netdev - Re: [PATCH net] net: fix race between napi kthread mode and busy poll

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Date:   Wed, 24 Feb 2021 16:49:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Wei Wang <weiwan@...gle.com>
Cc:     Alexander Duyck <alexanderduyck@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH net] net: fix race between napi kthread mode and busy
 poll

On Wed, 24 Feb 2021 16:44:55 -0800 Wei Wang wrote:
> On Wed, Feb 24, 2021 at 4:33 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Wed, 24 Feb 2021 16:16:58 -0800 Wei Wang wrote:  
> > > On Wed, Feb 24, 2021 at 4:11 PM Alexander Duyck <alexanderduyck@...com> wrote:  
>  [...]  
> > >
> > > Please help hold on to the patch for now. I think Martin is still
> > > seeing issues on his setup even with this patch applied. I have not
> > > yet figured out why. But I think we should not merge this patch until
> > > the issue is cleared. Will update this thread with progress.  
> >
> > If I'm looking right __busy_poll_stop() is only called if the last
> > napi poll used to re-enable IRQs consumed full budget. You need to
> > clear your new bit in busy_poll_stop(), not in __busy_poll_stop().
> > That will fix the case when hand off back to the normal poller (sirq,
> > or thread) happens without going thru __napi_schedule().  
> 
> If the budget is not fully consumed, napi_complete_done() should have
> been called by the driver which will clear SCHED_BUSY_POLL bit.

Ah, right.