linux-kernel - Re: [PATCH] fix vmstat per cpu usage

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Message-Id: <20060802074308.babd264e.akpm@osdl.org>
Date:	Wed, 2 Aug 2006 07:43:08 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Jan Blunck <jblunck@...e.de>
Cc:	linux-kernel@...r.kernel.org, paulus@...ba.org
Subject: Re: [PATCH] fix vmstat per cpu usage

On Wed, 2 Aug 2006 15:30:06 +0200
Jan Blunck <jblunck@...e.de> wrote:

> Here comes another idea. To find further wrong usage of percpu variables I
> wrote the following patch. It still needs some work for the other archs but
> I'm interested in your feedback about that.
> 
> ...
> 
> Index: linux-2.6/include/asm-generic/percpu.h
> ===================================================================
> --- linux-2.6.orig/include/asm-generic/percpu.h
> +++ linux-2.6/include/asm-generic/percpu.h
> @@ -14,7 +14,9 @@ extern unsigned long __per_cpu_offset[NR
>      __attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name
>  
>  /* var is in discarded region: offset to particular copy we want */
> -#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
> +#define per_cpu(var, cpu) (*({				\
> +	int user_error_##var __attribute__ ((unused));	\
> +	RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))

What's it do?  Forces a syntax error if `var' isn't a simple identifier?

Seems sane, although I'd check that the compiler doesn't accidentally
waste a stack slot for that local.  Perhaps it's be safer to make
it a non-existing function:

	extern int user_error#var(void);

-
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