linux-kernel - Re: [PATCH RESEND] bug: When !CONFIG_BUG, simplify WARN_ON

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Date:	Mon, 24 Feb 2014 15:17:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Josh Triplett <josh@...htriplett.org>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] bug: When !CONFIG_BUG, simplify WARN_ON_ONCE and
 family

On Mon, 24 Feb 2014 09:02:35 +0100 Arnd Bergmann <arnd@...db.de> wrote:

> On Saturday 22 February 2014, Josh Triplett wrote:
> > When !CONFIG_BUG, WARN_ON and family become simple passthroughs of their
> > condition argument; however, WARN_ON_ONCE and family still have
> > conditions and a boolean to detect one-time invocation, even though the
> > warning they'd emit doesn't exist.  Make the existing definitions
> > conditional on CONFIG_BUG, and map them all to the passthrough WARN_ON
> > when !CONFIG_BUG.
> > 
> > This saves 4.4k on a minimized configuration (smaller than
> > allnoconfig), and 20.6k with defconfig plus CONFIG_BUG=n.
> 
> This looks good, but it reminds me of a patch that I did a while ago
> and that got lost while I was on leave:
> 
> > +#else /* !CONFIG_BUG */
> > +#ifndef HAVE_ARCH_BUG
> > +#define BUG() do {} while(0)
> > +#endif
> > +
> > +#ifndef HAVE_ARCH_BUG_ON
> > +#define BUG_ON(condition) do { if (condition) ; } while(0)
> > +#endif
> 
> I've done some analysis of this before[1] and came to the conclusion that
> this definition (which I realize you are not changing) is bad.
> 
> For one thing, it will cause lots of gcc warnings about code that
> should have been unreachable being compiled. It also causes
> misoptimizations for code that should be detected as unused or
> (worse) lets us run into undefined behavior if we ever get into
> the BUG() case.
> 
> This means we actually want BUG() to end with __builtin_unreachable()
> as in the CONFIG_BUG=y case, and also ensure it actually is
> unreachable. As I have shown in [1], the there is a small overhead
> of doing this in terms of code size.

CONFIG_BUG=n causes all sorts of stupid problems.

And as kernel developers we don't *want* people disabling BUG - it
reduces our ability to detect and fix stuff and it adds all sorts of
hard-to-maintain nobody-tests-it things like this.

So...  how about we just do away with CONFIG_BUG?

- Do we know of anyone who is really using this and to good effect?

- Is their use case important/valuable enough for us to continue bothering?

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