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Date: Thu, 8 Mar 2012 15:51:39 -0800
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch] mm, mempolicy: make mempolicies robust against errors
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To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: David Rientjes <rientjes@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, linux-mm@kvack.org

On Wed, 07 Mar 2012 01:34:16 -0500
KOSAKI Motohiro <kosaki.motohiro@gmail.com> wrote:

> >> And, now BUG() has renreachable() annotation. why don't it work?
> >>
> >>
> >> #define BUG()                                                   \
> >> do {                                                            \
> >>          asm volatile("ud2");                                    \
> >>          unreachable();                                          \
> >> } while (0)
> >>
> >
> > That's not compiled for CONFIG_BUG=n; such a config fallsback to
> > include/asm-generic/bug.h which just does
> >
> > 	#define BUG()	do {} while (0)
> >
> > because CONFIG_BUG specifically _wants_ to bypass BUG()s and is reasonably
> > protected by CONFIG_EXPERT.
> 
> So, I strongly suggest to remove CONFIG_BUG=n. It is neglected very long time and
> much plenty code assume BUG() is not no-op. I don't think we can fix all place.
> 
> Just one instruction don't hurt code size nor performance.

Well yes, CONFIG_BUG=n is a crazy thing to do.  a) because programmers
universally assume that BUG() doesn't return and b) given that the
kernel KNOWS that it is about to fall off a cliff, why would anyone
want to deprive themselves of information about the forthcoming crash?

So perhaps a good compromise here is to do nothing: let the
CONFIG_BUG=n build spew a pile of warnings, and let the crazy
CONFIG_BUG=n people suffer.  That's if any such people exist...

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