linux-kernel - Re: Buiild error in i915/xe

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Message-ID: <87ed0xrcb8.fsf@intel.com>
Date: Mon, 20 Jan 2025 12:48:11 +0200
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: David Laight <david.laight.linux@...il.com>, Guenter Roeck
 <linux@...ck-us.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, David Laight
 <David.Laight@...lab.com>, Arnd Bergmann <arnd@...nel.org>,
 "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>, intel-xe@...ts.freedesktop.org,
 intel-gfx@...ts.freedesktop.org, David Airlie <airlied@...il.com>, Simona
 Vetter <simona@...ll.ch>, Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: Buiild error in i915/xe

On Sun, 19 Jan 2025, David Laight <david.laight.linux@...il.com> wrote:
> On Sat, 18 Jan 2025 14:58:48 -0800
> Guenter Roeck <linux@...ck-us.net> wrote:
>
>> On 1/18/25 14:11, David Laight wrote:
>> > On Sat, 18 Jan 2025 13:21:39 -0800
>> > Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> >   
>> >> On Sat, 18 Jan 2025 at 09:49, Guenter Roeck <linux@...ck-us.net> wrote:  
>> >>>
>> >>> No idea why the compiler would know that the values are invalid.  
>> >>
>> >> It's not that the compiler knows tat they are invalid, but I bet what
>> >> happens is in scale() (and possibly other places that do similar
>> >> checks), which does this:
>> >>
>> >>          WARN_ON(source_min > source_max);
>> >>          ...
>> >>          source_val = clamp(source_val, source_min, source_max);
>> >>
>> >> and the compiler notices that the ordering comparison in the first
>> >> WARN_ON() is the same as the one in clamp(), so it basically converts
>> >> the logic to
>> >>
>> >>          if (source_min > source_max) {
>> >>                  WARN(..);
>> >>                  /* Do the clamp() knowing that source_min > source_max */
>> >>                  source_val = clamp(source_val, source_min, source_max);
>> >>          } else {
>> >>                  /* Do the clamp knowing that source_min <= source_max */
>> >>                  source_val = clamp(source_val, source_min, source_max);
>> >>          }
>> >>
>> >> (obviously I dropped the other WARN_ON in the conversion, it wasn't
>> >> relevant for this case).
>> >>
>> >> And now that first clamp() case is done with source_min > source_max,
>> >> and it triggers that build error because that's invalid.
>> >>
>> >> So the condition is not statically true in the *source* code, but in
>> >> the "I have moved code around to combine tests" case it now *is*
>> >> statically true as far as the compiler is concerned.  
>> > 
>> > Well spotted :-)
>> > 
>> > One option would be to move the WARN_ON() below the clamp() and
>> > add an OPTIMISER_HIDE_VAR(source_max) between them.
>> > 
>> > Or do something more sensible than the WARN().
>> > Perhaps return target_min on any such errors?
>> >   
>> 
>> This helps:
>> 
>> -       WARN_ON(source_min > source_max);
>> -       WARN_ON(target_min > target_max);
>> -
>>          /* defensive */
>>          source_val = clamp(source_val, source_min, source_max);
>> 
>> +       WARN_ON(source_min > source_max);
>> +       WARN_ON(target_min > target_max);
>
> That is a 'quick fix' ...
>
> Much better would be to replace the WARN() with (say):
> 	if (target_min >= target_max)
> 		return target_min;
> 	if (source_min >= source_max)
> 		return target_min + (target_max - target_min)/2;
> So that the return values are actually in range (in as much as one is defined).
> Note that the >= cpmparisons also remove a divide by zero.

I want the loud and early warnings for clear bugs instead of
"gracefully" silencing the errors only to be found through debugging
user reports.

BR,
Jani.



-- 
Jani Nikula, Intel