netdev - Re: [Patch net] net_sched: fix a missing rcu barrier in mini_qdisc_pair

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Message-ID: <CAM_iQpVyycftiydUi3VJsZ1QMj45dWDvkmsOquT10JqF-zrwkw@mail.gmail.com>
Date:   Thu, 21 Dec 2017 12:54:06 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...lanox.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch net] net_sched: fix a missing rcu barrier in mini_qdisc_pair_swap()

On Thu, Dec 21, 2017 at 11:01 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Thu, Dec 21, 2017 at 1:03 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>>
>>
>> But again, we don't we just free qdisc in call_rcu and avoid the
>> barrier?
>
>
> Non-sense again. Why qdisc code should be adjusted for your
> miniq code? It is your own responsibility to take care of this shit.
> Don't spread it out of minq.

Also, in case you believe call_rcu to free qdisc is queued after
the call_rcu in miniq, you are wrong again:

https://www.kernel.org/pub/linux/kernel/people/paulmck/Answers/RCU/RCUCBordering.html

The rcu callbacks don't guarantee FIFO ordering.