Make rcutorture RNG use temporal entropy

Repost of http://lkml.org/lkml/2007/8/10/472 made available by request.

The locking used by get_random_bytes() can conflict with the
preempt_disable() and synchronize_sched() form of RCU.  This patch changes
rcutorture's RNG to gather entropy from the new cpu_clock() interface
(relying on interrupts, preemption, daemons, and rcutorture's reader
thread's rock-bottom scheduling priority to provide useful entropy), and
also adds and EXPORT_SYMBOL_GPL() to make that interface available to GPLed
kernel modules such as rcutorture.

Passes several hours of rcutorture.

[ego@in.ibm.com: Use raw_smp_processor_id() in rcu_random()]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/rcutorture.c

@@ -41,7 +41,6 @@
 #include <linux/notifier.h>
 #include <linux/freezer.h>
 #include <linux/cpu.h>
-#include <linux/random.h>
 #include <linux/delay.h>
 #include <linux/byteorder/swabb.h>
 #include <linux/stat.h>
@@ -165,16 +164,14 @@ struct rcu_random_state {
 
 /*
  * Crude but fast random-number generator.  Uses a linear congruential
- * generator, with occasional help from get_random_bytes().
+ * generator, with occasional help from cpu_clock().
  */
 static unsigned long
 rcu_random(struct rcu_random_state *rrsp)
 {
-	long refresh;
-
 	if (--rrsp->rrs_count < 0) {
-		get_random_bytes(&refresh, sizeof(refresh));
-		rrsp->rrs_state += refresh;
+		rrsp->rrs_state +=
+			(unsigned long)cpu_clock(raw_smp_processor_id());
 		rrsp->rrs_count = RCU_RANDOM_REFRESH;
 	}
 	rrsp->rrs_state = rrsp->rrs_state * RCU_RANDOM_MULT + RCU_RANDOM_ADD;