// SPDX-License-Identifier: GPL-2.0

  #include <linux/spinlock.h>
  #include <linux/task_work.h>
  #include <linux/tracehook.h>

  static struct callback_head work_exited; /* all we need is ->next == NULL */

  /**
   * task_work_add - ask the @task to execute @work->func()
   * @task: the task which should run the callback
   * @work: the callback to run

   * @notify: how to notify the targeted task

   *

   * Queue @work for task_work_run() below and notify the @task if @notify
   * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL works like signals, in that the
   * it will interrupt the targeted task and run the task_work. @TWA_RESUME
   * work is run only when the task exits the kernel and returns to user mode,
   * or before entering guest mode. Fails if the @task is exiting/exited and thus
   * it can't process this @work. Otherwise @work->func() will be called when the
   * @task goes through one of the aforementioned transitions, or exits.

   *

   * If the targeted task is exiting, then an error is returned and the work item
   * is not queued. It's up to the caller to arrange for an alternative mechanism
   * in that case.

   *

   * Note: there is no ordering guarantee on works queued here. The task_work
   * list is LIFO.

   *

   * RETURNS:
   * 0 if succeeds or -ESRCH.
   */

  int task_work_add(struct task_struct *task, struct callback_head *work,
  		  enum task_work_notify_mode notify)

  {

  	struct callback_head *head;

  	unsigned long flags;

  	do {

  		head = READ_ONCE(task->task_works);

  		if (unlikely(head == &work_exited))
  			return -ESRCH;

  		work->next = head;
  	} while (cmpxchg(&task->task_works, head, work) != head);

  	switch (notify) {

  	case TWA_NONE:
  		break;

  	case TWA_RESUME:

  		set_notify_resume(task);

  		break;
  	case TWA_SIGNAL:

  		/*
  		 * Only grab the sighand lock if we don't already have some
  		 * task_work pending. This pairs with the smp_store_mb()
  		 * in get_signal(), see comment there.
  		 */
  		if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
  		    lock_task_sighand(task, &flags)) {

  			task->jobctl |= JOBCTL_TASK_WORK;
  			signal_wake_up(task, 0);
  			unlock_task_sighand(task, &flags);
  		}
  		break;

  	default:
  		WARN_ON_ONCE(1);
  		break;

  	}

  	return 0;

  }

  /**
   * task_work_cancel - cancel a pending work added by task_work_add()
   * @task: the task which should execute the work
   * @func: identifies the work to remove
   *
   * Find the last queued pending work with ->func == @func and remove
   * it from queue.
   *
   * RETURNS:
   * The found work or NULL if not found.
   */

  struct callback_head *

  task_work_cancel(struct task_struct *task, task_work_func_t func)
  {

  	struct callback_head **pprev = &task->task_works;

  	struct callback_head *work;

  	unsigned long flags;

  
  	if (likely(!task->task_works))
  		return NULL;

  	/*
  	 * If cmpxchg() fails we continue without updating pprev.
  	 * Either we raced with task_work_add() which added the
  	 * new entry before this work, we will find it again. Or

  	 * we raced with task_work_run(), *pprev == NULL/exited.

  	 */

  	raw_spin_lock_irqsave(&task->pi_lock, flags);

  	while ((work = READ_ONCE(*pprev))) {

  		if (work->func != func)
  			pprev = &work->next;
  		else if (cmpxchg(pprev, work, work->next) == work)
  			break;

  	}

  	raw_spin_unlock_irqrestore(&task->pi_lock, flags);

  
  	return work;

  }

  /**
   * task_work_run - execute the works added by task_work_add()
   *
   * Flush the pending works. Should be used by the core kernel code.
   * Called before the task returns to the user-mode or stops, or when
   * it exits. In the latter case task_work_add() can no longer add the
   * new work after task_work_run() returns.
   */

  void task_work_run(void)
  {
  	struct task_struct *task = current;

  	struct callback_head *work, *head, *next;

  	for (;;) {

  		/*
  		 * work->func() can do task_work_add(), do not set
  		 * work_exited unless the list is empty.
  		 */
  		do {

  			head = NULL;

  			work = READ_ONCE(task->task_works);

  			if (!work) {
  				if (task->flags & PF_EXITING)
  					head = &work_exited;
  				else
  					break;
  			}

  		} while (cmpxchg(&task->task_works, work, head) != work);

  		if (!work)
  			break;

  		/*
  		 * Synchronize with task_work_cancel(). It can not remove
  		 * the first entry == work, cmpxchg(task_works) must fail.
  		 * But it can remove another entry from the ->next list.
  		 */
  		raw_spin_lock_irq(&task->pi_lock);
  		raw_spin_unlock_irq(&task->pi_lock);

  		do {
  			next = work->next;
  			work->func(work);
  			work = next;

  			cond_resched();

  		} while (work);

  	}
  }