Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats_inc()

[Eric Dumazet <edumazet@google.com>]

On Sat, Oct 7, 2023 at 8:34 AM Yajun Deng <yajun.deng@linux.dev> wrote:
>
>
> On 2023/10/7 13:29, Eric Dumazet wrote:
> > On Sat, Oct 7, 2023 at 7:06 AM Yajun Deng <yajun.deng@linux.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 ?
>
>
> Yes, I replied in v6.
>
> https://lore.kernel.org/all/e25b5f3c-bd97-56f0-de86-b93a3172870d@linux.dev/
>
> > 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);
> > +}
>
>
> This wouldn't trace anything even the rx_dropped is in increasing. It
> needs to add an extra operation, such as:

I honestly do not know what you are talking about.

Have you even tried to change your patch to use

field = (__force unsigned long __percpu *)((__force void *)p + offset);
this_cpu_inc(*field);

Instead of the clearly buggy code you had instead :

    field = (unsigned long *)((void *)this_cpu_ptr(p) + offset);
     WRITE_ONCE(*field, READ_ONCE(*field) + 1);

If your v7 submission was ok for tracing what you wanted,
I fail to see why a v8 with 3 lines changed would not work.