netdev - Re: RCU callback crashes

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Message-ID: <20171220160855.0c0fbcd7@cakuba.netronome.com>
Date:   Wed, 20 Dec 2017 16:08:55 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RCU callback crashes

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;
>         }

Mm.. I was running with this hack for the last two hours and it was OK:

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 876fab2604b8..d7e0c3ad0a1c 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1260,6 +1260,7 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
                 * are not seeing it.
                 */
                call_rcu_bh(&miniq_old->rcu, mini_qdisc_rcu_func);
+       rcu_barrier_bh();
 }
 EXPORT_SYMBOL(mini_qdisc_pair_swap);

Let me try to move the barrier...