Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 18 Jun 2019 16:23:43 -0700 | Subject | Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions |
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On Tue, Jun 18, 2019 at 4:04 PM Nathan Chancellor <natechancellor@gmail.com> wrote: > > On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke 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; > > > > As an FYI, this is also being fixed in clang: > > https://bugs.llvm.org/show_bug.cgi?id=42239 > > https://reviews.llvm.org/D63369
While I'm glad we found and fixed a bug in Clang; I'm still for fixing the underlying code from a readability perspective. If it takes longer than a few seconds to understand a statement to the non-author, the code as written may be too clever.
As a side note, I'm going to try to see if MAINTAINERS and scripts/get_maintainers.pl supports regexes on the commit messages in order to cc our mailing list (clang-built-linux <clang-built-linux@googlegroups.com>) automatically; but in the meantime please try to remember to cc the list manually. I usually find it quickly from the bookmarked https://clangbuiltlinux.github.io/ which lists it there) (but I should still automate this).
> > > Signed-off-by: Matthias Kaehlcke <mka@chromium.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--;
This change looks correct to me, so thanks for the patch and: Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
(Bikeshed; I prefer pre-inc/dec to post-inc/dec unless you really care about the previous value. Should be irrelevant with a sufficiently smart compiler though. ;) ) -- Thanks, ~Nick Desaulniers
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