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

From: Rusty Russell
Date: Sun May 10 2009 - 04:50:24 EST


On Sat, 9 May 2009 04:39:44 am Ingo Molnar wrote:
> * Jeff Garzik <jeff@xxxxxxxxxx> 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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