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

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Message-Id: <20190618211440.54179-1-mka@chromium.org>
Date:   Tue, 18 Jun 2019 14:14:40 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     "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>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

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(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 868c74771fa9..cf7788e336b7 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -338,12 +338,18 @@ static struct tnode *tnode_alloc(int bits)
 
 static inline void empty_child_inc(struct key_vector *n)
 {
-	++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;
+	tn_info(n)->empty_children++;
+
+	if (!tn_info(n)->empty_children)
+		tn_info(n)->full_children++;
 }
 
 static inline void empty_child_dec(struct key_vector *n)
 {
-	tn_info(n)->empty_children-- ? : tn_info(n)->full_children--;
+	if (!tn_info(n)->empty_children)
+		tn_info(n)->full_children--;
+
+	tn_info(n)->empty_children--;
 }
 
 static struct key_vector *leaf_new(t_key key, struct fib_alias *fa)
-- 
2.22.0.410.gd8fdbe21b5-goog