netdev - Re: 5.15-rc3+ crash in fq-codel?

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Date:   Mon, 27 Sep 2021 17:16:39 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: 5.15-rc3+ crash in fq-codel?

On 9/27/21 5:04 PM, Ben Greear wrote:
> On 9/27/21 4:49 PM, Eric Dumazet wrote:
>>
>>
>> On 9/27/21 4:30 PM, Ben Greear wrote:
>>> Hello,
>>>
>>> In a hacked upon kernel, I'm getting crashes in fq-codel when doing bi-directional
>>> pktgen traffic on top of mac-vlans.  Unfortunately for me, I've made big changes to
>>> pktgen so I cannot easily run this test on stock kernels, and there is some chance
>>> some of my hackings have caused this issue.
>>>
>>> But, in case others have seen similar, please let me know.  I shall go digging
>>> in the meantime...
>>>
>>> Looks to me like 'skb' is NULL in line 120 below.
>>
>>
>> pktgen must not be used in a mode where a single skb
>> is cloned and reused, if packet needs to be stored in a qdisc.
>>
>> qdisc of all sorts assume skb->next/prev can be used as
>> anchor in their list.
>>
>> If the same skb is queued multiple times, lists are corrupted.
>>
>> Please double check your clone_skb pktgen setup.
>>
>> I thought we had IFF_TX_SKB_SHARING for this, and that macvlan was properly clearing this bit.
> 
> My pktgen config was not using any duplicated queueing in this case.
> 
> I changed to pfifo fast and so far it is stable for ~10 minutes, where before it would crash
> within a minute.  I'll let it bake overnight....

Still running stable.  I also notice we have been using fq-codel for a while and haven't noticed
this problem (next most recent kernel we might have run similar test on would be 5.13-ish).

I'll duplicate this test on our older kernels tomorrow to see if it looks like a regression or
if we just haven't actually done this exact test in a while...

Thanks,
Ben