linux-kernel - Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

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Message-Id: <200905101819.41765.rusty@rustcorp.com.au>
Date:	Sun, 10 May 2009 18:19:40 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeff Garzik <jeff@...zik.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Travis <travis@....com>,
	LKML <linux-kernel@...r.kernel.org>, viro@...iv.linux.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>, roland@...hat.com
Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

On Sat, 9 May 2009 04:39:44 am Ingo Molnar wrote:
> * Jeff Garzik <jeff@...zik.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);
> > 	};

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.

> > 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 :(

Rusty.
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