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

[Yajun Deng]

On 2023/10/8 14:45, Eric Dumazet wrote:
> On Sat, Oct 7, 2023 at 8:34 AM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
> > On 2023/10/7 13:29, Eric Dumazet wrote:
> > > On Sat, Oct 7, 2023 at 7:06 AM Yajun Deng <yajun.deng@xxxxxxxxx> 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@xxxxxxxxx/
> >
> > > 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);


Yes, I tested this code. But the following couldn't show anything even 
if the rx_dropped is increasing.

'sudo python3 /usr/share/bcc/tools/trace netdev_core_stats_inc'

It needs to add anything else. The above command will show correctly.

> 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.


Me too.

If I add a pr_info in your code, the kprobe will be ok.