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

[Ingo Molnar]

* Jeff Garzik <jeff@xxxxxxxxxx> wrote:

> Ingo Molnar wrote:
>> * Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
>>
>>> 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.
>>
>> 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.
>
> Yeah, it was intended to spark debate...  definitely not to be applied  
> (hence "NOT-signed-off-by", among other hints).
>
>
>>>>> 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?
>
> I think cpumask[0] would be more clear and consistent with the 
> rest of the kernel.
>
> But unfortunately for the twin projects of (a) static analysis and 
> checking with 'sparse', and (b) compiling under another compiler, 
> VLA-in-middle-of-struct is a killer in either case.

even if at the end of the struct?

	Ingo
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