netdev - Re: RCU callback crashes

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Date:   Wed, 20 Dec 2017 16:03:49 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kubakici@...pl>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RCU callback crashes

On Wed, Dec 20, 2017 at 10:31 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Wed, Dec 20, 2017 at 10:17 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>
>> I guess it is q->miniqp which is freed in qdisc_graft() without properly
>> waiting for rcu readers?
>
> It is probably so, the call_rcu_bh(&miniq_old->rcu, mini_qdisc_rcu_func)
> in the end of mini_qdisc_pair_swap() is invoked on miniq_old->rcu,
> but miniq is being freed, no rcu barrier waits for it...
>
> You can try to add a rcu_barrier_bh() at the end to see if this crash
> is gone, but I don't think people like adding yet another rcu barrier...

Hi, Jakub

Can you test the following fix? I am not a fan of rcu barrier but we
already have one so...

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 876fab2604b8..1b68fedea124 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1240,6 +1240,8 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,

        if (!tp_head) {
                RCU_INIT_POINTER(*miniqp->p_miniq, NULL);
+               /* Wait for existing flying RCU callback before being freed. */
+               rcu_barrier_bh();
                return;
        }