Bug 12858 - rb_erase Kernel Panic
rb_erase Kernel Panic
Product: Networking
Classification: Unclassified
Component: Other
All Linux
: P1 high
Assigned To: Jarek Poplawski
Depends on:
  Show dependency treegraph
Reported: 2009-03-11 23:15 UTC by m0sia
Modified: 2010-01-19 21:37 UTC (History)
1 user (show)

See Also:
Kernel Version:
Tree: Mainline
Regression: No

Call trace from server remote control (376.46 KB, image/png)
2009-03-11 23:17 UTC, m0sia

Description m0sia 2009-03-11 23:15:51 UTC
Distribution: Debian
Problem Description: rb_erase Kernel Panic

EIP: [<c01c313b>] rb_erase+0xf6/Bx22f SS:ESP 0068:da3c3e34
Kernel panic - not syncing: Fatal exception in interrupt

Full call trace in attachment.

Steps to reproduce:
tc qdisc add dev eth1 root handle 1: htb 

tc class add dev eth1 parent 1: classid 1:1 htb rate 36000Kbit
tc class add dev eth1 parent 1:1 classid 1:11 htb rate 28000Kbit prio 0
tc class add dev eth1 parent 1:1 classid 1:15 htb rate 1000Kbit ceil 28000Kbit prio 0
tc class add dev eth1 parent 1:1 classid 1:19 htb rate 1000Kbit ceil 5000Kbit prio 2

N from 10 to 10k(One class and filter per user)
tc class add dev eth1 parent 1:{11,15,19} classid 1:$N htb rate 1Kbit ceil {$SPEED}Kbit
tc filter add dev eth1 parent parent 1: protocol ip pref $N handle $N fw flowid 1:$N

Classes and filters for users may change or add in real time.
Comment 1 m0sia 2009-03-11 23:17:21 UTC
Created attachment 20498 [details]
Call trace from server remote control
Comment 2 Jarek Poplawski 2009-03-12 05:02:44 UTC
Could you add some details, e.g.:
- is it vanilla kernel or debian built, or with some other
patches? (Could it be replaced with newer version BTW?)
- how often this bug happens or could it be reproduced reliably?
- is there any chance to get the beginning lines of this bug report
(before the call trace)?

Jarek P.

Comment 3 m0sia 2009-03-12 06:05:27 UTC
1) It is vanilla from kernel.org. Yes i can replace with new one.
2) This bug happened only once during half-year server usage. But if it happens only once it is still a bug. =) During last year there was another bug, that annoyed me: http://bugzilla.kernel.org/show_bug.cgi?id=11571 Thank you for fix.
3) There is no chance to get begining lines. There is nothing in syslog and i have only screenshot from iLo(hp remote console).

For now i wonder, what means 
"/* If this triggers, it is a bug in this code, but it need not be fatal */" in htb code near htb_safe_rb_erase.
Comment 4 Jarek Poplawski 2009-03-12 11:07:15 UTC
Of course, you are right this is a bug even if spotted only once, and it's very nice you've reported this. I only tried to assess chances of debugging it. Alas it doesn't look so nice. =(

This call trace is similar to reports I tracked long time ago and fixed just in ...2.6.26 (my own earlier bug BTW). I looked a lot for this at that time and if there was something left I guess it's really well hidden. So, I'll try to look again, but I'm not very optimistic if it's so unreproducible.

I'm glad you mentioned this 11571 bug: as a matter of fact I wasn't sure this patch worked for you, so I'll add this info to that report.

The htb_safe_rb_erase comment means somebody admitted the possibility of double erasing on some code path, and made it safe for debugging with WARN_ON. But I can't remember such reports, so probably it's better then expected.

Jarek P.
Comment 5 Jarek Poplawski 2009-03-14 05:51:07 UTC
After looking for this I think there really is at least one buggy place in sch_htb in kernels 2.6.26 and older, which can cause such oopses. I mean htb_destroy_class calling htb_safe_rb_erase without sch_tree_lock. Now I wonder why it happened so rarely...

Happily this place was fixed (BTW) in 2.6.27 by this patch:
"net-sched: sch_htb: move hash and sibling list removal to htb_delete"
commit	fbd8f1379aeeb3e44a59302a6b2850636130bb2a

My recommendation is to upgrade to 2.6.27 or later. Alternatively you could try to use this patch vs. 2.6.26, but I didn't test this, and 2.6.26 is rarely updated now.

So, this bug report is really helpful, but my proposal is to close it unless confirmed after some time on newer kernels.

Jarek P.

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