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

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Date:   Mon, 13 Aug 2018 12:16:52 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net-next] net: sched: act_ife: disable bh when taking ife_mod_lock

On Mon, Aug 13, 2018 at 10:20 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
> Lockdep reports deadlock for following locking scenario in ife action:
>
> Task one:
> 1) Executes ife action update.
> 2) Takes tcfa_lock.
> 3) Waits on ife_mod_lock which is already taken by task two.
>
> Task two:
>
> 1) Executes any path that obtains ife_mod_lock without disabling bh (any
> path that takes ife_mod_lock while holding tcfa_lock has bh disabled) like
> loading a meta module, or creating new action.
> 2) Takes ife_mod_lock.
> 3) Task is preempted by rate estimator timer.
> 4) Timer callback waits on tcfa_lock which is taken by task one.
>
> In described case tasks deadlock because they take same two locks in
> different order. To prevent potential deadlock reported by lockdep, always
> disable bh when obtaining ife_mod_lock.

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

The only time you need tcfa_lock is when adding it to ->metalist:

list_add_tail(&mi->metalist, &ife->metalist);

when it already exists.

Which means you can just take tcfa_lock after taking ife_mod_lock.