linux-kernel - Re: [PATCH] BUG(): CONFIG_BUG=n version of BUG() should be unreachable()

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Date:	Tue, 22 Dec 2009 17:26:55 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Alexander Beregalov <a.beregalov@...il.com>
CC:	arnd@...db.de, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>, sam@...nborg.org,
	dhowells@...hat.com
Subject: Re: [PATCH] BUG(): CONFIG_BUG=n version of BUG() should be unreachable()

Alexander Beregalov wrote:
> Previouss definition of BUG() as 'do {} while(0)' produced compilation
> warnings when BUG() was used in default branch of switch() statement
> (control reaches end of non-void function).
> 
> Example:
> unsigned long function()
> {
> 	switch() {
> 	case 1:
> 		return 1;
> 	case 2:
> 		return 2;
> 	default:
> 		BUG();
> }
> 
> Using unreachable() fixes the problem.
> 
> Signed-off-by: Alexander Beregalov <a.beregalov@...il.com>
> 

NAK.


> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index 18c435d..1106439 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -89,7 +89,7 @@ extern void warn_slowpath_null(const char *file, const int line);
>  
>  #else /* !CONFIG_BUG */
>  #ifndef HAVE_ARCH_BUG
> -#define BUG() do {} while(0)
> +#define BUG() unreachable() 
>  #endif
>  
>  #ifndef HAVE_ARCH_BUG_ON

You can only use 'unreachable()' in situations where it is truly 
unreachable.  In the case above you will reach it in the default case.

I would suggest one of the following instead:

#define BUG() BUILD_BUG_ON(1)

#define BUG() do {} while(1)


David Daney
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