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

[Alexander Duyck]

On Tue, Jun 18, 2019 at 4:22 PM Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote:
>
> On Tue, Jun 18, 2019 at 04:04:20PM -0700, Nathan Chancellor 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
>
> Great, thanks!
>
> In this case it was actually useful to get the warning, even though it
> didn't point out the actual bug. I think in general it would be
> preferable to avoid such constructs, even when they are correct. But
> then again, it's the reviewers/maintainers task to avoid unnecessarily
> cryptic code from slipping in, and this just happens to be one instance
> where the compiler could have helped.

So it took me a bit to remember/understand it as well since I haven't
touched the code in over 4 years, however part of that is because the
comment for this code is actually buried down in put_child.
Essentially this is just meant to be an add w/ carry and a sub w/
borrow to address a potential overflow if bits == KEYLENGTH.

If you want you can add:
Fixes: 95f60ea3e99a ("fib_trie: Add collapse() and should_collapse() to resize")
Acked-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>