rcutorture: Avoid no-test complaint if too few forward-progress tries

In a too-short test, random delays can cause each attempt to do
forward-progress testing to fail to complete, thus resulting in
spurious splats.  This commit therefore requires at least five tries
before complaining about rcutorture runs that failed to produce at
least one valid forward-progress testing attempt.  Note that actual
forward-progress failures will splat regardless of the number of tries.

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

kernel/rcu/rcutorture.c

@@ -1678,6 +1678,7 @@ static int rcu_torture_fwd_prog(void *args)
 	int idx;
 	unsigned long stopat;
 	bool tested = false;
+	int tested_tries = 0;
 
 	VERBOSE_TOROUT_STRING("rcu_torture_fwd_progress task started");
 	do {
@@ -1692,6 +1693,7 @@ static int rcu_torture_fwd_prog(void *args)
 			if (!fwd_progress_need_resched || need_resched())
 				cond_resched();
 		}
+		tested_tries++;
 		if (!time_before(jiffies, stopat) && !torture_must_stop()) {
 			tested = true;
 			cver = cver == READ_ONCE(rcu_torture_current_version);
@@ -1701,7 +1703,8 @@ static int rcu_torture_fwd_prog(void *args)
 		/* Avoid slow periods, better to test when busy. */
 		stutter_wait("rcu_torture_fwd_prog");
 	} while (!torture_must_stop());
-	WARN_ON(!tested);
+	/* Short runs might not contain a valid forward-progress attempt. */
+	WARN_ON(!tested && tested_tries >= 5);
 	torture_kthread_stopping("rcu_torture_fwd_prog");
 	return 0;
 }