rcu-tasks: Use rcuwait for the rcu_tasks_kthread()

The waitqueue used by rcu_tasks_kthread() has always only one waiter.
With a guaranteed only one waiter, this can be replaced with rcuwait
which is smaller and simpler. With rcuwait based wake counterpart, the
irqwork function (call_rcu_tasks_iw_wakeup()) can be invoked hardirq
context because it is only a wake up and no sleeping locks are involved
(unlike the wait_queue_head).
As a side effect, this is also one piece of the puzzle to pass the RCU
selftest at early boot on PREEMPT_RT.

Replace wait_queue_head with rcuwait and let the irqwork run in hardirq
context on PREEMPT_RT.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -46,7 +46,7 @@ struct rcu_tasks_percpu {
 
 /**
  * struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
- * @cbs_wq: Wait queue allowing new callback to get kthread's attention.
+ * @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
  * @cbs_gbl_lock: Lock protecting callback list.
  * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
  * @gp_func: This flavor's grace-period-wait function.
@@ -77,7 +77,7 @@ struct rcu_tasks_percpu {
  * @kname: This flavor's kthread name.
  */
 struct rcu_tasks {
-	struct wait_queue_head cbs_wq;
+	struct rcuwait cbs_wait;
 	raw_spinlock_t cbs_gbl_lock;
 	int gp_state;
 	int gp_sleep;
@@ -113,11 +113,11 @@ static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
 #define DEFINE_RCU_TASKS(rt_name, gp, call, n)						\
 static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {			\
 	.lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock),		\
-	.rtp_irq_work = IRQ_WORK_INIT(call_rcu_tasks_iw_wakeup),			\
+	.rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),			\
 };											\
 static struct rcu_tasks rt_name =							\
 {											\
-	.cbs_wq = __WAIT_QUEUE_HEAD_INITIALIZER(rt_name.cbs_wq),			\
+	.cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait),				\
 	.cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock),			\
 	.gp_func = gp,									\
 	.call_func = call,								\
@@ -266,7 +266,7 @@ static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
 	struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
 
 	rtp = rtpcp->rtpp;
-	wake_up(&rtp->cbs_wq);
+	rcuwait_wake_up(&rtp->cbs_wait);
 }
 
 // Enqueue a callback for the specified flavor of Tasks RCU.
@@ -514,7 +514,9 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
 
 		/* If there were none, wait a bit and start over. */
-		wait_event_idle(rtp->cbs_wq, (needgpcb = rcu_tasks_need_gpcb(rtp)));
+		rcuwait_wait_event(&rtp->cbs_wait,
+				   (needgpcb = rcu_tasks_need_gpcb(rtp)),
+				   TASK_IDLE);
 
 		if (needgpcb & 0x2) {
 			// Wait for one grace period.