1 // SPDX-License-Identifier: GPL-2.0
2 #include <linux/spinlock.h>
3 #include <linux/task_work.h>
4 #include <linux/tracehook.h>
5 
6 static struct callback_head work_exited; /* all we need is ->next == NULL */
7 
8 /**
9  * task_work_add - ask the @task to execute @work->func()
10  * @task: the task which should run the callback
11  * @work: the callback to run
12  * @notify: how to notify the targeted task
13  *
14  * Queue @work for task_work_run() below and notify the @task if @notify
15  * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL works like signals, in that the
16  * it will interrupt the targeted task and run the task_work. @TWA_RESUME
17  * work is run only when the task exits the kernel and returns to user mode,
18  * or before entering guest mode. Fails if the @task is exiting/exited and thus
19  * it can't process this @work. Otherwise @work->func() will be called when the
20  * @task goes through one of the aforementioned transitions, or exits.
21  *
22  * If the targeted task is exiting, then an error is returned and the work item
23  * is not queued. It's up to the caller to arrange for an alternative mechanism
24  * in that case.
25  *
26  * Note: there is no ordering guarantee on works queued here. The task_work
27  * list is LIFO.
28  *
29  * RETURNS:
30  * 0 if succeeds or -ESRCH.
31  */
task_work_add(struct task_struct * task,struct callback_head * work,enum task_work_notify_mode notify)32 int task_work_add(struct task_struct *task, struct callback_head *work,
33 		  enum task_work_notify_mode notify)
34 {
35 	struct callback_head *head;
36 
37 	/* record the work call stack in order to print it in KASAN reports */
38 	kasan_record_aux_stack(work);
39 
40 	do {
41 		head = READ_ONCE(task->task_works);
42 		if (unlikely(head == &work_exited))
43 			return -ESRCH;
44 		work->next = head;
45 	} while (cmpxchg(&task->task_works, head, work) != head);
46 
47 	switch (notify) {
48 	case TWA_NONE:
49 		break;
50 	case TWA_RESUME:
51 		set_notify_resume(task);
52 		break;
53 	case TWA_SIGNAL:
54 		set_notify_signal(task);
55 		break;
56 	default:
57 		WARN_ON_ONCE(1);
58 		break;
59 	}
60 
61 	return 0;
62 }
63 
64 /**
65  * task_work_cancel_match - cancel a pending work added by task_work_add()
66  * @task: the task which should execute the work
67  * @match: match function to call
68  *
69  * RETURNS:
70  * The found work or NULL if not found.
71  */
72 struct callback_head *
task_work_cancel_match(struct task_struct * task,bool (* match)(struct callback_head *,void * data),void * data)73 task_work_cancel_match(struct task_struct *task,
74 		       bool (*match)(struct callback_head *, void *data),
75 		       void *data)
76 {
77 	struct callback_head **pprev = &task->task_works;
78 	struct callback_head *work;
79 	unsigned long flags;
80 
81 	if (likely(!task->task_works))
82 		return NULL;
83 	/*
84 	 * If cmpxchg() fails we continue without updating pprev.
85 	 * Either we raced with task_work_add() which added the
86 	 * new entry before this work, we will find it again. Or
87 	 * we raced with task_work_run(), *pprev == NULL/exited.
88 	 */
89 	raw_spin_lock_irqsave(&task->pi_lock, flags);
90 	while ((work = READ_ONCE(*pprev))) {
91 		if (!match(work, data))
92 			pprev = &work->next;
93 		else if (cmpxchg(pprev, work, work->next) == work)
94 			break;
95 	}
96 	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
97 
98 	return work;
99 }
100 
task_work_func_match(struct callback_head * cb,void * data)101 static bool task_work_func_match(struct callback_head *cb, void *data)
102 {
103 	return cb->func == data;
104 }
105 
106 /**
107  * task_work_cancel - cancel a pending work added by task_work_add()
108  * @task: the task which should execute the work
109  * @func: identifies the work to remove
110  *
111  * Find the last queued pending work with ->func == @func and remove
112  * it from queue.
113  *
114  * RETURNS:
115  * The found work or NULL if not found.
116  */
117 struct callback_head *
task_work_cancel(struct task_struct * task,task_work_func_t func)118 task_work_cancel(struct task_struct *task, task_work_func_t func)
119 {
120 	return task_work_cancel_match(task, task_work_func_match, func);
121 }
122 
123 /**
124  * task_work_run - execute the works added by task_work_add()
125  *
126  * Flush the pending works. Should be used by the core kernel code.
127  * Called before the task returns to the user-mode or stops, or when
128  * it exits. In the latter case task_work_add() can no longer add the
129  * new work after task_work_run() returns.
130  */
task_work_run(void)131 void task_work_run(void)
132 {
133 	struct task_struct *task = current;
134 	struct callback_head *work, *head, *next;
135 
136 	for (;;) {
137 		/*
138 		 * work->func() can do task_work_add(), do not set
139 		 * work_exited unless the list is empty.
140 		 */
141 		do {
142 			head = NULL;
143 			work = READ_ONCE(task->task_works);
144 			if (!work) {
145 				if (task->flags & PF_EXITING)
146 					head = &work_exited;
147 				else
148 					break;
149 			}
150 		} while (cmpxchg(&task->task_works, work, head) != work);
151 
152 		if (!work)
153 			break;
154 		/*
155 		 * Synchronize with task_work_cancel(). It can not remove
156 		 * the first entry == work, cmpxchg(task_works) must fail.
157 		 * But it can remove another entry from the ->next list.
158 		 */
159 		raw_spin_lock_irq(&task->pi_lock);
160 		raw_spin_unlock_irq(&task->pi_lock);
161 
162 		do {
163 			next = work->next;
164 			work->func(work);
165 			work = next;
166 			cond_resched();
167 		} while (work);
168 	}
169 }
170