netdev - Re: RCU callback crashes

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Message-ID: <CAM_iQpVPUifm3rcXu8SP9ShSmm7z9z+8UjppdY_AxMYQwHE9YQ@mail.gmail.com>
Date:   Wed, 20 Dec 2017 10:31:25 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kubakici@...pl>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RCU callback crashes

On Wed, Dec 20, 2017 at 10:17 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> I guess it is q->miniqp which is freed in qdisc_graft() without properly
> waiting for rcu readers?

It is probably so, the call_rcu_bh(&miniq_old->rcu, mini_qdisc_rcu_func)
in the end of mini_qdisc_pair_swap() is invoked on miniq_old->rcu,
but miniq is being freed, no rcu barrier waits for it...

You can try to add a rcu_barrier_bh() at the end to see if this crash
is gone, but I don't think people like adding yet another rcu barrier...