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 = ACCESS_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 cmpxchg() fails we continue without updating pprev.
 62	 * Either we raced with task_work_add() which added the
 63	 * new entry before this work, we will find it again. Or
 64	 * we raced with task_work_run(), *pprev == NULL/exited.
 65	 */
 66	raw_spin_lock_irqsave(&task->pi_lock, flags);
 67	while ((work = ACCESS_ONCE(*pprev))) {
 68		smp_read_barrier_depends();
 69		if (work->func != func)
 70			pprev = &work->next;
 71		else if (cmpxchg(pprev, work, work->next) == work)
 72			break;
 73	}
 74	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 75
 76	return work;
 77}
 78
 79/**
 80 * task_work_run - execute the works added by task_work_add()
 81 *
 82 * Flush the pending works. Should be used by the core kernel code.
 83 * Called before the task returns to the user-mode or stops, or when
 84 * it exits. In the latter case task_work_add() can no longer add the
 85 * new work after task_work_run() returns.
 86 */
 87void task_work_run(void)
 88{
 89	struct task_struct *task = current;
 90	struct callback_head *work, *head, *next;
 91
 92	for (;;) {
 93		/*
 94		 * work->func() can do task_work_add(), do not set
 95		 * work_exited unless the list is empty.
 96		 */
 97		do {
 98			work = ACCESS_ONCE(task->task_works);
 99			head = !work && (task->flags & PF_EXITING) ?
100				&work_exited : NULL;
101		} while (cmpxchg(&task->task_works, work, head) != work);
102
103		if (!work)
104			break;
105		/*
106		 * Synchronize with task_work_cancel(). It can't remove
107		 * the first entry == work, cmpxchg(task_works) should
108		 * fail, but it can play with *work and other entries.
109		 */
110		raw_spin_unlock_wait(&task->pi_lock);
111		smp_mb();
112
113		do {
114			next = work->next;
115			work->func(work);
116			work = next;
117			cond_resched();
118		} while (work);
119	}
120}