netdev - Re: [PATCH] net: Revert "net_sched: no need to free qdisc in RCU callback"

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Message-ID: <CAM_iQpVkV0YbOaSfmXb_=OpzpxepmxkXHKibKUcmhELZBM-Abg@mail.gmail.com>
Date:   Thu, 21 Dec 2017 12:59:56 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kubakici@...pl>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] net: Revert "net_sched: no need to free qdisc in RCU callback"

On Thu, Dec 21, 2017 at 12:39 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>
> Why just moving qdisc_free to rcu is not enough? It would resolve this
> issue and also avoid using synchronize net. Something like:

If you mean Jakub's issue, apparently not:
https://www.kernel.org/pub/linux/kernel/people/paulmck/Answers/RCU/RCUCBordering.html

Jiri, you have to use a rcu barrier to wait for a rcu callback, not
queuing another rcu callback, the ordering is simply NOT guaranteed.

What's more importantly, you already have one rcu barrier in the
same function. Why keep believing you don't need it?