Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

Geert Uytterhoeven
2024-12-04T13:15:25Z

Hi Arnd et al,

People started seeing this in today's linux-next...

On Tue, Jul 30, 2024 at 4:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> On Tue, Jul 30, 2024, at 12:10, Arnd Bergmann wrote:
> > On Tue, Jul 30, 2024, at 05:59, Linus Torvalds wrote:
> >> On Mon, 29 Jul 2024 at 16:21, Linus Torvalds <torvalds@linuxfoundation.org> wrote:
> >
> > I'm giving this a spin on the randconfig test setup now to see
> > if there are some other cases like the bcachefs one. So far I've
> > seen one failure, but I can't make sense of it yet:
> >
> > drivers/gpu/drm/i915/display/intel_backlight.c: In function 'scale':
> > include/linux/compiler_types.h:510:45: error: call to
> > '__compiletime_assert_905' declared with attribute error: clamp() low
> > limit source_min greater than high limit source_max
> > include/linux/minmax.h:107:9: note: in expansion of macro
> > 'BUILD_BUG_ON_MSG'
> > 107 | BUILD_BUG_ON_MSG(statically_true(ulo > uhi),
> > \
> > drivers/gpu/drm/i915/display/intel_backlight.c:47:22: note: in
> > expansion of macro 'clamp'
> > 47 | source_val = clamp(source_val, source_min, source_max);
> >
> > See https://pastebin.com/raw/yLJ5ZqVw for the x86-64 .config
> > that triggered this.
>
> The above seems to happen only with gcc-13 and gcc-14, but not gcc-12
> and earlier, and it's the only one I've seen with a bit of randconfig
> testing on that version.
>
> There is another one that I see with gcc-8 randconfigs (arm64):
>
> net/netfilter/ipvs/ip_vs_conn.c: In function 'ip_vs_conn_init':
> include/linux/compiler_types.h:510:38: error: call to '__compiletime_assert_1040' declared with attribute error: clamp() low limit min greater than high limit max_avail
> 510 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> include/linux/minmax.h:182:28: note: in expansion of macro '__careful_clamp'
> 182 | #define clamp(val, lo, hi) __careful_clamp(val, lo, hi)
> | ^~~~~~~~~~~~~~~
> net/netfilter/ipvs/ip_vs_conn.c:1498:8: note: in expansion of macro 'clamp'
> 1498 | max = clamp(max, min, max_avail);
>
> I can reproduce this one with gcc-8/9/10, but not gcc-11
> or higher.
>
> This may be another case of __builtin_constant_p() being
> slightly unreliable when a local variable is constant-folded
> based on a condition, or with partial inlining.

Or perhaps the argument order is wrong, and it should be

max = clamp(max_avail, min, max);

instead?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds