netdev - Re: [PATCH net-next v6 2/3] net: implement threaded-able napi poll loop support

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Message-ID: <CAKgT0UeP2YWwim1QELj_6mp1R7HGPgtwcd_xruAZAmJk9ivR9A@mail.gmail.com>
Date:   Thu, 14 Jan 2021 19:38:49 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Wei Wang <weiwan@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>
Subject: Re: [PATCH net-next v6 2/3] net: implement threaded-able napi poll
 loop support

On Thu, Jan 14, 2021 at 7:14 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> On Thu, Jan 14, 2021 at 4:33 PM Wei Wang <weiwan@...gle.com> wrote:
> >

<snip>

> > +void napi_enable(struct napi_struct *n)
> > +{
> > +       BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> > +       smp_mb__before_atomic();
> > +       clear_bit(NAPI_STATE_SCHED, &n->state);
> > +       clear_bit(NAPI_STATE_NPSVC, &n->state);
> > +       WARN_ON(napi_set_threaded(n, n->dev->threaded));
>
> I am not sure what the point is in having a return value if you are
> just using it to trigger a WARN_ON. It might make more sense to
> actually set the WARN_ON inside of napi_set_threaded instead of having
> it here as you could then identify the error much more easily. Or for
> that matter you might be able to use something like pr_warn which
> would allow you a more detailed message about the specific netdev that
> experienced the failure.

One additional change I would make here. The call to napi_set_threaded
should be moved to before the smp_mb__before_atomic(). That way we can
guarantee that the threaded flag and task_struct pointer are visible
to all consumers before they can set NAPI_STATE_SCHED. Otherwise I
think we run the risk of a race where a napi request could fire before
we have finished configuring it.