linux-kernel - Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c

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Date:	Tue, 30 Sep 2008 11:02:30 -0700
From:	Mike Travis <travis@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>, Rusty Russell <rusty@...tcorp.com.au>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	David Miller <davem@...emloft.net>, Alan.Brunelle@...com,
	tglx@...utronix.de, rjw@...k.pl,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel-testers@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	arjan@...ux.intel.com, Jack Steiner <steiner@....com>
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

Linus Torvalds wrote:
> 
> On Tue, 30 Sep 2008, Mike Travis wrote:
>> One pain is:
>>
>> 	typedef struct __cpumask_s *cpumask_t;
>> 	const cpumask_t xxx;
>>
>> is not the same as:
>>
>> 	typedef const struct __cpumask_s *const_cpumask_t;
>> 	const_cpumask_t xxx;
>>
>> and I'm not exactly sure why.
> 
> Umm. The const has different 
> 
> One is
> 
> 	typedef const struct __cpumask_s *const_cpumask_t;
> 
> which becomes
> 
> 	(const struct __cpumask_s) *
> 
> while the other is
> 
> 	const cpumask_t xxx
> 
> which is
> 
> 	const (struct __cpumask_s *)
> 
> and if you look a bit more closely, you'll see that they are _obviously_ 
> not the same thing at all.

Thanks, yes that explains what I should have figured out.  (Nice way to
explain it btw... ;-)

> 
> Quite frankly, I personally do hate typedefs that end up being pointers, 
> and used as pointers, without showing that in the source code.
> 
> When you do
> 
> 	type_t a;
> 
> 	fn(a);
> 
> I expect the code to essentially do a pass-by-value. But when the type_t 
> is a pointer, that doesn't really work.

I agree, and as I mentioned, Rusty was working towards an alternative
method of declaring cpumask's which does not hide this.

My goal was to create an (apparent) opaque handle for cpumask_t and modify
the code so all changes to the contents of the cpumask are via functions.

> 
> Your issue with 'const' is just another version of the same. You don't 
> want the _pointer_ to be const, you want what it points _to_ to be const. 
> But because you hid the pointerness inside the typedef, you simply cannot 
> do that.
> 
> The problem with cpumask's, of course, is that for the "small mask" case, 
> we really don't want it to be a pointer. So now it's sometimes a pointer 
> and sometimes not. The typedef hides that, and I understand why it's a 
> good idea, but I'm surprised you didn't understand what the implications 
> were for 'const', and I'm now a bit more leery about this whole thing just 
> because the typedef ends up hiding so much - it doesn't just hide the 
> basic type, it hides a very basic *code* issue.

Perhaps then defining the cpumask as a list of unsigned longs (remove the
outer struct) would play more "naturally".  Lists by definition are always
referenced by pointers.  I have another version of the patchset that shows
this and I'll post just the cpumask.h and doc files.

> 
> 			Linus

Thanks!
Mike
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