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    SubjectRe: Circular dependency between <linux/kernel.h> and <asm/bug.h> on ARM
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    On Tue, Jan 3, 2012 at 9:45 AM, Russell King - ARM Linux
    <linux@arm.linux.org.uk> wrote:
    > 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@decadent.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.

    Actually, kernel.h doesn't need to include any variant of bug.h at all.

    I started on cleaning this up a few weeks back, but didn't have the time
    to get it ready for 3.3 -- so my intent is to do so for linux-next that will
    be the 3.4 release. The Work In Progress can be seen here:

    http://git.kernel.org/?p=linux/kernel/git/paulg/linux.git;a=shortlog;h=refs/heads/bug.h-cleanup-WIP

    I only mention it here since I'd hate to see anyone waste time on
    duplicating work that is already done.

    Thanks,
    Paul.

    ---

    >
    >> 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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