1/*
2 * Copyright (c) 2008 Intel Corporation
3 * Author: Matthew Wilcox <willy@linux.intel.com>
4 *
5 * Distributed under the terms of the GNU GPL, version 2
6 *
7 * This file implements counting semaphores.
8 * A counting semaphore may be acquired 'n' times before sleeping.
9 * See mutex.c for single-acquisition sleeping locks which enforce
10 * rules which allow code to be debugged more easily.
11 */
12
13/*
14 * Some notes on the implementation:
15 *
16 * The spinlock controls access to the other members of the semaphore.
17 * down_trylock() and up() can be called from interrupt context, so we
18 * have to disable interrupts when taking the lock.  It turns out various
19 * parts of the kernel expect to be able to use down() on a semaphore in
20 * interrupt context when they know it will succeed, so we have to use
21 * irqsave variants for down(), down_interruptible() and down_killable()
22 * too.
23 *
24 * The ->count variable represents how many more tasks can acquire this
25 * semaphore.  If it's zero, there may be tasks waiting on the wait_list.
26 */
27
28#include <linux/compiler.h>
29#include <linux/kernel.h>
30#include <linux/export.h>
31#include <linux/sched.h>
32#include <linux/semaphore.h>
33#include <linux/spinlock.h>
34#include <linux/ftrace.h>
35
36static noinline void __down(struct semaphore *sem);
37static noinline int __down_interruptible(struct semaphore *sem);
38static noinline int __down_killable(struct semaphore *sem);
39static noinline int __down_timeout(struct semaphore *sem, long timeout);
40static noinline void __up(struct semaphore *sem);
41
42/**
43 * down - acquire the semaphore
44 * @sem: the semaphore to be acquired
45 *
46 * Acquires the semaphore.  If no more tasks are allowed to acquire the
47 * semaphore, calling this function will put the task to sleep until the
48 * semaphore is released.
49 *
50 * Use of this function is deprecated, please use down_interruptible() or
51 * down_killable() instead.
52 */
53void down(struct semaphore *sem)
54{
55	unsigned long flags;
56
57	raw_spin_lock_irqsave(&sem->lock, flags);
58	if (likely(sem->count > 0))
59		sem->count--;
60	else
61		__down(sem);
62	raw_spin_unlock_irqrestore(&sem->lock, flags);
63}
64EXPORT_SYMBOL(down);
65
66/**
67 * down_interruptible - acquire the semaphore unless interrupted
68 * @sem: the semaphore to be acquired
69 *
70 * Attempts to acquire the semaphore.  If no more tasks are allowed to
71 * acquire the semaphore, calling this function will put the task to sleep.
72 * If the sleep is interrupted by a signal, this function will return -EINTR.
73 * If the semaphore is successfully acquired, this function returns 0.
74 */
75int down_interruptible(struct semaphore *sem)
76{
77	unsigned long flags;
78	int result = 0;
79
80	raw_spin_lock_irqsave(&sem->lock, flags);
81	if (likely(sem->count > 0))
82		sem->count--;
83	else
84		result = __down_interruptible(sem);
85	raw_spin_unlock_irqrestore(&sem->lock, flags);
86
87	return result;
88}
89EXPORT_SYMBOL(down_interruptible);
90
91/**
92 * down_killable - acquire the semaphore unless killed
93 * @sem: the semaphore to be acquired
94 *
95 * Attempts to acquire the semaphore.  If no more tasks are allowed to
96 * acquire the semaphore, calling this function will put the task to sleep.
97 * If the sleep is interrupted by a fatal signal, this function will return
98 * -EINTR.  If the semaphore is successfully acquired, this function returns
99 * 0.
100 */
101int down_killable(struct semaphore *sem)
102{
103	unsigned long flags;
104	int result = 0;
105
106	raw_spin_lock_irqsave(&sem->lock, flags);
107	if (likely(sem->count > 0))
108		sem->count--;
109	else
110		result = __down_killable(sem);
111	raw_spin_unlock_irqrestore(&sem->lock, flags);
112
113	return result;
114}
115EXPORT_SYMBOL(down_killable);
116
117/**
118 * down_trylock - try to acquire the semaphore, without waiting
119 * @sem: the semaphore to be acquired
120 *
121 * Try to acquire the semaphore atomically.  Returns 0 if the semaphore has
122 * been acquired successfully or 1 if it it cannot be acquired.
