Re: [PATCH] rcu: remove unused variable in boot_cpu_state_init

From: Arnd Bergmann
Date: Thu Jun 22 2017 - 03:42:09 EST

On Thu, Jun 22, 2017 at 9:26 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> On Wed, Jun 21, 2017 at 11:57:28PM +0200, Arnd Bergmann wrote:
>> > Without CONFIG_SMP, we get a harmless warning about
>> > an unused variable:
>> >
>> > kernel/cpu.c: In function 'boot_cpu_state_init':
>> > kernel/cpu.c:1778:6: error: unused variable 'cpu' [-Werror=unused-variable]
>> >
>> > This reworks the function to have the declaration inside
>> > of the #ifdef.
>> >
>> > Fixes: faeb334286b7 ("rcu: Migrate callbacks earlier in the CPU-offline timeline")
>> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
>> I simply added a __maybe_unused in 6441c656acde ("rcu: Migrate callbacks
>> earlier in the CPU-offline timeline") in my -rcu tree. However, your
>> approach does have the advantage of complaining if the code using that
>> variable is removed.
>> So, would you be OK with my folding your approach into my commit with
>> attribution?
> Also, note that __maybe_unused can be dangerous: it can hide a build warning where
> there's a _real_ unused variable bug now or due to future changes, causing a real
> runtime bug.
> So I think we should consider it a syntactic construct to avoid.

Unused variables are relatively harmless compared to used-uninitialized
variables that are always bugs (though they are provably impossible to
detect correctly in some cases).

For unused variables, we might want to enable "-Wunused-but-set" again.
This was introduced in gcc-5 or gcc-6 and moved to "make W=1" because
of too many new warnings getting introduced, but I already fixed a lot of
those. I'll give that a spin on my randconfig build test to see how many of
them we have remaining these days, if any.