linux-kernel - Re: [PATCH net] net: natsemi: fix `rx_dropped` double accounting on `netif

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Message-ID: <CANn89iLUTs4oKK30g8AjYhreM2Krwt5sAwzsO=xU--G7myt6WQ@mail.gmail.com>
Date: Fri, 12 Sep 2025 07:19:00 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Yeounsu Moon <yyyynoom@...il.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: natsemi: fix `rx_dropped` double accounting on
 `netif_rx()` failure

On Wed, Sep 10, 2025 at 10:35 PM Yeounsu Moon <yyyynoom@...il.com> wrote:
>
> `netif_rx()` already increments `rx_dropped` core stat when it fails.
> The driver was also updating `ndev->stats.rx_dropped` in the same path.
> Since both are reported together via `ip -s -s` command, this resulted
> in drops being counted twice in user-visible stats.
>
> Keep the driver update on `skb_put()` failure, but skip it after
> `netif_rx()` errors.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

I do not think this Fixes: is correct.

I think core networking got this accounting in netif_rx() in 2010

commit caf586e5f23c (" net: add a core netdev->rx_dropped counter")


> Signed-off-by: Yeounsu Moon <yyyynoom@...il.com>
> ---
>  drivers/net/ethernet/natsemi/ns83820.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/ethernet/natsemi/ns83820.c b/drivers/net/ethernet/natsemi/ns83820.c
> index 56d5464222d9..cdbf82affa7b 100644
> --- a/drivers/net/ethernet/natsemi/ns83820.c
> +++ b/drivers/net/ethernet/natsemi/ns83820.c
> @@ -820,7 +820,7 @@ static void rx_irq(struct net_device *ndev)
>         struct ns83820 *dev = PRIV(ndev);
>         struct rx_info *info = &dev->rx_info;
>         unsigned next_rx;
> -       int rx_rc, len;
> +       int len;
>         u32 cmdsts;
>         __le32 *desc;
>         unsigned long flags;
> @@ -881,8 +881,10 @@ static void rx_irq(struct net_device *ndev)
>                 if (likely(CMDSTS_OK & cmdsts)) {
>  #endif
>                         skb_put(skb, len);
> -                       if (unlikely(!skb))

I doubt this driver is used.

Notice that this test  about skb being NULL or not happens after
skb_put(skb, len)
which would have crashed anyway if skb was NULL.


> +                       if (unlikely(!skb)) {
> +                               ndev->stats.rx_dropped++;
>                                 goto netdev_mangle_me_harder_failed;
> +                       }
>                         if (cmdsts & CMDSTS_DEST_MULTI)
>                                 ndev->stats.multicast++;
>                         ndev->stats.rx_packets++;
> @@ -901,15 +903,12 @@ static void rx_irq(struct net_device *ndev)
>                                 __vlan_hwaccel_put_tag(skb, htons(ETH_P_IPV6), tag);
>                         }
>  #endif
> -                       rx_rc = netif_rx(skb);
> -                       if (NET_RX_DROP == rx_rc) {
> -netdev_mangle_me_harder_failed:
> -                               ndev->stats.rx_dropped++;
> -                       }
> +                       netif_rx(skb);
>                 } else {
>                         dev_kfree_skb_irq(skb);
>                 }
>
> +netdev_mangle_me_harder_failed:
>                 nr++;
>                 next_rx = info->next_rx;
>                 desc = info->descs + (DESC_SIZE * next_rx);
> --
> 2.51.0
>