Em 30-05-2010 00:42, Andrew Morton escreveu:
On Fri, 28 May 2010 07:53:13 -0300 Cesar Eduardo Barros<cesarb@xxxxxxxxxx> wrote:
Make it much harder to misuse, by moving it to level 9 on Rusty's
list[4] ("The compiler/linker won't let you get it wrong"). This is done
by refusing to build if the pointer passed to it is convertible to a
struct page * but it is not a void * (verified by trying to convert it
to a pointer to a dummy struct).
The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck()
(which is what you would call in case for some strange reason calling it
with a pointer to a struct page is not incorrect in your code).
Fair enough, that's a 99% fix. A long time ago I made kmap_atomic()
return a char * (iirc) and kunmap_atomic() is passed a char*. It
worked, but I ended up throwing it away. I don't precisely remember
why - I think it was intrusiveness and general hassle rather than
anything fundamental.
I vaguely recall reading something about that on LWN a long time ago.[1]
The advantage of my __builtin_types_compatible_p approach is that it
does not have to change the callers at all (except in the extremly
unlikely case that someone actually meant to call it with a struct page
*, which is something I did not find when looking at the whole kernel
with spatch[2]).
The disadvantage of my approach is that gcc's error message is
absolutely atrocious:
mm/swapfile.c: In function ‘foo’:
mm/swapfile.c:2501: error: negative width in bit-field ‘<anonymous>’
But that is a problem with BUILD_BUG_ON, not this code.
+/* Prevent people trying to call kunmap_atomic() as if it were kunmap() */
+struct __kunmap_atomic_dummy {};
+#define kunmap_atomic(addr, idx) do { \
+ BUILD_BUG_ON( \
+ __builtin_types_compatible_p(typeof(addr), struct page *)&& \
+ !__builtin_types_compatible_p(typeof(addr), struct __kunmap_atomic_dummy *)); \
+ kunmap_atomic_notypecheck((addr), (idx)); \
+ } while (0)
We have a little __same_type() helper for this. __must_be_array()
should be using it, too.
It would be great (shortening the long lines a lot), except that in this
case it is a complete misnomer, which would probably confuse people
reading the code. If __same_type(typeof(addr), void *) worked, I would
not need a dummy struct; but __same_type is actually looking for
compatible types, not same type (perhaps for non-pointers it actually
means "same type"). In the first part of the condition, I am actually
looking for "same type", but even there __same_type(void *, struct page
*) would return true (which is why I need the second part).
And now I am having second thoughts about the line breaks here; I should
have also broken between the parameters of __builtin_types_compatible_p,
to avoid long lines. If you want, I can resend the patch with it reindented.
[1] Yep, there it is: https://lwn.net/Articles/111226/
[2]
@@
struct page *page;
expression E;
@@
* kunmap_atomic(page, E)
--
Cesar Eduardo Barros
cesarb@xxxxxxxxxx
cesar.barros@xxxxxxxxx
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