netdev - Re: [Patch net] ipv4: restore rt->fi for reference counting

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Date:   Fri, 12 May 2017 10:49:13 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Julian Anastasov <ja@....bg>, David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] ipv4: restore rt->fi for reference counting

On Thu, May 11, 2017 at 9:55 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2017-05-11 at 18:22 -0700, Cong Wang wrote:
>> On Thu, May 11, 2017 at 5:07 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> > So, if I understand you correctly it is safe to NULL'ing
>> > nh_dev in NETDEV_UNREGISTER_FINAL, right?
>> >
>> > If still not, how about transfer nh_dev's to loopback_dev
>> > too in NETDEV_UNREGISTER? Like we transfer dst->dev.
>> >
>> > I don't want to touch the fast path to check for NULL, as
>> > it will change more code and slow down performance.
>>
>> Finally I come up with the attached patch. Please let me know if
>> I still miss anything.
>
> You have not addressed my prior feedback, unless I am mistaken ?
>
> fib_route_seq_show() and others do not expect fi->fib_dev suddenly
> becoming NULL.
>
> RCU contract is more complicated than simply adding rcu grace period
> before delete.
>

If you mean the lack of rcu_dereference() in fib_route_seq_show() ,
yeah, I don't fix it because I think it should be in a separate patch,
it is not this patch which introduces RCU to nh_dev as I explained
previously.

Or you mean anything else?