netdev - Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats

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Message-ID: <CANn89iL-zUw1FqjYRSC7BGB0hfQ5uKpJzUba3YFd--c=GdOoGg@mail.gmail.com>
Date: Sat, 7 Oct 2023 07:29:24 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alexander Lobakin <aleksander.lobakin@...el.com>
Subject: Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats_inc()

On Sat, Oct 7, 2023 at 7:06 AM Yajun Deng <yajun.deng@...ux.dev> wrote:
>
> Although there is a kfree_skb_reason() helper function that can be used to
> find the reason why this skb is dropped, but most callers didn't increase
> one of rx_dropped, tx_dropped, rx_nohandler and rx_otherhost_dropped.
>
...

> +
> +void netdev_core_stats_inc(struct net_device *dev, u32 offset)
> +{
> +       /* This READ_ONCE() pairs with the write in netdev_core_stats_alloc() */
> +       struct net_device_core_stats __percpu *p = READ_ONCE(dev->core_stats);
> +       unsigned long *field;
> +
> +       if (unlikely(!p))
> +               p = netdev_core_stats_alloc(dev);
> +
> +       if (p) {
> +               field = (unsigned long *)((void *)this_cpu_ptr(p) + offset);
> +               WRITE_ONCE(*field, READ_ONCE(*field) + 1);

This is broken...

As I explained earlier, dev_core_stats_xxxx(dev) can be called from
many different contexts:

1) process contexts, where preemption and migration are allowed.
2) interrupt contexts.

Adding WRITE_ONCE()/READ_ONCE() is not solving potential races.

I _think_ I already gave you how to deal with this ?

Please try instead:

+void netdev_core_stats_inc(struct net_device *dev, u32 offset)
+{
+       /* This READ_ONCE() pairs with the write in netdev_core_stats_alloc() */
+       struct net_device_core_stats __percpu *p = READ_ONCE(dev->core_stats);
+       unsigned long __percpu *field;
+
+       if (unlikely(!p)) {
+               p = netdev_core_stats_alloc(dev);
+               if (!p)
+                       return;
+       }
+       field = (__force unsigned long __percpu *)((__force void *)p + offset);
+       this_cpu_inc(*field);
+}