netdev - Re: RCU callback crashes

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Date:   Wed, 20 Dec 2017 14:38:49 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jakub Kicinski <kubakici@...pl>, Jiri Pirko <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RCU callback crashes

On Wed, Dec 20, 2017 at 12:23 PM, John Fastabend
<john.fastabend@...il.com> wrote:
> I'm trying to see how removing that rcu grace period was safe in the
> first place. The datapath is using rcu_read critical section to protect
> the qdisc but the control path (a) doesn't use rcu grace period and (b)
> doesn't use the qidisc lock. Going to go get a coffee and I'll think
> about it a bit more. Any ideas Cong?

qdisc_graft() -> dev_deactivate() -> synchronize_net() is the reason
you want to find?

Also, you can try `git log` to see why it was introduced in the beginning,
here it is:

commit 5d944c640b4ae5f37c537acf491c2f0eb89fa0d6
Author: Eric Dumazet <eric.dumazet@...il.com>
Date:   Wed Mar 31 07:06:04 2010 +0000

    gen_estimator: deadlock fix