netdev - Re: [PATCH net] net_sched: gen_estimator: fix lockdep splat

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Date:   Sun, 28 Jan 2018 19:44:36 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net] net_sched: gen_estimator: fix lockdep splat

On Sat, Jan 27, 2018 at 10:58 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> syzbot reported a lockdep splat in gen_new_estimator() /
> est_fetch_counters() when attempting to lock est->stats_lock.
>
> Since est_fetch_counters() is called from BH context from timer
> interrupt, we need to block BH as well when calling it from process
> context.
>
> Most qdiscs use per cpu counters and are immune to the problem,
> but net/sched/act_api.c and net/netfilter/xt_RATEEST.c are using
> a spinlock to protect their data. They both call gen_new_estimator()
> while object is created and not yet alive, so this bug could
> not trigger a deadlock, only a lockdep splat.
>
> Fixes: 1c0d32fde5bd ("net_sched: gen_estimator: complete rewrite of rate estimators")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>

Acked-by: Cong Wang <xiyou.wangcong@...il.com>

Nit: perhaps it is better to move the spin_lock out of est_fetch_counters()
and let callers decide to use spin_lock or spin_lock_bh.