[OpenWrt-Devel] AA on brcm47xx: Unhandled kernel unaligned access

Nikolai Zhubr n-a-zhubr at yandex.ru
Fri Jun 20 16:12:40 EDT 2014

Hello people,

I have asus wl-500W router (http://wiki.openwrt.org/toh/asus/wl500w). It 
is also very similar to wl-500gp.
Some few months ago I updated to 12.09. I can't recall now if it was 
backfire or kamikaze before, but I noticed 2 things immediately:

1. Maximum practically achievable download speed increased somewhat.
(From ~30Mbit to ~34mbit approx)

2. After reaching (and keeping) this max download speed, the device will 
always reboot soon. (Absolutely reproducible)

For some time I was thinking that's just hardware, like bad electrolytic 
capacitors and/or weak power supply. Finally I opened the router, 
replaced all 3 capacitors (2 of 3 appeared somewhat damaged indeed), 
attached a voltmeter to check for undervoltage. Still nothing: power 
supply is OK, reboots still happen.

So I had to turn to the software side, and found 2 new things again:

1. While uplink load goes up approaching 34Mbit, softirqs eat up more 
and more CPU, approaching 100% CPU.

2. At some point I get (on a serial link):
[  368.948000] sched: RT throttling activated
[  382.688000] Unhandled kernel unaligned access[#1]:
[  382.932000] Kernel panic - not syncing: Fatal exception in interrupt
[  382.940000] Rebooting in 3 seconds..
(Trimmed all in between because there is no debugging info for now)

I suppose this is something that should not normally happen, and I'd 
like to have it fixed somehow. I haven't tried trunk yet, but I will, if 
it could make some difference. I can provide serial logs, compile trunk, 
apply patches, redo testing etc. (As time permits)
Any hints appreciated.

Thank you,
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