From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754220AbZELIzg (ORCPT ); Tue, 12 May 2009 04:55:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751018AbZELIz0 (ORCPT ); Tue, 12 May 2009 04:55:26 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:50641 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750841AbZELIzZ (ORCPT ); Tue, 12 May 2009 04:55:25 -0400 Date: Tue, 12 May 2009 10:54:02 +0200 From: Ingo Molnar To: Jeff Garzik Cc: Rusty Russell , Peter Zijlstra , Mike Travis , LKML , viro@zeniv.linux.org.uk, Andrew Morton , roland@redhat.com Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct Message-ID: <20090512085402.GA18004@elte.hu> References: <20090508184838.GA11157@havoc.gtf.org> <20090508185015.GA11320@havoc.gtf.org> <20090508190944.GB12130@elte.hu> <200905101819.41765.rusty@rustcorp.com.au> <20090511105816.GG4648@elte.hu> <4A088DF4.7080305@garzik.org> <20090511204933.GA7737@elte.hu> <4A08DDE2.1080708@garzik.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A08DDE2.1080708@garzik.org> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Jeff Garzik wrote: > Ingo Molnar wrote: >>>>>> That cpumask[] should probably be cpumask[0], to document the >>>>>> aliasing to ->span and ->cpus properly. >>>>> If the comment wasn't sufficient documentation, I don't think >>>>> that would help :( >>>> It's a visual helper: it matches up with how we do these 'zero size >>>> array means dynamic structure continuation' tricks generally. >>>> >>>> I first mis-parsed the code for a second when seeing cpumask[]. >>>> cpumask[0] stands out like a sore thumb. And we dont read comments >>>> anyway ;-) >>>> >>>> Jeff, i suspect you found this because you are working on something >>>> rather interesting? :) If yes, would it help your project if we >>>> did the cpumask[0] cleanup and pushed it upstream immediately? >>> I think cpumask[0] would be more clear and consistent with the rest >>> of the kernel. >>> >>> But unfortunately for the twin projects of (a) static analysis and >>> checking with 'sparse', and (b) compiling under another compiler, >>> VLA-in-middle-of-struct is a killer in either case. >> >> even if at the end of the struct? > > Putting the VLA at the end of the struct would be a huge help, > yes. > > For example, struct sched_group and struct sched_domain are OK > as-is (though "[0]" would be preferred). > > It is the definition of struct static_sched_group and struct > static_sched_domain that creates the problem, because with the > bitmap following cpumask[] and span[], the VLA is no longer at the > end of the struct. > > VLA-in-the-middle raises the complexity required of the compiler > quite a bit. As a result, VLA-in-middle is not implemented in > sparse or clang (LLVM's C front-end and static analyzer). feel free to send patches for this - i dont have those build modes to test that it's sufficient. I'd suggest to go the simplest path: remove all the vla aliasing tricks: just make struct sched_domain use a plain struct cpumask and eliminate static_sched_domain altogether. The memory overhead is marginal as most of our sched domains are static allocated (and full size) anyway. Ingo