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

[Ingo Molnar <mingo@elte.hu>]

* Rusty Russell <rusty@rustcorp.com.au> wrote:

> On Sat, 9 May 2009 04:39:44 am Ingo Molnar wrote:
> > * 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);
> > > 	};
> 
> 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