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

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Message-ID: <CAM_iQpUxLDa1yQMDvCH760xGOsDMYz9wvHro64VYamnhXE=+yA@mail.gmail.com>
Date:   Thu, 8 Sep 2016 22:24:32 -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 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... ;-)