netdev - Re: [PATCH net-next v2 03/10] tun: switch to net core provided statistics counters

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Message-ID: <20201105171446.5f78f1a6@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 5 Nov 2020 17:14:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Harald Welte <laforge@...monks.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        osmocom-net-gprs@...ts.osmocom.org, wireguard@...ts.zx2c4.com,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH net-next v2 03/10] tun: switch to net core provided
 statistics counters

On Wed, 4 Nov 2020 15:25:24 +0100 Heiner Kallweit wrote:
> @@ -1066,7 +1054,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	return NETDEV_TX_OK;
>  
>  drop:
> -	this_cpu_inc(tun->pcpu_stats->tx_dropped);
> +	dev->stats.tx_dropped++;
>  	skb_tx_error(skb);
>  	kfree_skb(skb);
>  	rcu_read_unlock();

This is no longer atomic. Multiple CPUs may try to update it at the
same time.

Do you know what the story on dev->rx_dropped is? The kdoc says drivers
are not supposed to use it but:

drivers/net/ipvlan/ipvlan_core.c:               atomic_long_inc(&skb->dev->rx_dropped);
drivers/net/macvlan.c:  atomic_long_inc(&skb->dev->rx_dropped);
drivers/net/vxlan.c:            atomic_long_inc(&vxlan->dev->rx_dropped);

Maybe tun can use it, too?