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

[Oliver Hartkopp <socketcan@hartkopp.net>]

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+.

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?

Best regards,
Oliver

ps. vxcan should have a similar problem.

, 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@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=937a3a1fbfbc99d6fcaf
> Assisted-by: LLM
> Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
> ---
>   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 = {