From: Ingo Molnar <mingo@elte.hu>
To: Jeff Garzik <jeff@garzik.org>,
Rusty Russell <rusty@rustcorp.com.au>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Mike Travis <travis@sgi.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
viro@zeniv.linux.org.uk,
Andrew Morton <akpm@linux-foundation.org>,
roland@redhat.com
Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct
Date: Fri, 8 May 2009 21:09:44 +0200 [thread overview]
Message-ID: <20090508190944.GB12130@elte.hu> (raw)
In-Reply-To: <20090508185015.GA11320@havoc.gtf.org>
* Jeff Garzik <jeff@garzik.org> 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);
> };
>
> struct static_sched_domain {
> struct sched_domain sd; DECLARE_BITMAP(span,
> CONFIG_NR_CPUS);
> };
>
> Both sched_group and sched_domain have the following trailing struct
> member:
>
> unsigned long cpumask[];
>
> So this change is intended largely to spawn a discussion, because
> I'm not sure this VLA-in-middle-of-struct behavior is guaranteed to
> always behave as expected?
>
> Maybe a C expert can say whether cpumask[0] is better than cpumask[],
> or have other comments?
This bit of the code is a bit temporary, with a half-done cpumask
conversion ...
I'm not convinced at all that this code wants to be converted to
cpuvar_t. These are all either statically percpu or dynamically
allocated structures, so we wont allocate NR_CPUS of them. So i'd
lean towards embedding a full struct cpumask and that's it.
That cpumask[] should probably be cpumask[0], to document the
aliasing to ->span and ->cpus properly.
Rusty, what do you think?
Ingo
next prev parent reply other threads:[~2009-05-08 19:11 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-05-08 18:48 [PATCH 1/2] kernel/{sched,smp}.c: fix static decl prior to struct declaration Jeff Garzik
2009-05-08 18:50 ` [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct Jeff Garzik
2009-05-08 19:09 ` Ingo Molnar [this message]
2009-05-10 8:49 ` Rusty Russell
2009-05-10 15:09 ` Jeff Garzik
2009-05-12 13:34 ` Rusty Russell
2009-05-12 14:03 ` Al Viro
2009-05-13 2:12 ` Rusty Russell
2009-05-13 2:31 ` Jeff Garzik
2009-05-13 5:36 ` Al Viro
2009-05-13 6:49 ` [PATCH] sched: avoid flexible array member inside struct (gcc extension) Rusty Russell
2009-05-13 13:51 ` [tip:sched/urgent] " tip-bot for Rusty Russell
2009-05-11 10:58 ` [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct Ingo Molnar
2009-05-11 20:43 ` Jeff Garzik
2009-05-11 20:49 ` Ingo Molnar
2009-05-12 2:24 ` Jeff Garzik
2009-05-12 8:54 ` Ingo Molnar
2009-05-08 19:04 ` [PATCH 1/2] kernel/{sched,smp}.c: fix static decl prior to struct declaration Ingo Molnar
2009-05-08 19:08 ` Jeff Garzik
2009-05-08 19:13 ` Ingo Molnar
2009-05-08 19:38 ` [PATCH 1/2 v2] " Jeff Garzik
2009-05-11 11:26 ` Ingo Molnar
2009-05-11 20:06 ` Jeff Garzik
2009-05-11 20:51 ` Ingo Molnar
2009-05-11 11:30 ` [tip:sched/core] kernel/{sched, smp}.c: " tip-bot for Jeff Garzik
2009-05-11 20:02 ` [PATCH 1/2 v3] kernel/{sched,smp}.c: " Jeff Garzik
2009-05-11 21:51 ` [tip:sched/urgent] kernel/{sched, smp}.c: " tip-bot for Jeff Garzik
2009-05-11 21:56 ` Ingo Molnar
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