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    SubjectRe: 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@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.

    > 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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