linux-kernel - [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c

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Message-ID: <20080311013014.GA28682@basil.nowhere.org>
Date:	Tue, 11 Mar 2008 02:30:14 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c

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]

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@...e.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;
+}
+
 /* 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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