netdev - Re: [Intel-wired-lan] [PATCH 1/1 iwl-net] idpf: disable local BH when scheduling napi for marker packets

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Message-ID: <8fa3c29d-c7d3-421d-b567-e9bf997e6751@intel.com>
Date: Wed, 14 Feb 2024 07:39:53 -0800
From: "Tantilov, Emil S" <emil.s.tantilov@...el.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>, Alan Brady
	<alan.brady@...el.com>
CC: Jesse Brandeburg <jesse.brandeburg@...el.com>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, <intel-wired-lan@...ts.osuosl.org>,
	<netdev@...r.kernel.org>
Subject: Re: [Intel-wired-lan] [PATCH 1/1 iwl-net] idpf: disable local BH when
 scheduling napi for marker packets



On 2/14/2024 6:54 AM, Alexander Lobakin wrote:
> From: Alexander Lobakin <aleksander.lobakin@...el.com>
> Date: Tue, 13 Feb 2024 14:16:47 +0100
> 
>> From: Alan Brady <alan.brady@...el.com>
>> Date: Wed,  7 Feb 2024 16:42:43 -0800
>>
>>> From: Emil Tantilov <emil.s.tantilov@...el.com>
>>>
>>> Fix softirq's not being handled during napi_schedule() call when
>>> receiving marker packets for queue disable by disabling local bottom
>>> half.
>>
>> BTW, how exactly does this help?
>>
>> __napi_schedule() already disables interrupts (local_irq_save()).
>> napi_schedule_prep() only has READ_ONCE() and other atomic read/write
>> helpers.
>>
>> It's always been safe to call napi_schedule() with enabled BH, so I
>> don't really understand how this works.

It's been a while since I debugged this, I'll have to take a look again, 
but its not so much about being safe as it is about making sure the 
marker packets are received in those cases - like ifdown in the trace.

> This also needs to be dropped from the fixes queue until investigated.
> For now, it looks like a cheap hack (without the explanation how exactly
> it does help), not a proper fix.

It does fix the issue at hand. Looking at the kernel code I see multiple 
examples of napi_schedule() being wrapped with local_bh_disable/enable, 
so this appears to be a common (not uncommon?) technique.

Thanks,
Emil

> 
> Thanks,
> Olek