Re: [BUG] Kernel recieves DNS reply, but doesn't deliver it to a waiting application

[Andrew Savchenko <bircoph@gmail.com>]

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Hello,

On Wed, 16 Jan 2013 20:36:44 +0400 Andrew Savchenko wrote:
> On Fri, 28 Dec 2012 10:11:03 -0800 Eric Dumazet wrote:
[...]
> > Thanks a lot !
> > 
> > I see strange drops in dev_hard_start_xmit()
> > 
> > l2tp needs some care.
> > 
> > Please try the following patch, as skb_cow_head() API
> > doesnt really ease skb->truesize exact tracking anyway, better not mess
> > with it. 
> 
> Sorry for the delay, but I was able to reboot kernel only today.
> Your patch is applied on top of the 3.7.2 vanilla kernel.
> 
> l2tp works fine and /proc/net/udp tx_queue values are normal now, see
> attached /prot/net/udp output. This is a good hint that problem is
> probably solved, but we need to wait at least several weeks to be
> sure.

With 16-days uptime system works fine.

Also I was able to reproduce this bug on another box: an embedded
system running openwrt with 3.7.5 kernel with openl2tpd client and
dnsmasq server. Due to limited memory resources this bug happened to
be easily reproducible: several thousands of dns queries were
sufficient to reproduce this bug. Full debug on embedded box was not
possible due to constrained resources, but bug appearance was the same
and /proc/net/udp is apparently broken (see attached log).

I applied your patch on openwrt's 3.7.5 kernel and it fixed the bug
on this box too.

So we've found a solution and I'm looking forward for it in the main
kernel :)

Best regards,
Andrew Savchenko

[-- Attachment #1.2: proc.net.udp --]
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# cat /proc/net/udp
  sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops             
   42: 00000000:7B2A 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6239 2 816bb760 1                  
   53: 00000000:0035 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 1476 2 81b6e440 0                  
   67: 00000000:0043 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 1471 2 81b6e220 0                  
   76: 00000000:1B4C 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6225 2 816bacc0 1                  
   95: 00000000:165F 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6221 2 816ba880 1                  
  101: 0A34182E:AE65 0A320014:06A5 01 00000000:00000000 00:00000000 00000000     0        0 1538 4 81b6ecc0 0                  
  123: 00000000:007B 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 1530 2 81b6e880 0                  
  126: 00000000:847E 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6223 2 816baaa0 1                  
  153: 00000000:CF99 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6233 2 816bb540 1                  
  163: 00000000:09A3 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6227 2 816baee0 1                  
  165: 00000000:06A5 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 1523 2 81b6e660 0                  
  184: 00000000:70B8 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6231 2 816bb320 1                  
  226: 00000000:CCE2 00000000:0000 07 FFFFFFB0:00000000 00:00000000 00000000 65534        0 6229 2 816bb100 1

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