1#include <linux/spinlock.h>
  2#include <linux/task_work.h>
  3#include <linux/tracehook.h>
  4
  5static struct callback_head work_exited; /* all we need is ->next == NULL */
  6
  7/**
  8 * task_work_add - ask the @task to execute @work->func()
  9 * @task: the task which should run the callback
 10 * @work: the callback to run
 11 * @notify: send the notification if true
 12 *
 13 * Queue @work for task_work_run() below and notify the @task if @notify.
 14 * Fails if the @task is exiting/exited and thus it can't process this @work.
 15 * Otherwise @work->func() will be called when the @task returns from kernel
 16 * mode or exits.
 17 *
 18 * This is like the signal handler which runs in kernel mode, but it doesn't
 19 * try to wake up the @task.
 20 *
 21 * Note: there is no ordering guarantee on works queued here.
 22 *
 23 * RETURNS:
 24 * 0 if succeeds or -ESRCH.
 25 */
 26int
 27task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 28{
 29	struct callback_head *head;
 30
 31	do {
 32		head = READ_ONCE(task->task_works);
 33		if (unlikely(head == &work_exited))
 34			return -ESRCH;
 35		work->next = head;
 36	} while (cmpxchg(&task->task_works, head, work) != head);
 37
 38	if (notify)
 39		set_notify_resume(task);
 40	return 0;
 41}
 42
 43/**
 44 * task_work_cancel - cancel a pending work added by task_work_add()
 45 * @task: the task which should execute the work
 46 * @func: identifies the work to remove
 47 *
 48 * Find the last queued pending work with ->func == @func and remove
 49 * it from queue.
 50 *
 51 * RETURNS:
 52 * The found work or NULL if not found.
 53 */
 54struct callback_head *
 55task_work_cancel(struct task_struct *task, task_work_func_t func)
 56{
 57	struct callback_head **pprev = &task->task_works;
 58	struct callback_head *work;
 59	unsigned long flags;
 60
 61	if (likely(!task->task_works))
 62		return NULL;
 63	/*
 64	 * If cmpxchg() fails we continue without updating pprev.
 65	 * Either we raced with task_work_add() which added the
 66	 * new entry before this work, we will find it again. Or
 67	 * we raced with task_work_run(), *pprev == NULL/exited.
 68	 */
 69	raw_spin_lock_irqsave(&task->pi_lock, flags);
 70	while ((work = lockless_dereference(*pprev))) {
 71		if (work->func != func)
 72			pprev = &work->next;
 73		else if (cmpxchg(pprev, work, work->next) == work)
 74			break;
 75	}
 76	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 77
 78	return work;
 79}
 80
 81/**
 82 * task_work_run - execute the works added by task_work_add()
 83 *
 84 * Flush the pending works. Should be used by the core kernel code.
 85 * Called before the task returns to the user-mode or stops, or when
 86 * it exits. In the latter case task_work_add() can no longer add the
 87 * new work after task_work_run() returns.
 88 */
 89void task_work_run(void)
 90{
 91	struct task_struct *task = current;
 92	struct callback_head *work, *head, *next;
 93
 94	for (;;) {
 95		/*
 96		 * work->func() can do task_work_add(), do not set
 97		 * work_exited unless the list is empty.
 98		 */
 99		do {
100			work = READ_ONCE(task->task_works);
101			head = !work && (task->flags & PF_EXITING) ?
102				&work_exited : NULL;
103		} while (cmpxchg(&task->task_works, work, head) != work);
104
105		if (!work)
106			break;
107		/*
108		 * Synchronize with task_work_cancel(). It can't remove
109		 * the first entry == work, cmpxchg(task_works) should
110		 * fail, but it can play with *work and other entries.
111		 */
112		raw_spin_unlock_wait(&task->pi_lock);
113
114		do {
115			next = work->next;
116			work->func(work);
117			work = next;
118			cond_resched();
119		} while (work);
120	}
121}