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SubjectRe: 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@nuclearcat.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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