netdev - Re: [PATCH net-next] net: sched: act_ife: disable bh when taking ife_mod

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Message-Id: <20180813.155328.1817591638806945356.davem@davemloft.net>
Date:   Mon, 13 Aug 2018 15:53:28 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     xiyou.wangcong@...il.com
Cc:     vladbu@...lanox.com, netdev@...r.kernel.org, jhs@...atatu.com,
        jiri@...nulli.us
Subject: Re: [PATCH net-next] net: sched: act_ife: disable bh when taking
 ife_mod_lock

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Mon, 13 Aug 2018 12:16:52 -0700

> Your fix doesn't make sense, because what ife_mod_lock protects
> is absolutely not touched in BH context, they have no race.

It does make sense, the problem is if you acquire ife_mod_lock and
take a software interrupt while you hold it.

If that software interrupt takes the tcfa_lock, we're setup for an
AB-BA deadlock.

And there is also no easy way to reverse the lock ordering to
avoid this either.

I therefore think his fix is perfectly fine and that's why I
applied it.