linux-kernel - Re: [PATCH net-next 4/6] net: ipa: ensure hardware has power in ipa_start

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Message-ID: <ebc4e7af-8300-307c-9278-25fdd6bf1e65@linaro.org>
Date:   Mon, 16 Aug 2021 12:56:40 -0500
From:   Alex Elder <elder@...aro.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, bjorn.andersson@...aro.org,
        evgreen@...omium.org, cpratapa@...eaurora.org,
        subashab@...eaurora.org, elder@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 4/6] net: ipa: ensure hardware has power in
 ipa_start_xmit()

On 8/16/21 9:20 AM, Alex Elder wrote:
> On 8/16/21 9:15 AM, Jakub Kicinski wrote:
>> On Fri, 13 Aug 2021 21:25:23 -0500 Alex Elder wrote:
>>>> This is racy, what if the pm work gets scheduled on another CPU and
>>>> calls wake right here (i.e. before you call netif_stop_queue())?
>>>> The queue may never get woken up?
>>>
>>> I haven't been seeing this happen but I think you may be right.
>>>
>>> I did think about this race, but I think I was relying on the
>>> PM work queue to somehow avoid the problem.  I need to think
>>> about this again after a good night's sleep.  I might need
>>> to add an atomic flag or something.
>>
>> Maybe add a spin lock?  Seems like the whole wake up path will be
>> expensive enough for a spin lock to be in the noise. You can always
>> add complexity later.
> 
> Exactly what I just decided after trying to work out a
> clever way without using a spinlock...  I'll be sending
> out a fix today.  Thanks.

I'm finding this isn't an easy problem to solve (or even think
about).  While I ponder the best course of action I'm going
to send out another series (i.e., *before* I send a fix for
this issue) because I'd like to get everything I have out
for review this week.  I *will* address this potential race
one way or another, possibly later today.

					-Alex

> 
>                      -Alex
>