Re: [PATCH] Fix possible race with 4level-fixup.h

[Nick Piggin <nickpiggin@yahoo.com.au>]

Benjamin Herrenschmidt wrote:
> Hi !
> 
> When using 4level-fixup.h, a PMD page may end up beeing freed before the
> matching PGD entry is cleared due to the way the compatibility macros
> work. This can cause nasty races on some architectures.
> 
> This patch fixes it by defining pud_clear() to be pgd_clear(). That
> means we'll actually write 0 twice, a small price to pay here,
> especially seeing how easy it is to convert to the new headers anyway
> (hint hint, ppc & ppc64 patches as soon as 2.6.11 is out).
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
> Index: linux-work/include/asm-generic/4level-fixup.h
> ===================================================================
> --- linux-work.orig/include/asm-generic/4level-fixup.h	2005-01-24 17:09:49.000000000 +1100
> +++ linux-work/include/asm-generic/4level-fixup.h	2005-02-17 18:10:38.000000000 +1100
> @@ -24,7 +24,7 @@
>  #define pud_bad(pud)			0
>  #define pud_present(pud)		1
>  #define pud_ERROR(pud)			do { } while (0)
> -#define pud_clear(pud)			do { } while (0)
> +#define pud_clear(pud)			pgd_clear((pgd_t *)(pud))
>  

Just a small nit - no cast needed here.