Re: [peterz-queue:sched/core 13/19] kernel/sched/fair.c:892:34: warning: variable 'stats' set but not used

From: Segher Boessenkool
Date: Sat Sep 11 2021 - 17:37:36 EST


Hi!

On Sat, Sep 11, 2021 at 02:20:49PM +0200, Peter Zijlstra wrote:
> On Sat, Sep 11, 2021 at 02:21:26AM +0800, kernel test robot wrote:
> > compiler: m68k-linux-gcc (GCC) 11.2.0
>
> > kernel/sched/fair.c: In function 'update_curr':
> > kernel/sched/fair.c:860:42: warning: unused variable 'stats' [-Wunused-variable]
> > 860 | struct sched_statistics *stats = __schedstats_from_se(curr);
> > | ^~~~~
>
> OK, compiler guys, this code reads like:
>
> #define schedstats_enabled() (0)
> #define __schedstat_set(x, y) do { } while (0)
>
>
> if (schedstats_enabled()) {
> struct sched_statistics *stats = __schedstats_from_se(curr);
>
> __schedstat_set(stats->exec_max,
> max(delta_exec, stats->exec_max));
> }
>
> So yes, we initialize a variable that then isn't used, but the whole
> bloody thing is inside if (0) which will not ever get ran *anyway*.
>
> This is a crap warning if ever I saw one...

Yes, we really should warn "do not use a preprocessor macro if what you
want is a function"? The variable really *is* unused, with this macro.

If we would remove dead code before warning about unused variables
there would be many *more* false positives, fwiw.


Segher