netdev - Re: RCU callback crashes

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Date:   Wed, 20 Dec 2017 23:24:50 -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 4:37 PM, Jakub Kicinski <kubakici@...pl> wrote:
> On Wed, 20 Dec 2017 16:03:49 -0800, Cong Wang wrote:
>> 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;
>>         }
>
> Looks good after 30 minutes, feel free to add if you post officially:
>
> Tested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>

Thanks for testing! I just sent a formal patch out.