linux-kernel - Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

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Message-ID: <CAD=FV=V6TqT93Lb2UoQdkyO2j7OHrggCn-4qwDLEFw=N7RZ2Eg@mail.gmail.com>
Date:   Tue, 18 Jun 2019 14:45:34 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Alexander Duyck <alexander.h.duyck@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

Hi,

On Tue, Jun 18, 2019 at 2:14 PM Matthias Kaehlcke <mka@...omium.org> wrote:
>
> empty_child_inc/dec() use the ternary operator for conditional
> operations. The conditions involve the post/pre in/decrement
> operator and the operation is only performed when the condition
> is *not* true. This is hard to parse for humans, use a regular
> 'if' construct instead and perform the in/decrement separately.
>
> This also fixes two warnings that are emitted about the value
> of the ternary expression being unused, when building the kernel
> with clang + "kbuild: Remove unnecessary -Wno-unused-value"
> (https://lore.kernel.org/patchwork/patch/1089869/):
>
> CC      net/ipv4/fib_trie.o
> net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value]
>         ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;
>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
> I have no good understanding of the fib_trie code, but the
> disentangled code looks wrong, and it should be equivalent to the
> cryptic version, unless I messed it up. In empty_child_inc()
> 'full_children' is only incremented when 'empty_children' is -1. I
> suspect a bug in the cryptic code, but am surprised why it hasn't
> blown up yet. Or is it intended behavior that is just
> super-counterintuitive?
>
> For now I'm leaving it at disentangling the cryptic expressions,
> if there is a bug we can discuss what action to take.
> ---
>  net/ipv4/fib_trie.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)

I have no knowledge of this code either but Matthias's patch looks
sane to me and I agree with the disentangling before making functional
changes.

My own personal belief is that this is pointing out a bug somewhere.
Since "empty_children" ends up being an unsigned type it doesn't feel
like it was by-design that -1 is ever a value that should be in there.

In any case:

Reviewed-by: Douglas Anderson <dianders@...omium.org>