123 *
124 * NOTE: This return value is inverted from both spin_trylock and
125 * mutex_trylock!  Be careful about this when converting code.
126 *
127 * Unlike mutex_trylock, this function can be used from interrupt context,
128 * and the semaphore can be released by any task or interrupt.
129 */
130int down_trylock(struct semaphore *sem)
131{
132	unsigned long flags;
133	int count;
134
135	raw_spin_lock_irqsave(&sem->lock, flags);
136	count = sem->count - 1;
137	if (likely(count >= 0))
138		sem->count = count;
139	raw_spin_unlock_irqrestore(&sem->lock, flags);
140
141	return (count < 0);
142}
143EXPORT_SYMBOL(down_trylock);
144
145/**
146 * down_timeout - acquire the semaphore within a specified time
147 * @sem: the semaphore to be acquired
148 * @timeout: how long to wait before failing
149 *
150 * Attempts to acquire the semaphore.  If no more tasks are allowed to
151 * acquire the semaphore, calling this function will put the task to sleep.
152 * If the semaphore is not released within the specified number of jiffies,
153 * this function returns -ETIME.  It returns 0 if the semaphore was acquired.
154 */
155int down_timeout(struct semaphore *sem, long timeout)
156{
157	unsigned long flags;
158	int result = 0;
159
160	raw_spin_lock_irqsave(&sem->lock, flags);
161	if (likely(sem->count > 0))
162		sem->count--;
163	else
164		result = __down_timeout(sem, timeout);
165	raw_spin_unlock_irqrestore(&sem->lock, flags);
166
167	return result;
168}
169EXPORT_SYMBOL(down_timeout);
170
171/**
172 * up - release the semaphore
173 * @sem: the semaphore to release
174 *
175 * Release the semaphore.  Unlike mutexes, up() may be called from any
176 * context and even by tasks which have never called down().
177 */
178void up(struct semaphore *sem)
179{
180	unsigned long flags;
181
182	raw_spin_lock_irqsave(&sem->lock, flags);
183	if (likely(list_empty(&sem->wait_list)))
184		sem->count++;
185	else
186		__up(sem);
187	raw_spin_unlock_irqrestore(&sem->lock, flags);
188}
189EXPORT_SYMBOL(up);
190
191/* Functions for the contended case */
192
193struct semaphore_waiter {
194	struct list_head list;
195	struct task_struct *task;
196	bool up;
197};
198
199/*
200 * Because this function is inlined, the 'state' parameter will be
201 * constant, and thus optimised away by the compiler.  Likewise the
202 * 'timeout' parameter for the cases without timeouts.
203 */
204static inline int __sched __down_common(struct semaphore *sem, long state,
205								long timeout)
206{
207	struct task_struct *task = current;
208	struct semaphore_waiter waiter;
209
210	list_add_tail(&waiter.list, &sem->wait_list);
211	waiter.task = task;
212	waiter.up = false;
213
214	for (;;) {
215		if (signal_pending_state(state, task))
216			goto interrupted;
217		if (unlikely(timeout <= 0))
218			goto timed_out;
219		__set_task_state(task, state);
220		raw_spin_unlock_irq(&sem->lock);
221		timeout = schedule_timeout(timeout);
222		raw_spin_lock_irq(&sem->lock);
223		if (waiter.up)
224			return 0;
225	}
226
227 timed_out:
228	list_del(&waiter.list);
229	return -ETIME;
230
231 interrupted:
232	list_del(&waiter.list);
233	return -EINTR;
234}
235
236static noinline void __sched __down(struct semaphore *sem)
237{
238	__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
239}
240
241static noinline int __sched __down_interruptible(struct semaphore *sem)
242{
243	return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
244}
245
246static noinline int __sched __down_killable(struct semaphore *sem)
247{
248	return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
249}
250
251static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
252{
253	return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
254}
255
256static noinline void __sched __up(struct semaphore *sem)
257{
258	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
259						struct semaphore_waiter, list);
260	list_del(&waiter->list);
261	waiter->up = true;
262	wake_up_process(waiter->task);
263}
264