linux-kernel - Re: [PATCH v2] compiler.h: Include <linux/bug.h> to avoid build breakage with ARRAY

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Message-ID: <20120405185207.GE27274@pengutronix.de>
Date:	Thu, 5 Apr 2012 20:52:07 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	Fabio Estevam <festevam@...il.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, linux@....linux.org.uk,
	sam@...nborg.org, Fabio Estevam <fabio.estevam@...escale.com>,
	kernel@...gutronix.de
Subject: Re: [PATCH v2] compiler.h: Include <linux/bug.h> to avoid build
 breakage with ARRAY_SIZE()

On Thu, Apr 05, 2012 at 02:35:55PM -0400, Paul Gortmaker wrote:
> On 12-04-04 10:29 AM, Uwe Kleine-König wrote:
> > Hello,
> > 
> > On Fri, Mar 02, 2012 at 09:22:02AM -0500, Paul Gortmaker wrote:
> >> On 12-03-01 10:13 PM, Fabio Estevam wrote:
> >>> On Thu, Mar 1, 2012 at 11:49 PM, Paul Gortmaker
> >>> <paul.gortmaker@...driver.com> wrote:
> >>>
> >>>> Thanks, but no.
> >>>>
> >>>> You missed the whole point of my previous comments -- that being
> >>>> that we don't want to just jam headers into always-used headers.
> >>>
> >>> Yes, it is not clear for me how to fix this build error. I got
> >>> different feedbacks from you, Russell and Sam.
> >>
> >> Understood, there was some discussion there.  Anyways it is already
> >> dealt with in yesterday's linux-next tree, so you won't have the
> >> build failure anymore.
> > I hit that same problem in an not yet mainlined source file. A simple
> > file containing only:
> 
> A quick check shows about 15000 instances in mainline.  Even if 1% of
> those were blowing up, I'd expect a full mailbox.
> 
> > 
> > 	#include <linux/kernel.h>
> > 
> > 	int array[3];
> > 
> > 	int func(void)
> > 	{
> > 		return ARRAY_SIZE(array);
> 
> Well, ARRAY_SIZE is just a convenient macro that uses BUG content.
> Hiding it behind a name doesn't change the fact that you've
> implicitly decided to use bug.h content.  Maybe you really don't
> want to be using it.  Maybe we should have:
> 
> --------------------------------------
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -43,6 +43,7 @@
>  #define PTR_ALIGN(p, a)                ((typeof(p))ALIGN((unsigned long)(p), (a)))
>  #define IS_ALIGNED(x, a)               (((x) & ((typeof(x))(a) - 1)) == 0)
>  
> +#define __ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
>  
>  /*
> ------------------------------------
> 
> and give people a choice?
> 
> Don't get me wrong.  I'm not all extremist about this.  If it turns
> out that it seems to cause way too much grief, and someone like Andrew
> says "Yeah, lets put #include <linux/bug.h> back in kernel.h" then I
> won't hesitate to do that.  But given that code currently in mainline
> isn't blowing up all over, I wasn't yet convinced we needed to do that.
I didn't intend to readd bug.h to kernel.h. Just wanted to know if
adding #include <linux/bug.h> to my source file is the right thing to
do after your change.

Another alternative is to put ARRAY_SIZE into it's own header and let
that include <linux/bug.h>. Not sure this is a sane approach though.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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