Re: kernel panic in 4.2.3, rb_erase in sch_fq

[Denys Fedoryshchenko]

On 2015-11-04 06:58, Eric Dumazet wrote:
> On Tue, 2015-11-03 at 20:46 -0800, Eric Dumazet wrote:
> > On Wed, 2015-11-04 at 06:25 +0200, Denys Fedoryshchenko wrote:
> > > On 2015-11-04 00:06, Cong Wang wrote:
> > > > On Mon, Nov 2, 2015 at 6:11 AM, Denys Fedoryshchenko
> > > > <nuclearcat@xxxxxxxxxxxxxx> wrote:
> > > >> Hi!
> > > >>
> > > >> Actually seems i was getting this panic for a while (once per week) on
> > > >> loaded pppoe server, but just now was able to get full panic message.
> > > >> After checking commit logs on sch_fq.c i didnt seen any fixes, so
> > > >> probably
> > > >> upgrading to newer kernel wont help?
> > > >
> > > >
> > > > Can you share your `tc qdisc show dev xxxx` with us? And how to
> > > > reproduce
> > > > it? I tried to setup htb+fq and then flip the interface back and forth
> > > > but I don't
> > > > see any crash.
> > > My guess it wont be easy to reproduce, it is happening on box with 4.5k
> > > interfaces, that constantly create/delete interfaces,
> > > and even with that this problem may happen once per day, or may not
> > > happen for 1 week.
> > >
> > > Here is script that is being fired after new ppp interface detected. But
> > > pppoe process are independent from
> > > process that are "establishing" shapers.
> >
> >
> > It is probably a generic bug. sch_fq seems OK to me.
> >
> > Somehow nobody tries to change qdisc hundred times per second ;)
> >
> > Could you try following patch ?
> >
> > It seems to 'fix' the issue for me.
>
> Following patch would be more appropriate.
> Prior one was meant to 'show' the issue.
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index cb5d4ad32946..7f5f3e8a10f5 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -706,9 +706,11 @@ struct Qdisc *dev_graft_qdisc(struct netdev_queue
> *dev_queue,
>  	spin_lock_bh(root_lock);
>
>  	/* Prune old scheduler */
> -	if (oqdisc && atomic_read(&oqdisc->refcnt) <= 1)
> -		qdisc_reset(oqdisc);
> -
> +	if (oqdisc) {
> +		if (atomic_read(&oqdisc->refcnt) <= 1)
> +			qdisc_reset(oqdisc);
> +		set_bit(__QDISC_STATE_DEACTIVATED, &oqdisc->state);
> +	}
>  	/* ... and graft new one */
>  	if (qdisc == NULL)
>  		qdisc = &noop_qdisc;

Applied, will test it, but this bug might be triggered rarely.
I will try to push it to more pppoe servers in order to stress test them 
(and 4.3) more.
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