netdev - Re: kernel panic in 4.2.3, rb_erase in sch

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Date:	Wed, 04 Nov 2015 07:13:24 +0200
From:	Denys Fedoryshchenko <nuclearcat@...learcat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Cong Wang <cwang@...pensource.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: kernel panic in 4.2.3, rb_erase in sch_fq

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@...learcat.com> 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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