Re: kernel panic in 4.2.3, rb_erase in sch_fq

[Eric Dumazet <eric.dumazet@gmail.com>]

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.

diff --git a/net/core/dev.c b/net/core/dev.c
index 8ce3f74cd6b9..bf136103bc7b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2880,6 +2880,12 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 		spin_lock(&q->busylock);
 
 	spin_lock(root_lock);
+	if (unlikely(q != rcu_dereference_bh(txq->qdisc))) {
+		pr_err_ratelimited("Arg, qdisc changed ! state %lx\n", q->state);
+		kfree_skb(skb);
+		rc = NET_XMIT_DROP;
+		goto end;
+	}
 	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
 		kfree_skb(skb);
 		rc = NET_XMIT_DROP;
@@ -2913,6 +2919,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 			__qdisc_run(q);
 		}
 	}
+end:
 	spin_unlock(root_lock);
 	if (unlikely(contended))
 		spin_unlock(&q->busylock);