On Thu, Jun 22, 2017 at 9:26 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Paul E. McKenney <paul...@linux.vnet.ibm.com> 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 <a...@arndb.de> >> >> 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. Arnd