rcu: Allow rcutorture to starve grace-period kthread

This commit provides an rcutorture.stall_gp_kthread module parameter
to allow rcutorture to starve the grace-period kthread.  This allows
testing the code that detects such starvation.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -1486,6 +1486,31 @@ static void rcu_gp_slow(int delay)
 		schedule_timeout_uninterruptible(delay);
 }
 
+static unsigned long sleep_duration;
+
+/* Allow rcutorture to stall the grace-period kthread. */
+void rcu_gp_set_torture_wait(int duration)
+{
+	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST) && duration > 0)
+		WRITE_ONCE(sleep_duration, duration);
+}
+EXPORT_SYMBOL_GPL(rcu_gp_set_torture_wait);
+
+/* Actually implement the aforementioned wait. */
+static void rcu_gp_torture_wait(void)
+{
+	unsigned long duration;
+
+	if (!IS_ENABLED(CONFIG_RCU_TORTURE_TEST))
+		return;
+	duration = xchg(&sleep_duration, 0UL);
+	if (duration > 0) {
+		pr_alert("%s: Waiting %lu jiffies\n", __func__, duration);
+		schedule_timeout_uninterruptible(duration);
+		pr_alert("%s: Wait complete\n", __func__);
+	}
+}
+
 /*
  * Initialize a new grace period.  Return false if no grace period required.
  */
@@ -1686,6 +1711,7 @@ static void rcu_gp_fqs_loop(void)
 		rcu_state.gp_state = RCU_GP_WAIT_FQS;
 		ret = swait_event_idle_timeout_exclusive(
 				rcu_state.gp_wq, rcu_gp_fqs_check_wake(&gf), j);
+		rcu_gp_torture_wait();
 		rcu_state.gp_state = RCU_GP_DOING_FQS;
 		/* Locking provides needed memory barriers. */
 		/* If grace period done, leave loop. */
@@ -1834,6 +1860,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 			swait_event_idle_exclusive(rcu_state.gp_wq,
 					 READ_ONCE(rcu_state.gp_flags) &
 					 RCU_GP_FLAG_INIT);
+			rcu_gp_torture_wait();
 			rcu_state.gp_state = RCU_GP_DONE_GPS;
 			/* Locking provides needed memory barrier. */
 			if (rcu_gp_init())