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}
121