netdev - Re: [PATCH net] net_sched: act_mirred: full rcu conversion

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Message-ID: <CAM_iQpXeNY1Kxo1MvZPW5JqB+wEvUXoD0oh-LoB2CdOL=qh9CQ@mail.gmail.com>
Date:   Sun, 11 Sep 2016 22:46:02 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        John Fastabend <john.fastabend@...il.com>,
        Hadar Hen Zion <hadarh@...lanox.com>,
        Amir Vadai <amirva@...lanox.com>
Subject: Re: [PATCH net] net_sched: act_mirred: full rcu conversion

On Fri, Sep 9, 2016 at 5:23 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2016-09-08 at 22:24 -0700, Cong Wang wrote:
>> On Thu, Sep 8, 2016 at 8:35 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > From: Eric Dumazet <edumazet@...gle.com>
>> >
>> > As reported by Cong Wang, I was lazy when I did initial RCU conversion
>> > of tc_mirred, as I thought I could avoid allocation/freeing of a
>> > parameter block.
>>
>> Quote from Eric Dumazet:
>>
>> https://www.mail-archive.com/netdev@vger.kernel.org/msg115482.html
>>
>> <Quote>
>> Well, I added a READ_ONCE() to read tcf_action once.
>>
>> Adding rcu here would mean adding a pointer and extra cache line, to
>> deref the values.
>>
>> IMHO the race here has no effect . You either read the old or new value.
>> </Quote>
>>
>> Me with facepalm... ;-)
>
>
> Point is still valid. Show me a real case where it was a serious
> problem, instead of simply theoretical.
>
> tc_mirred + ifb patches allowed us to reach a milestone, removing the
> last contended spinlocks, and you are catching up with this one year
> later.
>
> I wont backport this fix in Google prod kernels, because there is
> absolutely no way we need it, and the extra memory cache line might hurt
> latencies.
>
> Since you did not write a fix on your side since June 17th, I presume
> you do not care that much.

Sounds like I were the author of this patch.... Why are you questioning
your own patch? Did I ask you to care about it? ;-)

Please drop this patch.

Thanks.