[PATCH] [1/2] Use an own random generator for pageattr-test.c

[PATCH] [1/2] Use an own random generator for pageattr-test.c

[Andi Kleen]

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>

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;

[PATCH] [2/2] srandom32 fixes

[Andi Kleen]

- Rename it to a different name because it does something quite
different from a traditional user land srandom32 -- it doesn't
reset state but just adds to it.

- Let it update the state of all CPUs. The network stack goes
into pains to feed the current IP addresses in, but it is not very
effective if that is only done for some random CPU instead of all.
So change it to feed bits into all CPUs.  I decided to do that lockless 
because well somewhat random results are ok.

Signed-off-by: Andi Kleen <ak@suse.de>
Index: linux/include/linux/random.h
===================================================================
--- linux.orig/include/linux/random.h
+++ linux/include/linux/random.h
@@ -70,7 +70,7 @@ unsigned int get_random_int(void);
 unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
 
 u32 random32(void);
-void srandom32(u32 seed);
+void random32_add_bits(u32 seed);
 
 #endif /* __KERNEL___ */
 
Index: linux/lib/random32.c
===================================================================
--- linux.orig/lib/random32.c
+++ linux/lib/random32.c
@@ -93,19 +93,24 @@ u32 random32(void)
 EXPORT_SYMBOL(random32);
 
 /**
- *	srandom32 - add entropy to pseudo random number generator
+ *	random32_add_bits - add entropy to pseudo random number generator
  *	@seed: seed value
  *
  *	Add some additional seeding to the random32() pool.
- *	Note: this pool is per cpu so it only affects current CPU.
  */
-void srandom32(u32 entropy)
+void random32_add_bits(u32 entropy)
 {
-	struct rnd_state *state = &get_cpu_var(net_rand_state);
-	__set_random32(state, state->s1 ^ entropy);
-	put_cpu_var(state);
+	int i;
+	/*
+	 * No locking on the CPUs, but then somewhat random results are, well,
+	 * expected.
+	 */
+	for_each_possible_cpu (i) {
+		struct rnd_state *state = &per_cpu(net_rand_state, i);
+		__set_random32(state, state->s1 ^ entropy);
+	}
 }
-EXPORT_SYMBOL(srandom32);
+EXPORT_SYMBOL(random32_add_bits);
 
 /*
  *	Generate some initially weak seeding values to allow
Index: linux/include/linux/net.h
===================================================================
--- linux.orig/include/linux/net.h
+++ linux/include/linux/net.h
@@ -213,7 +213,7 @@ extern struct socket *sockfd_lookup(int 
 extern int	     net_ratelimit(void);
 
 #define net_random()		random32()
-#define net_srandom(seed)	srandom32((__force u32)seed)
+#define net_srandom(seed)	random32_add_bits((__force u32)seed)
 
 extern int   	     kernel_sendmsg(struct socket *sock, struct msghdr *msg,
 				    struct kvec *vec, size_t num, size_t len);