linux-kernel - Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when comparing in percpu_add

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Message-ID: <ZxAD0-kQJ0_lDxje@smile.fi.intel.com>
Date: Wed, 16 Oct 2024 21:20:03 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Ingo Molnar <mingo@...nel.org>, Uros Bizjak <ubizjak@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, Dennis Zhou <dennis@...nel.org>,
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when
 comparing in percpu_add_op()

On Wed, Oct 16, 2024 at 09:06:13PM +0300, Andy Shevchenko wrote:
> On Wed, Oct 16, 2024 at 08:44:56AM -0700, Dave Hansen wrote:

...

> > This doesn't _look_ right.

See below.

...

> Maybe more readable, but wouldn't it be theoretically buggy for u64?
> I'm talking about the case when u64 == UINT_MAX, which will be true
> in your case and false in mine.
> 
> >         const int pao_ID__ = (__builtin_constant_p(val) &&
> >                               ((val) == 1 || (int)(val) == -1)) ?
> > 
> >                                 (int)(val) : 0;

This code _is_ buggy, thanks to my new test case.

[   66.161375] pcp -1 (0xffffffffffffffff) != expected 4294967295 (0xffffffff)

Hence, I'll send a v2 with the test case and updated Subject.

-- 
With Best Regards,
Andy Shevchenko