rcutorture: Ignore attempts to test preemption and forward progress

Use of the rcutorture preempt_duration and the default-on fwd_progress
kernel parameters can result in preemption of callback processing during
forward-progress testing, which is an excellent way to OOM your test
if your kernel offloads RCU callbacks.  This commit therefore treats
preempt_duration in the same way as stall_cpu in CONFIG_RCU_NOCB_CPU=y
kernels, prohibiting fwd_progress testing and splatting when rcutorture
is built in (as opposed to being a loadable module).

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Author: paulmckrcu

kernel/rcu/rcutorture.c

@@ -3145,12 +3145,12 @@ static int __init rcu_torture_fwd_prog_init(void)
 		fwd_progress = 0;
 		return 0;
 	}
-	if (stall_cpu > 0) {
-		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, conflicts with CPU-stall testing");
+	if (stall_cpu > 0 || (preempt_duration > 0 && IS_ENABLED(CONFIG_RCU_NOCB_CPU))) {
+		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, conflicts with CPU-stall and/or preemption testing");
 		fwd_progress = 0;
 		if (IS_MODULE(CONFIG_RCU_TORTURE_TEST))
 			return -EINVAL; /* In module, can fail back to user. */
-		WARN_ON(1); /* Make sure rcutorture notices conflict. */
+		WARN_ON(1); /* Make sure rcutorture scripting notices conflict. */
 		return 0;
 	}
 	if (fwd_progress_holdoff <= 0)