Re: [PATCH 1/2] mm: convert k{un}map_atomic(p, KM_type) to k{un}map_atomic(p)

Nick Bowler
2011-08-26T20:41:12Z

On 2011-08-26 12:42 -0700, Andrew Morton wrote:
> On Fri, 26 Aug 2011 10:58:16 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, 2011-08-26 at 16:17 +0800, Lin Ming wrote:
> > >
> > > The KM_type parameter for kmap_atomic/kunmap_atomic is not used
> > > anymore since commit 3e4d3af(mm: stack based kmap_atomic()).
[...]
> > yet-another-massive patch.. (you're the third or fourth to do so) if
> > Andrew wants to take this one I won't mind, however previously he
> > didn't want flag day patches..
>
> I'm OK with cleaning all these up, but I suggest that we leave the
> back-compatibility macros in place for a while, to make sure that
> various stragglers get converted. Extra marks will be awarded for
> working out how to make unconverted code generate a compile warning ;)

It's possible to (ab)use the C preprocessor to accomplish this sort of
thing. For instance, consider the following:

#include <stdio.h>

int foo(int x)
{
return x;
}

/* Deprecated; call foo instead. */
static inline int __attribute__((deprecated)) foo_unconverted(int x, int unused)
{
return foo(x);
}

#define PASTE(a, b) a ## b
#define PASTE2(a, b) PASTE(a, b)

#define NARG_(_9, _8, _7, _6, _5, _4, _3, _2, _1, n, ...) n
#define NARG(...) NARG_(__VA_ARGS__, 9, 8, 7, 6, 5, 4, 3, 2, 1, :)

#define foo1(...) foo(__VA_ARGS__)
#define foo2(...) foo_unconverted(__VA_ARGS__)
#define foo(...) PASTE2(foo, NARG(__VA_ARGS__)(__VA_ARGS__))

int main(void)
{
printf("%d\n", foo(42));
printf("%d\n", foo(54, 42));
return 0;
}

The preprocessor will select between "foo" and "foo_unconverted"
depending on the number of arguments passed; and gcc will emit a warning
for the latter case:

% gcc test.c
test.c: In function a??maina??:
test.c:27: warning: a??foo_unconverteda?? is deprecated (declared at test.c:9)

Cheers,
--
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)