rcu-tasks: Move RCU Tasks self-tests to core_initcall()

The timer and hrtimer softirq processing has moved to dedicated threads
for kernels built with CONFIG_IRQ_FORCED_THREADING=y.  This results in
timers not expiring until later in early boot, which in turn causes the
RCU Tasks self-tests to hang in kernels built with CONFIG_PROVE_RCU=y,
which further causes the entire kernel to hang.  One fix would be to
make timers work during this time, but there are no known users of RCU
Tasks grace periods during that time, so no justification for the added
complexity.  Not yet, anyway.

This commit therefore moves the call to rcu_init_tasks_generic() from
kernel_init_freeable() to a core_initcall().  This works because the
timer and hrtimer kthreads are created at early_initcall() time.

Fixes: 49a1763 ("softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.")
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: <[email protected]>
Tested-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>

Author: paulmckrcu

include/linux/rcupdate.h

@@ -121,12 +121,6 @@ void rcu_init(void);
 extern int rcu_scheduler_active;
 void rcu_sched_clock_irq(int user);
 
-#ifdef CONFIG_TASKS_RCU_GENERIC
-void rcu_init_tasks_generic(void);
-#else
-static inline void rcu_init_tasks_generic(void) { }
-#endif
-
 #ifdef CONFIG_RCU_STALL_COMMON
 void rcu_sysrq_start(void);
 void rcu_sysrq_end(void);

init/main.c

@@ -1553,7 +1553,6 @@ static noinline void __init kernel_init_freeable(void)
 
 	init_mm_internals();
 
-	rcu_init_tasks_generic();
 	do_pre_smp_initcalls();
 	lockup_detector_init();
 

kernel/rcu/tasks.h

@@ -2256,7 +2256,7 @@ void __init tasks_cblist_init_generic(void)
 #endif
 }
 
-void __init rcu_init_tasks_generic(void)
+static int __init rcu_init_tasks_generic(void)
 {
 #ifdef CONFIG_TASKS_RCU
 	rcu_spawn_tasks_kthread();
@@ -2272,7 +2272,10 @@ void __init rcu_init_tasks_generic(void)
 
 	// Run the self-tests.
 	rcu_tasks_initiate_self_tests();
+
+	return 0;
 }
+core_initcall(rcu_init_tasks_generic);
 
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void rcu_tasks_bootup_oddness(void) {}