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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/config.h>
16#include <linux/sched.h>
17#include <linux/err.h>
18#include <linux/init.h>
19#include <asm/semaphore.h>
20
21/*
22 * Semaphores are implemented using a two-way counter:
23 * The "count" variable is decremented for each process
24 * that tries to acquire the semaphore, while the "sleeping"
25 * variable is a count of such acquires.
26 *
27 * Notably, the inline "up()" and "down()" functions can
28 * efficiently test if they need to do any extra work (up
29 * needs to do something only if count was negative before
30 * the increment operation.
31 *
32 * "sleeping" and the contention routine ordering is protected
33 * by the spinlock in the semaphore's waitqueue head.
34 *
35 * Note that these functions are only called when there is
36 * contention on the lock, and as such all this is the
37 * "non-critical" part of the whole semaphore business. The
38 * critical part is the inline stuff in <asm/semaphore.h>
39 * where we want to avoid any extra jumps and calls.
40 */
41
42/*
43 * Logic:
44 * - only on a boundary condition do we need to care. When we go
45 * from a negative count to a non-negative, we wake people up.
46 * - when we go from a non-negative count to a negative do we
47 * (a) synchronize with the "sleeper" count and (b) make sure
48 * that we're on the wakeup list before we synchronize so that
49 * we cannot lose wakeup events.
50 */
51
52fastcall void __up(struct semaphore *sem)
53{
54 wake_up(&sem->wait);
55}
56
57fastcall void __sched __down(struct semaphore * sem)
58{
59 struct task_struct *tsk = current;
60 DECLARE_WAITQUEUE(wait, tsk);
61 unsigned long flags;
62
63 tsk->state = TASK_UNINTERRUPTIBLE;
64 spin_lock_irqsave(&sem->wait.lock, flags);
65 add_wait_queue_exclusive_locked(&sem->wait, &wait);
66
67 sem->sleepers++;
68 for (;;) {
69 int sleepers = sem->sleepers;
70
71 /*
72 * Add "everybody else" into it. They aren't
73 * playing, because we own the spinlock in
74 * the wait_queue_head.
75 */
76 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
77 sem->sleepers = 0;
78 break;
79 }
80 sem->sleepers = 1; /* us - see -1 above */
81 spin_unlock_irqrestore(&sem->wait.lock, flags);
82
83 schedule();
84
85 spin_lock_irqsave(&sem->wait.lock, flags);
86 tsk->state = TASK_UNINTERRUPTIBLE;
87 }
88 remove_wait_queue_locked(&sem->wait, &wait);
89 wake_up_locked(&sem->wait);
90 spin_unlock_irqrestore(&sem->wait.lock, flags);
91 tsk->state = TASK_RUNNING;
92}
93
94fastcall int __sched __down_interruptible(struct semaphore * sem)
95{
96 int retval = 0;
97 struct task_struct *tsk = current;
98 DECLARE_WAITQUEUE(wait, tsk);
99 unsigned long flags;
100
101 tsk->state = TASK_INTERRUPTIBLE;
102 spin_lock_irqsave(&sem->wait.lock, flags);
103 add_wait_queue_exclusive_locked(&sem->wait, &wait);
104
105 sem->sleepers++;
106 for (;;) {
107 int sleepers = sem->sleepers;
108
109 /*
110 * With signals pending, this turns into
111 * the trylock failure case - we won't be
112 * sleeping, and we* can't get the lock as
113 * it has contention. Just correct the count
114 * and exit.
115 */
116 if (signal_pending(current)) {
117 retval = -EINTR;
118 sem->sleepers = 0;
119 atomic_add(sleepers, &sem->count);
120 break;
121 }
122
123 /*
124 * Add "everybody else" into it. They aren't
125 * playing, because we own the spinlock in
126 * wait_queue_head. The "-1" is because we're
127 * still hoping to get the semaphore.
128 */
129 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
130 sem->sleepers = 0;
131 break;
132 }
133 sem->sleepers = 1; /* us - see -1 above */
134 spin_unlock_irqrestore(&sem->wait.lock, flags);
135
136 schedule();
137
138 spin_lock_irqsave(&sem->wait.lock, flags);
139 tsk->state = TASK_INTERRUPTIBLE;
140 }
141 remove_wait_queue_locked(&sem->wait, &wait);
142 wake_up_locked(&sem->wait);
143 spin_unlock_irqrestore(&sem->wait.lock, flags);
144
145 tsk->state = TASK_RUNNING;
146 return retval;
147}
148
149/*
150 * Trylock failed - make sure we correct for
151 * having decremented the count.
152 *
153 * We could have done the trylock with a
154 * single "cmpxchg" without failure cases,
155 * but then it wouldn't work on a 386.
156 */
157fastcall int __down_trylock(struct semaphore * sem)
158{
159 int sleepers;
160 unsigned long flags;
161
162 spin_lock_irqsave(&sem->wait.lock, flags);
163 sleepers = sem->sleepers + 1;
164 sem->sleepers = 0;
165
166 /*
167 * Add "everybody else" and us into it. They aren't
168 * playing, because we own the spinlock in the
169 * wait_queue_head.
170 */
171 if (!atomic_add_negative(sleepers, &sem->count)) {
172 wake_up_locked(&sem->wait);
173 }
174
175 spin_unlock_irqrestore(&sem->wait.lock, flags);
176 return 1;
177}