rcutorture: Limit reader duration if irq or bh disabled

There are debug checks in some environments that will complain if the
duration of a bh-disabled region of code exceeds about 50 milliseconds.
Because rcu_read_delay() can produce a 50-millisecond delay and because
there could be up to eight reader segments with such delays, this commit
limits the maximum delay to 10 milliseconds if either interrupts or
softirqs are disabled.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -323,7 +323,7 @@ static void rcu_read_delay(struct torture_random_state *rrsp)
 	unsigned long started;
 	unsigned long completed;
 	const unsigned long shortdelay_us = 200;
-	const unsigned long longdelay_ms = 300;
+	unsigned long longdelay_ms = 300;
 	unsigned long long ts;
 
 	/* We want a short delay sometimes to make a reader delay the grace
@@ -333,6 +333,8 @@ static void rcu_read_delay(struct torture_random_state *rrsp)
 	if (!(torture_random(rrsp) % (nrealreaders * 2000 * longdelay_ms))) {
 		started = cur_ops->get_gp_seq();
 		ts = rcu_trace_clock_local();
+		if (preempt_count() & (SOFTIRQ_MASK | HARDIRQ_MASK))
+			longdelay_ms = 5; /* Avoid triggering BH limits. */
 		mdelay(longdelay_ms);
 		completed = cur_ops->get_gp_seq();
 		do_trace_rcu_torture_read(cur_ops->name, NULL, ts,