rcutorture: Emphasize testing of single reader protection type

For RCU implementations supporting multiple types of reader protection,
rcutorture currently randomly selects the combinations of types of
protection for each phase of each reader.  The problem with this,
for example, given the four kinds of protection for RCU-sched
(local_irq_disable(), local_bh_disable(), preempt_disable(), and
rcu_read_lock_sched()), the reader will be protected by a single
mechanism only 25% of the time.  We really heavier testing of single
read-side mechanisms.

This commit therefore uses only a single mechanism about 60% of the time,
half of the time explicitly and one-eighth of the time by chance.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -69,6 +69,7 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@us.ibm.com> and Josh Triplett <josh@jos
 #define RCUTORTURE_RDR_IRQ	 0x2	/*  ... disabling interrupts. */
 #define RCUTORTURE_RDR_PREEMPT	 0x4	/*  ... disabling preemption. */
 #define RCUTORTURE_RDR_RCU	 0x8	/*  ... entering another RCU reader. */
+#define RCUTORTURE_RDR_NBITS	 4	/* Number of bits defined above. */
 #define RCUTORTURE_MAX_EXTEND	 (RCUTORTURE_RDR_BH | RCUTORTURE_RDR_IRQ | \
 				  RCUTORTURE_RDR_PREEMPT)
 #define RCUTORTURE_RDR_MAX_LOOPS 0x7	/* Maximum reader extensions. */
@@ -1198,9 +1199,15 @@ static int
 rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
 {
 	int mask = rcutorture_extend_mask_max();
+	unsigned long randmask1 = torture_random(trsp) >> 8;
+	unsigned long randmask2 = randmask1 >> 1;
 
 	WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
-	mask = mask & (torture_random(trsp) >> RCUTORTURE_RDR_SHIFT);
+	/* Half the time lots of bits, half the time only one bit. */
+	if (randmask1 & 0x1)
+		mask = mask & randmask2;
+	else
+		mask = mask & (1 << (randmask2 % RCUTORTURE_RDR_NBITS));
 	if ((mask & RCUTORTURE_RDR_IRQ) &&
 	    !(mask & RCUTORTURE_RDR_BH) &&
 	    (oldmask & RCUTORTURE_RDR_BH))