netdev - Re: [PATCH net] net: netem: fix error path for corrupted GSO frames

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Date:   Thu, 17 Oct 2019 11:10:06 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com,
        Stephen Hemminger <stephen@...workplumber.org>,
        kbuild test robot <lkp@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Ben Hutchings <ben@...adent.org.uk>,
        Simon Horman <simon.horman@...ronome.com>
Subject: Re: [PATCH net] net: netem: fix error path for corrupted GSO frames

On Wed, Oct 16, 2019 at 4:22 PM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> On Wed, 16 Oct 2019 15:42:28 -0700, Cong Wang wrote:
> > > @@ -612,7 +613,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> > >                         }
> > >                         segs = skb2;
> > >                 }
> > > -               qdisc_tree_reduce_backlog(sch, -nb, prev_len - len);
> > > +               qdisc_tree_reduce_backlog(sch, !skb - nb, prev_len - len);
> >
> > Am I the only one has trouble to understand the expression
> > "!skb - nb"?
>
> The backward logic of qdisc_tree_reduce_backlog() always gives me a
> pause :S

Yeah, reducing with a negative value is actually an add. Feel free
to add a wrapper for this if you think it is better.

>
> Is
> -nb + !skb
> any better?

I don't see how they are different. :-/


>
> The point is we have a "credit" for the "head" skb we dropped. If we
> didn't manage to queue any of the segs then the expression becomes
> ...reduce_backlog(sch, 1, prev_len) basically cleaning up after the
> head.
>
> My knee jerk reaction was -> we should return DROP if head got dropped,
> but that just makes things more nasty because we requeue the segs
> directly into netem so if we say DROP we have to special case all the
> segs which succeeded, that gets even more hairy.

Hmm? My understanding is that !skb is either 0 or 1, so you end up
with either "-nb" or "1 - nb". The formal is easy to understand, while
the later is harder as I don't see why you need to plus 1.

>
> I'm open to suggestions.. :(

Why not write the code in a more readable way, for instance with the :?
operator? And, adding a comment in the code?

Thanks.