linux-kernel - Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

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Date: Wed, 17 Apr 2024 13:23:18 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched


* Matthew Wilcox <willy@...radead.org> wrote:

> On Tue, Apr 16, 2024 at 05:29:02PM -0700, Linus Torvalds wrote:
> > On Tue, 16 Apr 2024 at 14:15, Kees Cook <keescook@...omium.org> wrote:
> > >
> > > I was looking at -Wshadow=local again, and remembered this series. It
> > > sounded like things were close, but a tweak was needed. What would be
> > > next to get this working?
> > 
> > So what is the solution to
> > 
> >     #define MAX(a,b) ({ \
> >         typeof(a) __a = (a); \
> >         typeof(b) __b = (b); \
> >         __a > __b ? __a : __b; \
> >     })
> 
> #define __MAX(a, __a, b, __b) ({	\
> 	typeof(a) __a = (a);		\
> 	typeof(b) __b = (b);		\
> 	__a > __b ? __a : __b;		\
> })
> 
> #define MAX(a, b)	__MAX(a, UNIQUE_ID(a), b, UNIQUE_ID(b))
> 
> At least, I think that was the plan.  This was two years ago and I've
> mostly forgotten.

I think as long as we can keep any additional complexity inside macros it 
would be acceptable, at least from the scheduler's POV. A UNIQUE_ID() layer 
of indirection for names doesn't sound look a too high price.

I had good reasults with -Wshadow in user-space projects: once the false 
positives got ironed out, the vast percentage of new warnings was for 
genuinely problematic new code. But they rarely used block-nested macros 
like the kernel does.

Thanks,

	Ingo