linux-kernel - Re: [PATCH v3 0/2] Stop some of the abuse of BUG() where compile time checks should be used.

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Message-ID: <4ECD91E4.5090504@gmail.com>
Date:	Wed, 23 Nov 2011 16:37:56 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	David Daney <ddaney.cavm@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>, DM <dm.n9107@...il.com>,
	David Howells <dhowells@...hat.com>,
	David Daney <david.daney@...ium.com>,
	"Pinski, Andrew" <Andrew.Pinski@...iumnetworks.com>
Subject: Re: [PATCH v3 0/2] Stop some of the abuse of BUG() where compile
 time checks should be used.

On 11/23/2011 03:57 PM, Linus Torvalds wrote:
> Btw, would it possibly make sense to make the string more useful?
>
> For example using __FILE__ and __LINE__, or possibly letting the user
> of the BUILD_BUG() give a string ("Using HMASK without
> CONFIG_HUGEPAGE").

We thought about doing that, but without doing some complex preprocessor 
fu, the GCC attribute ((error())) thing doesn't do what we want.

It appears that if more than a single instance of the construct is used 
in a compilation unit, the string emitted by the compiler for any of the 
violations will be the last string encountered.

So if you did something like:

.
.
.
Line 99:  BUILD_BUG("You failed on line 99");
.
.
.
.
Line 666: BUILD_BUG("You failed on line 666");
.
.
.

The message emitted for a failure at line 99 would be "You failed on 
line 666".  Which is probably worse than no message at all.

It may be possible to do something like:

#define _LINENAME_CONCAT( _name_, _line_ ) _name_##_line_
#define _LINENAME(_name_, _line_) _LINENAME_CONCAT(_name_,_line_)

#define _BUILD_BUG(MSG,FUBAR)		\
do {					\
   extern void FUBAR (void)		\
      __linktime_error("BUILD_BUG failed: " MSG); \
   FUBAR ();				\
} while (0)

#define BUILD_BUG(M,A) _BUILD_BUG(M, _LINENAME(__build_bug_failed,__LINE__))

But it didn't seem worth it.

>
> Whatever. It's bikeshedding - what would probably be more important
> would be to get this into linux-next so that we find out whether there
> are any compile issues with it on other platforms or compiler
> versions.
>
>                    Linus
>

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