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SubjectRe: [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c
On Tue, 11 Mar 2008 02:30:14 +0100 Andi Kleen <andi@firstfloor.org> wrote:

> Use an own random generator for pageattr-test.c
>
> [Repost. Please ack/nack. This is a bug fix and imho a .25 late merge
> candidate because it fixes a subtle bug]

An Ingo/Thomas thing.

> pageattr-test attempts to be repeatable and uses srandom32 to get a
> repeatable random number sequence.
>
> Using srandom32() wasn't a good idea for various reasons:
> - it is per cpu and if the cpu changes on a preemptible kernel it gets lost
> - networking and random32 puts in some own state
> - srandom32() does not actually reset the state, but just adds bits to
> it
>
> Instead use a very simple private standard rnd that gives repeatable
> results. I took the reference rand() from ISO-C.
>
> Signed-off-by: Andi Kleen <ak@suse.de>
>
> ---
> arch/x86/mm/pageattr-test.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> Index: linux/arch/x86/mm/pageattr-test.c
> ===================================================================
> --- linux.orig/arch/x86/mm/pageattr-test.c
> +++ linux/arch/x86/mm/pageattr-test.c
> @@ -101,6 +101,14 @@ static int print_split(struct split_stat
> static unsigned long addr[NTEST];
> static unsigned int len[NTEST];
>
> +static int next = 100;
> +
> +static unsigned my_rand(void)
> +{
> + next = next * 1103515245 + 12345;
> + return next;
> +}

`next' should be local to my_rand().

> /* Change the global bit on random pages in the direct mapping */
> static int pageattr_test(void)
> {
> @@ -123,10 +131,9 @@ static int pageattr_test(void)
> memset(bm, 0, (max_pfn_mapped + 7) / 8);
>
> failed += print_split(&sa);
> - srandom32(100);
>
> for (i = 0; i < NTEST; i++) {
> - unsigned long pfn = random32() % max_pfn_mapped;
> + unsigned long pfn = my_rand() % max_pfn_mapped;
>
> addr[i] = (unsigned long)__va(pfn << PAGE_SHIFT);
> len[i] = random32() % 100;


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