rcuscale: fix building with RCU_TINY

Both the CONFIG_TASKS_RCU and CONFIG_TASKS_RUDE_RCU options
are broken when RCU_TINY is enabled as well, as some functions
are missing a declaration.

In file included from kernel/rcu/update.c:649:
kernel/rcu/tasks.h:1271:21: error: no previous prototype for 'get_rcu_tasks_rude_gp_kthread' [-Werror=missing-prototypes]
 1271 | struct task_struct *get_rcu_tasks_rude_gp_kthread(void)
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/rcu/rcuscale.c:330:27: error: 'get_rcu_tasks_rude_gp_kthread' undeclared here (not in a function); did you mean 'get_rcu_tasks_trace_gp_kthread'?
  330 |         .rso_gp_kthread = get_rcu_tasks_rude_gp_kthread,
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                           get_rcu_tasks_trace_gp_kthread

In file included from /home/arnd/arm-soc/kernel/rcu/update.c:649:
kernel/rcu/tasks.h:1113:21: error: no previous prototype for 'get_rcu_tasks_gp_kthread' [-Werror=missing-prototypes]
 1113 | struct task_struct *get_rcu_tasks_gp_kthread(void)
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~

Also, building with CONFIG_TASKS_RUDE_RCU but not CONFIG_TASKS_RCU is
broken because of some missing stub functions:

kernel/rcu/rcuscale.c:322:27: error: 'tasks_scale_read_lock' undeclared here (not in a function); did you mean 'srcu_scale_read_lock'?
  322 |         .readlock       = tasks_scale_read_lock,
      |                           ^~~~~~~~~~~~~~~~~~~~~
      |                           srcu_scale_read_lock
kernel/rcu/rcuscale.c:323:27: error: 'tasks_scale_read_unlock' undeclared here (not in a function); did you mean 'srcu_scale_read_unlock'?
  323 |         .readunlock     = tasks_scale_read_unlock,
      |                           ^~~~~~~~~~~~~~~~~~~~~~~
      |                           srcu_scale_read_unlock

Move the declarations outside of the RCU_TINY #ifdef and duplicate the
shared stub functions to address all of the above.

Fixes: 88d7ff818f0ce ("rcuscale: Add RCU Tasks Rude testing")
Fixes: 755f1c5eb416b ("rcuscale: Measure RCU Tasks Trace grace-period kthread CPU time")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: arndb

kernel/rcu/rcu.h

@@ -505,18 +505,20 @@ void rcu_async_relax(void);
 void rcupdate_announce_bootup_oddness(void);
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);
-# ifdef CONFIG_TASKS_RCU
-struct task_struct *get_rcu_tasks_gp_kthread(void);
-# endif // # ifdef CONFIG_TASKS_RCU
-# ifdef CONFIG_TASKS_RUDE_RCU
-struct task_struct *get_rcu_tasks_rude_gp_kthread(void);
-# endif // # ifdef CONFIG_TASKS_RUDE_RCU
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void show_rcu_tasks_gp_kthreads(void) {}
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 void rcu_request_urgent_qs_task(struct task_struct *t);
 #endif /* #else #ifdef CONFIG_TINY_RCU */
 
+#ifdef CONFIG_TASKS_RCU
+struct task_struct *get_rcu_tasks_gp_kthread(void);
+#endif // # ifdef CONFIG_TASKS_RCU
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+struct task_struct *get_rcu_tasks_rude_gp_kthread(void);
+#endif // # ifdef CONFIG_TASKS_RUDE_RCU
+
 #define RCU_SCHEDULER_INACTIVE	0
 #define RCU_SCHEDULER_INIT	1
 #define RCU_SCHEDULER_RUNNING	2

kernel/rcu/rcuscale.c

@@ -316,11 +316,20 @@ static struct rcu_scale_ops tasks_ops = {
  * Definitions for RCU-tasks-rude scalability testing.
  */
 
+static int tasks_rude_scale_read_lock(void)
+{
+	return 0;
+}
+
+static void tasks_rude_scale_read_unlock(int idx)
+{
+}
+
 static struct rcu_scale_ops tasks_rude_ops = {
 	.ptype		= RCU_TASKS_RUDE_FLAVOR,
 	.init		= rcu_sync_scale_init,
-	.readlock	= tasks_scale_read_lock,
-	.readunlock	= tasks_scale_read_unlock,
+	.readlock	= tasks_rude_scale_read_lock,
+	.readunlock	= tasks_rude_scale_read_unlock,
 	.get_gp_seq	= rcu_no_completed,
 	.gp_diff	= rcu_seq_diff,
 	.async		= call_rcu_tasks_rude,