Re: [PATCH net-next] net: use in_softirq() to indicate the NAPI context in napi_consume_skb()

[Eric Dumazet <edumazet@google.com>]

On Mon, Sep 21, 2020 at 4:08 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> When napi_consume_skb() is called in the tx desc cleaning process,
> it is usually in the softirq context(BH disabled, or are processing
> softirqs), but it may also be in the task context, such as in the
> netpoll or loopback selftest process.
>
> Currently napi_consume_skb() uses non-zero budget to indicate the
> NAPI context, the driver writer may provide the wrong budget when
> tx desc cleaning function is reused for both NAPI and non-NAPI
> context, see [1].
>
> So this patch uses in_softirq() to indicate the NAPI context, which
> doesn't necessarily mean in NAPI context, but it shouldn't care if
> NAPI context or not as long as it runs in softirq context or with BH
> disabled, then _kfree_skb_defer() will push the skb to the particular
> cpu' napi_alloc_cache atomically.
>
> [1] https://lkml.org/lkml/2020/9/15/38
>
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> ---
> note that budget parameter is not removed in this patch because it
> involves many driver changes, we can remove it in separate patch if
> this patch is accepted.
> ---
>  net/core/skbuff.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index e077447..03d0d28 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -895,8 +895,10 @@ void __kfree_skb_defer(struct sk_buff *skb)
>
>  void napi_consume_skb(struct sk_buff *skb, int budget)
>  {
> -       /* Zero budget indicate non-NAPI context called us, like netpoll */
> -       if (unlikely(!budget)) {
> +       /* called by non-softirq context, which usually means non-NAPI
> +        * context, like netpoll.
> +        */
> +       if (unlikely(!in_softirq())) {
>                 dev_consume_skb_any(skb);
>                 return;
>         }
> --


I do not think we should add this kind of fuzzy logic, just because
_one_ driver author made a mistake.

Add a disable_bh() in the driver slow path, and accept the _existing_
semantic, the one that was understood by dozens.