text: /*
   * kernel/freezer.c - Function to freeze a process
   *
   * Originally from kernel/power/process.c
   */
  
  #include <linux/interrupt.h>
  #include <linux/suspend.h>
  #include <linux/export.h>
  #include <linux/syscalls.h>
  #include <linux/freezer.h>
  #include <linux/kthread.h>
  
  /* total number of freezing conditions in effect */
  atomic_t system_freezing_cnt = ATOMIC_INIT(0);
  EXPORT_SYMBOL(system_freezing_cnt);
  
  /* indicate whether PM freezing is in effect, protected by pm_mutex */
  bool pm_freezing;
  bool pm_nosig_freezing;
  
  /* protects freezing and frozen transitions */
  static DEFINE_SPINLOCK(freezer_lock);
  
  /**
   * freezing_slow_path - slow path for testing whether a task needs to be frozen
   * @p: task to be tested
   *
   * This function is called by freezing() if system_freezing_cnt isn't zero
   * and tests whether @p needs to enter and stay in frozen state.  Can be
   * called under any context.  The freezers are responsible for ensuring the
   * target tasks see the updated state.
   */
  bool freezing_slow_path(struct task_struct *p)
  {
  	if (p->flags & PF_NOFREEZE)
  		return false;
  
  	if (pm_nosig_freezing || cgroup_freezing(p))
  		return true;
  
  	if (pm_freezing && !(p->flags & PF_KTHREAD))
  		return true;
  
  	return false;
  }
  EXPORT_SYMBOL(freezing_slow_path);
  
  /* Refrigerator is place where frozen processes are stored :-). */
  bool __refrigerator(bool check_kthr_stop)
  {
  	/* Hmm, should we be allowed to suspend when there are realtime
  	   processes around? */
  	bool was_frozen = false;
  	long save = current->state;
  
  	pr_debug("%s entered refrigerator\n", current->comm);
  
  	for (;;) {
  		set_current_state(TASK_UNINTERRUPTIBLE);
  
  		spin_lock_irq(&freezer_lock);
  		current->flags |= PF_FROZEN;
  		if (!freezing(current) ||
  		    (check_kthr_stop && kthread_should_stop()))
  			current->flags &= ~PF_FROZEN;
  		spin_unlock_irq(&freezer_lock);
  
  		if (!(current->flags & PF_FROZEN))
  			break;
  		was_frozen = true;
  		schedule();
  	}
  
  	pr_debug("%s left refrigerator\n", current->comm);
  
  	/*
  	 * Restore saved task state before returning.  The mb'd version
  	 * needs to be used; otherwise, it might silently break
  	 * synchronization which depends on ordered task state change.
  	 */
  	set_current_state(save);
  
  	return was_frozen;
  }
  EXPORT_SYMBOL(__refrigerator);
  
  static void fake_signal_wake_up(struct task_struct *p)
  {
  	unsigned long flags;
  
  	if (lock_task_sighand(p, &flags)) {
  		signal_wake_up(p, 0);
  		unlock_task_sighand(p, &flags);
  	}
  }
  
  /**
   * freeze_task - send a freeze request to given task
   * @p: task to send the request to
   *
   * If @p is freezing, the freeze request is sent either by sending a fake
   * signal (if it's not a kernel thread) or waking it up (if it's a kernel
   * thread).
   *
   * RETURNS:
   * %false, if @p is not freezing or already frozen; %true, otherwise
   */
  bool freeze_task(struct task_struct *p)
  {
  	unsigned long flags;
  
  	/*
  	 * This check can race with freezer_do_not_count, but worst case that
  	 * will result in an extra wakeup being sent to the task.  It does not
  	 * race with freezer_count(), the barriers in freezer_count() and
  	 * freezer_should_skip() ensure that either freezer_count() sees
  	 * freezing == true in try_to_freeze() and freezes, or
  	 * freezer_should_skip() sees !PF_FREEZE_SKIP and freezes the task
  	 * normally.
  	 */
  	if (freezer_should_skip(p))
  		return false;
  
  	spin_lock_irqsave(&freezer_lock, flags);
  	if (!freezing(p) || frozen(p)) {
  		spin_unlock_irqrestore(&freezer_lock, flags);
  		return false;
  	}
  
  	if (!(p->flags & PF_KTHREAD)) {
  		fake_signal_wake_up(p);
  		/*
  		 * fake_signal_wake_up() goes through p's scheduler
  		 * lock and guarantees that TASK_STOPPED/TRACED ->
  		 * TASK_RUNNING transition can't race with task state
  		 * testing in try_to_freeze_tasks().
  		 */
  	} else {
  		wake_up_state(p, TASK_INTERRUPTIBLE);
  	}
  
  	spin_unlock_irqrestore(&freezer_lock, flags);
  	return true;
  }
  
  void __thaw_task(struct task_struct *p)
  {
  	unsigned long flags;
  
  	/*
  	 * Clear freezing and kick @p if FROZEN.  Clearing is guaranteed to
  	 * be visible to @p as waking up implies wmb.  Waking up inside
  	 * freezer_lock also prevents wakeups from leaking outside
  	 * refrigerator.
  	 */
  	spin_lock_irqsave(&freezer_lock, flags);
  	if (frozen(p))
  		wake_up_process(p);
  	spin_unlock_irqrestore(&freezer_lock, flags);
  }
  
  /**
   * set_freezable - make %current freezable
   *
   * Mark %current freezable and enter refrigerator if necessary.
   */
  bool set_freezable(void)
  {
  	might_sleep();
  
  	/*
  	 * Modify flags while holding freezer_lock.  This ensures the
  	 * freezer notices that we aren't frozen yet or the freezing
  	 * condition is visible to try_to_freeze() below.
  	 */
  	spin_lock_irq(&freezer_lock);
  	current->flags &= ~PF_NOFREEZE;
  	spin_unlock_irq(&freezer_lock);
  
  	return try_to_freeze();
  }
  EXPORT_SYMBOL(set_freezable);
  