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

From: Yajun Deng
Date: Sun Oct 08 2023 - 03:00:20 EST



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.