linux-kernel - Re: Circular dependency between <linux/kernel.h> and <asm/bug.h> on ARM

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Message-ID: <20120103144538.GU2914@n2100.arm.linux.org.uk>
Date:	Tue, 3 Jan 2012 14:45:38 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Olof Johansson <olof@...om.net>
Cc:	Ben Hutchings <ben@...adent.org.uk>,
	Simon Glass <sjg@...omium.org>,
	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Circular dependency between <linux/kernel.h> and <asm/bug.h>
	on ARM

On Sun, Jan 01, 2012 at 11:01:27PM -0800, Olof Johansson wrote:
> On Sun, Jan 1, 2012 at 6:44 PM, Ben Hutchings <ben@...adent.org.uk> wrote:
> > Commit 87e040b6456fd3416a1f6831c1eedaef5c0a94ff ("ARM: 7017/1: Use
> > generic BUG() handler") makes BUG() use BUILD_BUG_ON().  However,
> > BUILD_BUG_ON() is not defined in <linux/bug.h> but in <linux/kernel.h>.
> >
> > arch/include/asm/bug.h does not include <linux/kernel.h> and *cannot* do
> > so because the latter already includes <asm/bug.h>.
> 
> Sure it can, but it's not ideal. Since the BUILD_BUG_ON is only used
> in bug.h in a #define, it will be resolved below the includes of
> either so there should be no ordering issue between the two.

I disagree - we should not be creating circular dependencies.  This
creates a mess, and uncertain results.  For instance, if we include
linux/bug.h or asm/bug.h before linux/kernel.h has been included,
then we end up with linux/kernel.h being parsed without a definition
for BUG_ON().

However, if linux/kernel.h is included first, we start parsing that,
include asm/bug.h, asm/bug.h then includes linux/kernel.h which produces
an empty file, and then we continue parsing asm/bug.h _without_
BUILD_BUG_ON() defined.

So, adding linux/kernel.h does _not_ solve the problem.  It solves the
problem for _some_ cases only.

> > Maybe BUILD_BUG_ON() should be moved out to a header of its own, or else
> > this particular use should be moved to some other file.  This needs to
> > be fixed somehow, as it obviously leads to build failures, e.g.:
> >
> > https://buildd.debian.org/status/fetch.php?pkg=linux-2.6&arch=armel&ver=3.2~rc7-1~experimental.1&stamp=1325142904
> 
> Or perhaps moving BUILD_BUG_ON to include/linux/bug.h?

and change linux/kernel.h to include linux/bug.h rather than asm/bug.h.

> The quickest fix for now might be to take out the BUILD_BUG_ON(),
> especially so close to 3.2-final.

I think just remove the BUILD_BUG_ON.  Other architectures have done
without it, so I see no reason we can't do as well.
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