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

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Message-Id: <8111159a-c571-4c71-b731-184af56b5cb1@app.fastmail.com>
Date: Tue, 30 Jul 2024 16:14:48 +0200
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Linus Torvalds" <torvalds@...uxfoundation.org>
Cc: "David Laight" <David.Laight@...lab.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Jens Axboe" <axboe@...nel.dk>, "Matthew Wilcox" <willy@...radead.org>,
 "Christoph Hellwig" <hch@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
 "Dan Carpenter" <dan.carpenter@...aro.org>,
 "Jason A . Donenfeld" <Jason@...c4.com>,
 "pedro.falcato@...il.com" <pedro.falcato@...il.com>,
 "Mateusz Guzik" <mjguzik@...il.com>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

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@...uxfoundation.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.

     Arnd