netdev - Re: pegged softirq and NAPI race (?)

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Message-ID: <C6C872D4-ABC4-4740-918D-ADD09442B330@fb.com>
Date:   Tue, 18 Sep 2018 21:36:37 +0000
From:   Song Liu <songliubraving@...com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     Florian Fainelli <f.fainelli@...il.com>,
        Alexei Starovoitov <ast@...com>,
        netdev <netdev@...r.kernel.org>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: pegged softirq and NAPI race (?)



> On Sep 18, 2018, at 2:28 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> 
> On Tue, Sep 18, 2018 at 2:25 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>> 
>> 
>> This would not be the only driver doing this unfortunately... should we
>> add a __must_check annotation to help catch those (mis)uses? Though that
>> could cause false positives for drivers using NAPI to clean their TX ring.
>> 
> 
> Well, before adding __must_check we would need to cook a (big) patch
> series to change all occurrences.
> 
> 
> Not clear why netpoll is the trigger ?

>From my observation, the trigger path is:

netpoll_poll_dev() => ndo_poll_controller() => napi_schedule_prep(). 

I guess it is a race between IRQ/netpoll=>napi_schedule_prep() and 
not handled napi_complete_done(). netpoll just makes it triggers more
often? 

Unfortunately for me, bnxt already checked napi_complete_done(), so 
there should be another bug with bnxt (at least with our 4.11 based
kernel). 

Song