From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753366AbaBXXSD (ORCPT ); Mon, 24 Feb 2014 18:18:03 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:51532 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752639AbaBXXSA (ORCPT ); Mon, 24 Feb 2014 18:18:00 -0500 Date: Mon, 24 Feb 2014 15:17:58 -0800 From: Andrew Morton To: Arnd Bergmann Cc: Josh Triplett , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND] bug: When !CONFIG_BUG, simplify WARN_ON_ONCE and family Message-Id: <20140224151758.b06125f6ddef2086d2b2a969@linux-foundation.org> In-Reply-To: <201402240902.35977.arnd@arndb.de> References: <20140222192325.GA12587@thin> <201402240902.35977.arnd@arndb.de> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 24 Feb 2014 09:02:35 +0100 Arnd Bergmann 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?