linux-kernel - Re: [PATCH 00/16] mm: prepare for converting vm->vm

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Date:	Fri, 23 Mar 2012 02:52:05 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Minchan Kim <minchan@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>
Subject: Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit

Linus Torvalds wrote:
> On Thu, Mar 22, 2012 at 3:24 PM, Konstantin Khlebnikov
> <khlebnikov@...nvz.org>  wrote:
>>
>> # define __nocast       __attribute__((nocast))
>>
>> typedef long __nocast long_t;
>
> So the intention is that this really creates a *new* type.
>
> So "long_t" really is a different type from "long", but because
> __nocast is so weak, it happily casts to another integer type of the
> same size.
>
> But a pointer to it is different, the same way "int *" is different
> from "long *" even if "int" and "long" happen to have the same size.
> So I do think that the warning you quote is correct and expected:
>
>> 1.c:13:12: warning: incorrect type in argument 1 (different modifiers)
>> 1.c:13:12:    expected int [nocast] [usertype] *x
>> 1.c:13:12:    got int *<noident>
>> 1.c:13:12: warning: implicit cast to nocast type
>>
>> Is this ok?
>
> Yes.
>
> The thing about __nocast is that it's so *very* very easy to lose it.
> For example, do this:
>
>    typedef long __nocast long_t;
>
>    int main(long_t a)
>    {
>          return a;
>    }
>
> and you get the (expected) warning.
>
> HOWEVER. Now do "return a+1" instead, and the warning goes away. Why?
> Because the expression ends up having just the type "long", because
> the "a" mixed happily with the "1" (that was cast from 'int' to 'long'
> by the normal C type rules).
>
> That is arguably a bug, but this kind of thing really wasn't what
> __nocast was designed for. The __nocast design ended up being too
> weak, though, and we hardly use it in the kernel.
>

Thanks. Looks like "__nocast" totally undocumented.
It would be nice to add something about this into Documentation/sparse.txt
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