netdev - Re: [PATCH] net-fq: Add WARN

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Date:   Thu, 7 Jun 2018 09:33:11 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net-fq: Add WARN_ON check for null flow.

On 06/07/2018 09:17 AM, Eric Dumazet wrote:
>
>
> On 06/07/2018 09:06 AM, greearb@...delatech.com wrote:
>> From: Ben Greear <greearb@...delatech.com>
>>
>> While testing an ath10k firmware that often crashed under load,
>> I was seeing kernel crashes as well.  One of them appeared to
>> be a dereference of a NULL flow object in fq_tin_dequeue.
>>
>> I have since fixed the firmware flaw, but I think it would be
>> worth adding the WARN_ON in case the problem appears again.
>>
>>  common_interrupt+0xf/0xf
>>  </IRQ>
>>
>
> Please find the exact commit that brought this bug,
> and add a corresponding Fixes: tag

It will be a total pain to bisect this problem since my test
case that causes this is running my modified firmware (and a buggy one at that),
modified ath10k driver (to work with this firmware and support my test case easily),
and the failure case appears to cause multiple different-but-probably-related
crashes and often hangs or reboots the test system.

Probably this is all caused by some nasty race or buggy logic related to
dealing with a crashed ath10k firmware tearing down txq logic from the
bottom up.  There have been many such bugs in the past, I and others fixed a few,
and very likely more remain.

For what it is worth, I didn't see this crash in 4.13, and I spent some time
testing buggy firmware there occasionally.

If someone else has interest in debugging the ath10k driver, I will be happy to generate
a mostly-stock firmware image with ability to crash in the TX path and give it to them.
It will crash the stock upstream code reliably in my experience.

Thanks,
Ben

>
>> Signed-off-by: Ben Greear <greearb@...delatech.com>
>> ---
>>  include/net/fq_impl.h | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/include/net/fq_impl.h b/include/net/fq_impl.h
>> index be7c0fa..e40354d 100644
>> --- a/include/net/fq_impl.h
>> +++ b/include/net/fq_impl.h
>> @@ -80,6 +80,9 @@ static struct sk_buff *fq_tin_dequeue(struct fq *fq,
>>
>>  	flow = list_first_entry(head, struct fq_flow, flowchain);
>>
>> +	if (WARN_ON_ONCE(!flow))
>> +		return NULL;
>> +
>>  	if (flow->deficit <= 0) {
>>  		flow->deficit += fq->quantum;
>>  		list_move_tail(&flow->flowchain,
>>
>

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com