netdev - Re: [Patch net v2 2/2] net_sched: fix all the madness of tc filter chain

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Message-ID: <CAM_iQpWT6sRWGTt+nPtj=dcfzeL6np6nOPOzrM7kdtGh5TL_gQ@mail.gmail.com>
Date:   Mon, 11 Sep 2017 14:58:16 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [Patch net v2 2/2] net_sched: fix all the madness of tc filter chain

On Mon, Sep 11, 2017 at 2:14 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> Looks like all due to the lack of locking on block->chain_list.
> I thought the rcu_barrier() could properly handle this,
> but seems still not, probably I need to move it in the loop,
> I am still not 100% sure if it is totally safe with
> list_for_each_safe():
>
>
> -       list_for_each_entry(chain, &block->chain_list, list)
> +       list_for_each_entry_safe(chain, tmp, &block->chain_list, list) {
>                 tcf_chain_flush(chain);
> -       rcu_barrier();
> +               rcu_barrier(); // are we safe now???
> +       }
>

Answer myself: No, this is not safe either, because we may
list_del() the next node, and apparently _safe() can't guarantee
that...

So either we have to use locking or use the trick you suggested.