text: // SPDX-License-Identifier: GPL-2.0
  /*
   * <linux/swait.h> (simple wait queues ) implementation:
   */
  #include "sched.h"
  
  void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
  			     struct lock_class_key *key)
  {
  	raw_spin_lock_init(&q->lock);
  	lockdep_set_class_and_name(&q->lock, key, name);
  	INIT_LIST_HEAD(&q->task_list);
  }
  EXPORT_SYMBOL(__init_swait_queue_head);
  
  /*
   * The thing about the wake_up_state() return value; I think we can ignore it.
   *
   * If for some reason it would return 0, that means the previously waiting
   * task is already running, so it will observe condition true (or has already).
   */
  void swake_up_locked(struct swait_queue_head *q)
  {
  	struct swait_queue *curr;
  
  	if (list_empty(&q->task_list))
  		return;
  
  	curr = list_first_entry(&q->task_list, typeof(*curr), task_list);
  	wake_up_process(curr->task);
  	list_del_init(&curr->task_list);
  }
  EXPORT_SYMBOL(swake_up_locked);
  
  void swake_up_one(struct swait_queue_head *q)
  {
  	unsigned long flags;
  
  	raw_spin_lock_irqsave(&q->lock, flags);
  	swake_up_locked(q);
  	raw_spin_unlock_irqrestore(&q->lock, flags);
  }
  EXPORT_SYMBOL(swake_up_one);
  
  /*
   * Does not allow usage from IRQ disabled, since we must be able to
   * release IRQs to guarantee bounded hold time.
   */
  void swake_up_all(struct swait_queue_head *q)
  {
  	struct swait_queue *curr;
  	LIST_HEAD(tmp);
  
  	raw_spin_lock_irq(&q->lock);
  	list_splice_init(&q->task_list, &tmp);
  	while (!list_empty(&tmp)) {
  		curr = list_first_entry(&tmp, typeof(*curr), task_list);
  
  		wake_up_state(curr->task, TASK_NORMAL);
  		list_del_init(&curr->task_list);
  
  		if (list_empty(&tmp))
  			break;
  
  		raw_spin_unlock_irq(&q->lock);
  		raw_spin_lock_irq(&q->lock);
  	}
  	raw_spin_unlock_irq(&q->lock);
  }
  EXPORT_SYMBOL(swake_up_all);
  
  static void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait)
  {
  	wait->task = current;
  	if (list_empty(&wait->task_list))
  		list_add_tail(&wait->task_list, &q->task_list);
  }
  
  void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state)
  {
  	unsigned long flags;
  
  	raw_spin_lock_irqsave(&q->lock, flags);
  	__prepare_to_swait(q, wait);
  	set_current_state(state);
  	raw_spin_unlock_irqrestore(&q->lock, flags);
  }
  EXPORT_SYMBOL(prepare_to_swait_exclusive);
  
  long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state)
  {
  	unsigned long flags;
  	long ret = 0;
  
  	raw_spin_lock_irqsave(&q->lock, flags);
  	if (unlikely(signal_pending_state(state, current))) {
  		/*
  		 * See prepare_to_wait_event(). TL;DR, subsequent swake_up_one()
  		 * must not see us.
  		 */
  		list_del_init(&wait->task_list);
  		ret = -ERESTARTSYS;
  	} else {
  		__prepare_to_swait(q, wait);
  		set_current_state(state);
  	}
  	raw_spin_unlock_irqrestore(&q->lock, flags);
  
  	return ret;
  }
  EXPORT_SYMBOL(prepare_to_swait_event);
  
  void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
  {
  	__set_current_state(TASK_RUNNING);
  	if (!list_empty(&wait->task_list))
  		list_del_init(&wait->task_list);
  }
  
  void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
  {
  	unsigned long flags;
  
  	__set_current_state(TASK_RUNNING);
  
  	if (!list_empty_careful(&wait->task_list)) {
  		raw_spin_lock_irqsave(&q->lock, flags);
  		list_del_init(&wait->task_list);
  		raw_spin_unlock_irqrestore(&q->lock, flags);
  	}
  }
  EXPORT_SYMBOL(finish_swait);
  