linux-kernel - Re: [PATCH 11/11] Use unreachable() in asm-generic/bug.h for !CONFIG

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Date:	Mon, 14 Sep 2009 19:12:45 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 11/11] Use unreachable() in asm-generic/bug.h for 
	!CONFIG_BUG case.

On Mon, Sep 14, 2009 at 5:55 PM, David Daney <ddaney@...iumnetworks.com> wrote:
> The subject says it all (most).  The only drawback here is that for a
> pre-GCC-5.4 compiler, instead of expanding to nothing we now expand
> BUG() to an endless loop.  Before the patch when configured with
> !CONFIG_BUG() you might get some warnings, but the code would be
> small.  After the patch there are no warnings, but there is an endless
> loop at each BUG() site.
>
> Of course for the GCC-4.5 case we get the best of both worlds.
>
> Signed-off-by: David Daney <ddaney@...iumnetworks.com>
> Suggested-by: Ingo Molnar <mingo@...e.hu>
> CC: Ingo Molnar <mingo@...e.hu>
> ---
>  include/asm-generic/bug.h |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index 4b67559..e952242 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -89,11 +89,11 @@ extern void warn_slowpath_null(const char *file, const int line);
>
>  #else /* !CONFIG_BUG */
>  #ifndef HAVE_ARCH_BUG
> -#define BUG() do {} while(0)
> +#define BUG() unreachable()
>  #endif
>
>  #ifndef HAVE_ARCH_BUG_ON
> -#define BUG_ON(condition) do { if (condition) ; } while(0)
> +#define BUG_ON(condition) do { if (condition) unreachable(); } while (0)
>  #endif
>
>  #ifndef HAVE_ARCH_WARN_ON
> --

This seems wrong to me.  Wouldn't you always want to do the endless
loop?  In the absence of an arch-specific method to jump to an
exception handler, it isn't really unreachable.  On gcc 4.5 this would
essentially become a no-op.

--
Brian Gerst
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