From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752954AbZEKK7e (ORCPT ); Mon, 11 May 2009 06:59:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751542AbZEKK7Z (ORCPT ); Mon, 11 May 2009 06:59:25 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:53596 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751142AbZEKK7Y (ORCPT ); Mon, 11 May 2009 06:59:24 -0400 Date: Mon, 11 May 2009 12:58:16 +0200 From: Ingo Molnar To: Rusty Russell Cc: Jeff Garzik , 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: <20090511105816.GG4648@elte.hu> References: <20090508184838.GA11157@havoc.gtf.org> <20090508185015.GA11320@havoc.gtf.org> <20090508190944.GB12130@elte.hu> <200905101819.41765.rusty@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200905101819.41765.rusty@rustcorp.com.au> 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=3.2.5 -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 * Rusty Russell wrote: > On Sat, 9 May 2009 04:39:44 am Ingo Molnar wrote: > > * Jeff Garzik wrote: > > > The semantics for variable-length arrays __in the middle of structs__ > > > are quite muddy, and a case in sched.c presents an interesting case, > > > as the preceding code comment indicates: > > > > > > /* > > > * The cpus mask in sched_group and sched_domain hangs off > > > the end. * FIXME: use cpumask_var_t or dynamic percpu alloc > > > to avoid * wasting space for nr_cpu_ids < CONFIG_NR_CPUS. */ > > > struct static_sched_group { > > > struct sched_group sg; DECLARE_BITMAP(cpus, > > > CONFIG_NR_CPUS); > > > }; > > Yeah, it's kinda nasty. Generally, sched_group is dynamically > allocated, so we just allocate sizeof(struct sched_group) + size > of nr_cpu_ids bits. > > These ones are static, and it was easier to put this hack in than > make them dynamic. There's nothing wrong with it, until we really > want NR_CPUS == bignum, or we want to get rid of NR_CPUS > altogether for CONFIG_CPUMASKS_OFFSTACK (which would be very > clean, but not clearly worthwhile). > > But more importantly, my comment is obviously unclear, since your > patch shows you didn't understand the purpose of the field: The > cpus bitmap *is* the sg-cpumask[] array. I dont think Jeff misunderstood this code (hey, he found it! :), his patch is a demonstration of why this code is a problem: a seemingly innocious invariant modification (his patch) kills the kernel dead. > > > Maybe a C expert can say whether cpumask[0] is better than cpumask[], > > > or have other comments? > > [0] is a gcc extension, but it should be equivalent. > > > 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? Ingo