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

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Date:   Mon, 13 Aug 2018 12:21:48 -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 12:16 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> 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.

BTW, there is an obvious deadlock:

use_all_metadata() acquires read_lock(&ife_mod_lock), then calls
add_metainfo() which calls find_ife_oplist() which acquires the same
lock....

But this is _irreverent_ to your fix, just want to point it out.