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Linux kernel mirror (for testing)
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linux
1/*
2 * i386 and x86-64 semaphore implementation.
3 *
4 * (C) Copyright 1999 Linus Torvalds
5 *
6 * Portions Copyright 1999 Red Hat, Inc.
7 *
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License
10 * as published by the Free Software Foundation; either version
11 * 2 of the License, or (at your option) any later version.
12 *
13 * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
14 */
15#include <linux/sched.h>
16#include <linux/err.h>
17#include <linux/init.h>
18#include <asm/semaphore.h>
19
20/*
21 * Semaphores are implemented using a two-way counter:
22 * The "count" variable is decremented for each process
23 * that tries to acquire the semaphore, while the "sleeping"
24 * variable is a count of such acquires.
25 *
26 * Notably, the inline "up()" and "down()" functions can
27 * efficiently test if they need to do any extra work (up
28 * needs to do something only if count was negative before
29 * the increment operation.
30 *
31 * "sleeping" and the contention routine ordering is protected
32 * by the spinlock in the semaphore's waitqueue head.
33 *
34 * Note that these functions are only called when there is
35 * contention on the lock, and as such all this is the
36 * "non-critical" part of the whole semaphore business. The
37 * critical part is the inline stuff in <asm/semaphore.h>
38 * where we want to avoid any extra jumps and calls.
39 */
40
41/*
42 * Logic:
43 * - only on a boundary condition do we need to care. When we go
44 * from a negative count to a non-negative, we wake people up.
45 * - when we go from a non-negative count to a negative do we
46 * (a) synchronize with the "sleeper" count and (b) make sure
47 * that we're on the wakeup list before we synchronize so that
48 * we cannot lose wakeup events.
49 */
50
51void __up(struct semaphore *sem)
52{
53 wake_up(&sem->wait);
54}
55
56void __sched __down(struct semaphore *sem)
57{
58 struct task_struct *tsk = current;
59 DECLARE_WAITQUEUE(wait, tsk);
60 unsigned long flags;
61
62 tsk->state = TASK_UNINTERRUPTIBLE;
63 spin_lock_irqsave(&sem->wait.lock, flags);
64 add_wait_queue_exclusive_locked(&sem->wait, &wait);
65
66 sem->sleepers++;
67 for (;;) {
68 int sleepers = sem->sleepers;
69
70 /*
71 * Add "everybody else" into it. They aren't
72 * playing, because we own the spinlock in
73 * the wait_queue_head.
74 */
75 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
76 sem->sleepers = 0;
77 break;
78 }
79 sem->sleepers = 1; /* us - see -1 above */
80 spin_unlock_irqrestore(&sem->wait.lock, flags);
81
82 schedule();
83
84 spin_lock_irqsave(&sem->wait.lock, flags);
85 tsk->state = TASK_UNINTERRUPTIBLE;
86 }
87 remove_wait_queue_locked(&sem->wait, &wait);
88 wake_up_locked(&sem->wait);
89 spin_unlock_irqrestore(&sem->wait.lock, flags);
90 tsk->state = TASK_RUNNING;
91}
92
93int __sched __down_interruptible(struct semaphore *sem)
94{
95 int retval = 0;
96 struct task_struct *tsk = current;
97 DECLARE_WAITQUEUE(wait, tsk);
98 unsigned long flags;
99
100 tsk->state = TASK_INTERRUPTIBLE;
101 spin_lock_irqsave(&sem->wait.lock, flags);
102 add_wait_queue_exclusive_locked(&sem->wait, &wait);
103
104 sem->sleepers++;
105 for (;;) {
106 int sleepers = sem->sleepers;
107
108 /*
109 * With signals pending, this turns into
110 * the trylock failure case - we won't be
111 * sleeping, and we* can't get the lock as
112 * it has contention. Just correct the count
113 * and exit.
114 */
115 if (signal_pending(current)) {
116 retval = -EINTR;
117 sem->sleepers = 0;
118 atomic_add(sleepers, &sem->count);
119 break;
120 }
121
122 /*
123 * Add "everybody else" into it. They aren't
124 * playing, because we own the spinlock in
125 * wait_queue_head. The "-1" is because we're
126 * still hoping to get the semaphore.
127 */
128 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
129 sem->sleepers = 0;
130 break;
131 }
132 sem->sleepers = 1; /* us - see -1 above */
133 spin_unlock_irqrestore(&sem->wait.lock, flags);
134
135 schedule();
136
137 spin_lock_irqsave(&sem->wait.lock, flags);
138 tsk->state = TASK_INTERRUPTIBLE;
139 }
140 remove_wait_queue_locked(&sem->wait, &wait);
141 wake_up_locked(&sem->wait);
142 spin_unlock_irqrestore(&sem->wait.lock, flags);
143
144 tsk->state = TASK_RUNNING;
145 return retval;
146}
147
148/*
149 * Trylock failed - make sure we correct for
150 * having decremented the count.
151 *
152 * We could have done the trylock with a
153 * single "cmpxchg" without failure cases,
154 * but then it wouldn't work on a 386.
155 */
156int __down_trylock(struct semaphore *sem)
157{
158 int sleepers;
159 unsigned long flags;
160
161 spin_lock_irqsave(&sem->wait.lock, flags);
162 sleepers = sem->sleepers + 1;
163 sem->sleepers = 0;
164
165 /*
166 * Add "everybody else" and us into it. They aren't
167 * playing, because we own the spinlock in the
168 * wait_queue_head.
169 */
170 if (!atomic_add_negative(sleepers, &sem->count)) {
171 wake_up_locked(&sem->wait);
172 }
173
174 spin_unlock_irqrestore(&sem->wait.lock, flags);
175 return 1;
176}