netdev - Re: [PATCH v2 net] net: solve a NAPI race

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Message-ID: <1488304037.9415.263.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 28 Feb 2017 09:47:17 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [PATCH v2 net] net: solve a NAPI race

On Tue, 2017-02-28 at 09:20 -0800, Alexander Duyck wrote:
> On Mon, Feb 27, 2017 at 6:21 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:

> > +bool napi_schedule_prep(struct napi_struct *n)
> > +{
> > +       unsigned long val, new;
> > +
> > +       do {
> > +               val = READ_ONCE(n->state);
> > +               if (unlikely(val & NAPIF_STATE_DISABLE))
> > +                       return false;
> > +               new = val | NAPIF_STATE_SCHED;
> > +               if (unlikely(val & NAPIF_STATE_SCHED))
> > +                       new |= NAPIF_STATE_MISSED;
> 
> You might want to consider just using a combination AND, divide,
> multiply, and OR to avoid having to have any conditional branches
> being added due to this code path.  Basically the logic would look
> like:


>     new = val | NAPIF_STATE_SCHED;
>     new |= (val & NAPIF_STATE_SCHED) / NAPIF_STATE_SCHED * NAPIF_STATE_MISSED;
> 
> In assembler that all ends up getting translated out to AND, SHL, OR.
> You avoid the branching, or MOV/OR/TEST/CMOV type code you would end
> up with otherwise.

Sure, I can try to optimize this a bit ;)


> > +       } while (cmpxchg(&n->state, val, new) != val);
> > +
> > +       if (unlikely(val & NAPIF_STATE_MISSED))
> > +               __napi_schedule(n);
> > +
> >         return true;
> >  }
> 
> If you rescheduled napi should you really be returning true?  Seems
> like you should be returning "!(val & NAPIF_STATE_MISSED)" to try to
> avoid letting this occur again.

Good idea.

Hmm... you mean that many drivers test napi_complete_done() return
value ?

;)

Thanks !