Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757470Ab0BRCH5 (ORCPT ); Wed, 17 Feb 2010 21:07:57 -0500 Received: from mail-gx0-f227.google.com ([209.85.217.227]:35135 "EHLO mail-gx0-f227.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756093Ab0BRCHz convert rfc822-to-8bit (ORCPT ); Wed, 17 Feb 2010 21:07:55 -0500 MIME-Version: 1.0 In-Reply-To: References: <20080725083943.GC19310@cs181140183.pp.htv.fi> <20080725015537.564e3397.akpm@linux-foundation.org> <20080725091455.GD19310@cs181140183.pp.htv.fi> <20080725092748.GF19310@cs181140183.pp.htv.fi> <20080725023455.dde3eb27.akpm@linux-foundation.org> <20080725122454.GE6701@parisc-linux.org> From: Grant Likely Date: Wed, 17 Feb 2010 19:07:34 -0700 X-Google-Sender-Auth: 5ce9203173b29d52 Message-ID: Subject: Re: __weak vs ifdef To: Linus Torvalds Cc: Matthew Wilcox , Andrew Morton , Adrian Bunk , Andrea Righi , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2054 Lines: 58 Reaching back into an old discussion.... On Sat, Jul 26, 2008 at 12:38 PM, Linus Torvalds wrote: > > > On Fri, 25 Jul 2008, Matthew Wilcox wrote: > >> On Fri, Jul 25, 2008 at 02:34:55AM -0700, Andrew Morton wrote: >> > We should make arch_pick_mmap_layout __weak and nuke that ifdef. >> >> I strongly disagree. ?I find it makes it harder to follow code flow >> when __weak functions are involved. ?Ifdefs are ugly, no question, but >> they're easier to grep for > > Hell no, they're not. > > Our use of random HAVE_ARCH_xyz or ARCH_SUPPORTS_xyz etc stuff makes > things _totally_ impossible to grep for. > > In contrast, it we did this code as > > ? ? ? ?#ifndef arch_pick_mmap_layout > ? ? ? ?void __weak arch_pick_mmap_layout(struct mm_struct *mm) > ? ? ? ?{ > ? ? ? ? ? ? ? ?mm->mmap_base = TASK_UNMAPPED_BASE; > ? ? ? ? ? ? ? ?mm->get_unmapped_area = arch_get_unmapped_area; > ? ? ? ? ? ? ? ?mm->unmap_area = arch_unmap_area; > ? ? ? ?} > ? ? ? ?#endif > > then trying to grep for arch_pick_mmap_layout() would show EVERY SINGLE > RELEVANT CASE! And it would show the "__weak" there too, so that once > people get used to this convention, they'd have a really easy time > figuring out the rules from just the output of the 'grep'. [...] Question. If I use this pattern, and use the __weak attribute on core code functions wrapped with a #ifndef, then how does it mesh with EXPORT_SYMBOL*() statements? Do both the core code, and the arch override need to do EXPORT_SYMBOL(), or should EXPORT_SYMBOL() only appear at the core code site? I also assume that at the core code site, the EXPORT_SYMBOL() must appear inside the #ifndef block so that a #define override doesn't break things. Correct? Cheers, g. -- Grant Likely, B.Sc., P.Eng. Secret Lab Technologies Ltd. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/