1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * Copyright (c) 2008 Intel Corporation
  4 * Author: Matthew Wilcox <willy@linux.intel.com>
  5 *
  6 * This file implements counting semaphores.
  7 * A counting semaphore may be acquired 'n' times before sleeping.
  8 * See mutex.c for single-acquisition sleeping locks which enforce
  9 * rules which allow code to be debugged more easily.
 10 */
 11
 12/*
 13 * Some notes on the implementation:
 14 *
 15 * The spinlock controls access to the other members of the semaphore.
 16 * down_trylock() and up() can be called from interrupt context, so we
 17 * have to disable interrupts when taking the lock.  It turns out various
 18 * parts of the kernel expect to be able to use down() on a semaphore in
 19 * interrupt context when they know it will succeed, so we have to use
 20 * irqsave variants for down(), down_interruptible() and down_killable()
 21 * too.
 22 *
 23 * The ->count variable represents how many more tasks can acquire this
 24 * semaphore.  If it's zero, there may be tasks waiting on the wait_list.
 25 */
 26
 27#include <linux/compiler.h>
 28#include <linux/kernel.h>
 29#include <linux/export.h>
 30#include <linux/sched.h>
 31#include <linux/sched/debug.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 semaphore_waiter waiter;
208
209	list_add_tail(&waiter.list, &sem->wait_list);
210	waiter.task = current;
211	waiter.up = false;
212
213	for (;;) {
214		if (signal_pending_state(state, current))
215			goto interrupted;
216		if (unlikely(timeout <= 0))
217			goto timed_out;
218		__set_current_state(state);
219		raw_spin_unlock_irq(&sem->lock);
220		timeout = schedule_timeout(timeout);
221		raw_spin_lock_irq(&sem->lock);
222		if (waiter.up)
223			return 0;
224	}
225
226 timed_out:
227	list_del(&waiter.list);
228	return -ETIME;
229
230 interrupted:
231	list_del(&waiter.list);
232	return -EINTR;
233}
234
235static noinline void __sched __down(struct semaphore *sem)
236{
237	__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
238}
239
240static noinline int __sched __down_interruptible(struct semaphore *sem)
241{
242	return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
243}
244
245static noinline int __sched __down_killable(struct semaphore *sem)
246{
247	return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
248}
249
250static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
251{
252	return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
253}
254
255static noinline void __sched __up(struct semaphore *sem)
256{
257	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
258						struct semaphore_waiter, list);
259	list_del(&waiter->list);
260	waiter->up = true;
261	wake_up_process(waiter->task);
262}