[PATCH 1/2] rcu-tasks: move RCU-tasks initialization out of core_initcall()

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Initialize the RCU-tasks earlier, before *_initcall() callbacks are
invoked. Do it after the workqueue subsytem is up and running. That
gives us a possibility to make use of synchronize_rcu_tasks*() wait
API in early_initcall() callbacks.

Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 include/linux/rcupdate.h |  6 ++++++
 init/main.c              |  1 +
 kernel/rcu/tasks.h       | 26 ++++++++++++++++++++++----
 3 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 3c3efa4d6ab5..340c7d5344a4 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -88,6 +88,12 @@ void rcu_sched_clock_irq(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);
 
+#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);
diff --git a/init/main.c b/init/main.c
index 130376ec10ba..e253e87bdf58 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1502,6 +1502,7 @@ static noinline void __init kernel_init_freeable(void)
 
 	init_mm_internals();
 
+	rcu_init_tasks_generic();
 	do_pre_smp_initcalls();
 	lockup_detector_init();
 
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 35bdcfd84d42..67a162949763 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -241,7 +241,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 	}
 }
 
-/* Spawn RCU-tasks grace-period kthread, e.g., at core_initcall() time. */
+/* Spawn RCU-tasks grace-period kthread. */
 static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
 {
 	struct task_struct *t;
@@ -564,7 +564,6 @@ static int __init rcu_spawn_tasks_kthread(void)
 	rcu_spawn_tasks_kthread_generic(&rcu_tasks);
 	return 0;
 }
-core_initcall(rcu_spawn_tasks_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_classic_gp_kthread(void)
@@ -692,7 +691,6 @@ static int __init rcu_spawn_tasks_rude_kthread(void)
 	rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
 	return 0;
 }
-core_initcall(rcu_spawn_tasks_rude_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_rude_gp_kthread(void)
@@ -968,6 +966,12 @@ static void rcu_tasks_trace_pregp_step(void)
 static void rcu_tasks_trace_pertask(struct task_struct *t,
 				    struct list_head *hop)
 {
+	// During early boot when there is only one boot-CPU,
+	// an idle_task is not set for other CPUs. In this case
+	// just revert.
+	if (unlikely(t == NULL))
+		return;
+
 	WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
 	WRITE_ONCE(t->trc_reader_checked, false);
 	t->trc_ipi_to_cpu = -1;
@@ -1193,7 +1197,6 @@ static int __init rcu_spawn_tasks_trace_kthread(void)
 	rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
 	return 0;
 }
-core_initcall(rcu_spawn_tasks_trace_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_trace_gp_kthread(void)
@@ -1222,6 +1225,21 @@ void show_rcu_tasks_gp_kthreads(void)
 }
 #endif /* #ifndef CONFIG_TINY_RCU */
 
+void __init rcu_init_tasks_generic(void)
+{
+#ifdef CONFIG_TASKS_RCU
+	rcu_spawn_tasks_kthread();
+#endif
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+	rcu_spawn_tasks_rude_kthread();
+#endif
+
+#ifdef CONFIG_TASKS_TRACE_RCU
+	rcu_spawn_tasks_trace_kthread();
+#endif
+}
+
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void rcu_tasks_bootup_oddness(void) {}
 void show_rcu_tasks_gp_kthreads(void) {}
-- 
2.20.1