba/0x5b0 net/can/bcm.c:367
> >    read-write to 0xffff88811aad8228 of 8 bytes by interrupt on cpu 0:
> >     vcan_tx+0x325/0x5d0 drivers/net/can/vcan.c:110
> >    value changed: 0x00000000000025ef -> 0x00000000000025f0
> >
> > Switch the packet and byte counters to the core's per-CPU dstats.
> > The core allocates them for NETDEV_PCPU_STAT_DSTATS and folds them
> > together with dev->stats in dev_get_stats(), so tx_dropped from
> > can_dropped_invalid_skb() is still reported.
> >
> > Reported-by: syzbot+937a3a1fbfbc99d6fcaf@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=937a3a1fbfbc99d6fcaf
> > Assisted-by: LLM
> > Signed-off-by: Yogesh Gaur <yogeshgaur.83@xxxxxxxxx>
> > ---
> >   drivers/net/can/vcan.c | 13 ++++---------
> >   1 file changed, 4 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/net/can/vcan.c b/drivers/net/can/vcan.c
> > index 76e6b7b5c6a1..2ff90cb7eee9 100644
> > --- a/drivers/net/can/vcan.c
> > +++ b/drivers/net/can/vcan.c
> > @@ -71,10 +71,7 @@ MODULE_PARM_DESC(echo, "Echo sent frames (for testing). Default: 0 (Off)");
> >
> >   static void vcan_rx(struct sk_buff *skb, struct net_device *dev)
> >   {
> > -     struct net_device_stats *stats = &dev->stats;
> > -
> > -     stats->rx_packets++;
> > -     stats->rx_bytes += can_skb_get_data_len(skb);
> > +     dev_dstats_rx_add(dev, can_skb_get_data_len(skb));
> >
> >       skb->pkt_type  = PACKET_BROADCAST;
> >       skb->dev       = dev;
> > @@ -85,7 +82,6 @@ static void vcan_rx(struct sk_buff *skb, struct net_device *dev)
> >
> >   static netdev_tx_t vcan_tx(struct sk_buff *skb, struct net_device *dev)
> >   {
> > -     struct net_device_stats *stats = &dev->stats;
> >       unsigned int len;
> >       int loop;
> >
> > @@ -93,8 +89,7 @@ static netdev_tx_t vcan_tx(struct sk_buff *skb, struct net_device *dev)
> >               return NETDEV_TX_OK;
> >
> >       len = can_skb_get_data_len(skb);
> > -     stats->tx_packets++;
> > -     stats->tx_bytes += len;
> > +     dev_dstats_tx_add(dev, len);
> >
> >       /* set flag whether this packet has to be looped back */
> >       loop = skb->pkt_type == PACKET_LOOPBACK;
> > @@ -107,8 +102,7 @@ static netdev_tx_t vcan_tx(struct sk_buff *skb, struct net_device *dev)
> >                       /* only count the packets here, because the
> >                        * CAN core already did the echo for us
> >                        */
> > -                     stats->rx_packets++;
> > -                     stats->rx_bytes += len;
> > +                     dev_dstats_rx_add(dev, len);
> >               }
> >               consume_skb(skb);
> >               return NETDEV_TX_OK;
> > @@ -185,6 +179,7 @@ static void vcan_setup(struct net_device *dev)
> >       dev->netdev_ops         = &vcan_netdev_ops;
> >       dev->ethtool_ops        = &vcan_ethtool_ops;
> >       dev->needs_free_netdev  = true;
> > +     dev->pcpu_stat_type     = NETDEV_PCPU_STAT_DSTATS;
> >   }
> >
> >   static struct rtnl_link_ops vcan_link_ops __read_mostly = {
>