linux-kernel - Re: [PATCH] Documentation: coding-style: ask function-like macros to evaluate parameters

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Date: Thu, 21 Mar 2024 10:44:27 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Barry Song <21cnbao@...il.com>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>, corbet@....net,
 workflows@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, Chris
 Zankel <chris@...kel.net>, Huacai Chen <chenhuacai@...ngson.cn>, Herbert Xu
 <herbert@...dor.apana.org.au>, Guenter Roeck <linux@...ck-us.net>, Max
 Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH] Documentation: coding-style: ask function-like macros
 to evaluate parameters

On Thu, 21 Mar 2024 07:48:36 +1300 Barry Song <21cnbao@...il.com> wrote:

> > Stronger than that please.  Just tell people not to use macros in such
> > situations.  Always code it in C.
> 
> While I appreciate the consistency of always using "static inline"
> instead of macros,
> I've noticed numerous instances of (void) macros throughout the kernel.
> 
> arch/arm64/include/asm/cpuidle.h:#define arm_cpuidle_save_irq_context(c) (void)c
> arch/arm64/include/asm/cpuidle.h:#define
> arm_cpuidle_restore_irq_context(c) (void)c
> arch/loongarch/include/asm/io.h:#define iounmap(addr) ((void)(addr))
> arch/mips/include/asm/cop2.h:#define cop2_save(r) do { (void)(r); } while (0)
> arch/mips/include/asm/cop2.h:#define cop2_restore(r) do { (void)(r); } while (0)
> arch/mips/include/asm/cop2.h:#define cop2_save(r) do { (void)(r); } while (0)
> arch/mips/include/asm/cop2.h:#define cop2_restore(r) do { (void)(r); } while (0)
> ....
> 
> I'm uncertain whether people would find it disconcerting if they completely
> deviate from the current approach.
> 
> If you believe it won't pose an issue, I can proceed with v3 to eliminate
> the first option, casting to (void).

I think so.  My overall view is that we should write things in C.  Only
use macros if the thing we're trying to do simply cannot be done in a C
function.

- inline functions don't have the "expression with side effects
  evaluated more than once" problem.

- inline functions avoid the unused-variable issue which started this thread

- inline functions look better

- for some reason, people are more inclined to document inline
  functions than macros.