rcu: Make rcu_torture_fqs() exit loops at end of test

The rcu_torture_fqs() function can prevent the rcutorture tests from
completing, resulting in a hang.  This commit therefore ensures that
rcu_torture_fqs() will exit its inner loops at the end of the test,
and also applies the newish ULONG_CMP_LT() macro to time comparisons.

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

kernel/rcutorture.c

@@ -741,7 +741,7 @@ static int rcu_torture_boost(void *arg)
 	do {
 		/* Wait for the next test interval. */
 		oldstarttime = boost_starttime;
-		while (jiffies - oldstarttime > ULONG_MAX / 2) {
+		while (ULONG_CMP_LT(jiffies, oldstarttime)) {
 			schedule_timeout_uninterruptible(1);
 			rcu_stutter_wait("rcu_torture_boost");
 			if (kthread_should_stop() ||
@@ -752,7 +752,7 @@ static int rcu_torture_boost(void *arg)
 		/* Do one boost-test interval. */
 		endtime = oldstarttime + test_boost_duration * HZ;
 		call_rcu_time = jiffies;
-		while (jiffies - endtime > ULONG_MAX / 2) {
+		while (ULONG_CMP_LT(jiffies, endtime)) {
 			/* If we don't have a callback in flight, post one. */
 			if (!rbi.inflight) {
 				smp_mb(); /* RCU core before ->inflight = 1. */
@@ -818,11 +818,13 @@ rcu_torture_fqs(void *arg)
 	VERBOSE_PRINTK_STRING("rcu_torture_fqs task started");
 	do {
 		fqs_resume_time = jiffies + fqs_stutter * HZ;
-		while (jiffies - fqs_resume_time > LONG_MAX) {
+		while (ULONG_CMP_LT(jiffies, fqs_resume_time) &&
+		       !kthread_should_stop()) {
 			schedule_timeout_interruptible(1);
 		}
 		fqs_burst_remaining = fqs_duration;
-		while (fqs_burst_remaining > 0) {
+		while (fqs_burst_remaining > 0 &&
+		       !kthread_should_stop()) {
 			cur_ops->fqs();
 			udelay(fqs_holdoff);
 			fqs_burst_remaining -= fqs_holdoff;