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1 | /* |
2 | * 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/init.h> | |
18 | #include <asm/errno.h> | |
19 | ||
20 | #include <asm/semaphore.h> | |
21 | ||
22 | /* | |
23 | * Semaphores are implemented using a two-way counter: | |
24 | * The "count" variable is decremented for each process | |
25 | * that tries to acquire the semaphore, while the "sleeping" | |
26 | * variable is a count of such acquires. | |
27 | * | |
28 | * Notably, the inline "up()" and "down()" functions can | |
29 | * efficiently test if they need to do any extra work (up | |
30 | * needs to do something only if count was negative before | |
31 | * the increment operation. | |
32 | * | |
33 | * "sleeping" and the contention routine ordering is protected | |
34 | * by the spinlock in the semaphore's waitqueue head. | |
35 | * | |
36 | * Note that these functions are only called when there is | |
37 | * contention on the lock, and as such all this is the | |
38 | * "non-critical" part of the whole semaphore business. The | |
39 | * critical part is the inline stuff in <asm/semaphore.h> | |
40 | * where we want to avoid any extra jumps and calls. | |
41 | */ | |
42 | ||
43 | /* | |
44 | * Logic: | |
45 | * - only on a boundary condition do we need to care. When we go | |
46 | * from a negative count to a non-negative, we wake people up. | |
47 | * - when we go from a non-negative count to a negative do we | |
48 | * (a) synchronize with the "sleeper" count and (b) make sure | |
49 | * that we're on the wakeup list before we synchronize so that | |
50 | * we cannot lose wakeup events. | |
51 | */ | |
52 | ||
53 | void __up(struct semaphore *sem) | |
54 | { | |
55 | wake_up(&sem->wait); | |
56 | } | |
57 | ||
58 | void __sched __down(struct semaphore * sem) | |
59 | { | |
60 | struct task_struct *tsk = current; | |
61 | DECLARE_WAITQUEUE(wait, tsk); | |
62 | unsigned long flags; | |
63 | ||
64 | tsk->state = TASK_UNINTERRUPTIBLE; | |
65 | spin_lock_irqsave(&sem->wait.lock, flags); | |
66 | add_wait_queue_exclusive_locked(&sem->wait, &wait); | |
67 | ||
68 | sem->sleepers++; | |
69 | for (;;) { | |
70 | int sleepers = sem->sleepers; | |
71 | ||
72 | /* | |
73 | * Add "everybody else" into it. They aren't | |
74 | * playing, because we own the spinlock in | |
75 | * the wait_queue_head. | |
76 | */ | |
77 | if (!atomic_add_negative(sleepers - 1, &sem->count)) { | |
78 | sem->sleepers = 0; | |
79 | break; | |
80 | } | |
81 | sem->sleepers = 1; /* us - see -1 above */ | |
82 | spin_unlock_irqrestore(&sem->wait.lock, flags); | |
83 | ||
84 | schedule(); | |
85 | ||
86 | spin_lock_irqsave(&sem->wait.lock, flags); | |
87 | tsk->state = TASK_UNINTERRUPTIBLE; | |
88 | } | |
89 | remove_wait_queue_locked(&sem->wait, &wait); | |
90 | wake_up_locked(&sem->wait); | |
91 | spin_unlock_irqrestore(&sem->wait.lock, flags); | |
92 | tsk->state = TASK_RUNNING; | |
93 | } | |
94 | ||
95 | int __sched __down_interruptible(struct semaphore * sem) | |
96 | { | |
97 | int retval = 0; | |
98 | struct task_struct *tsk = current; | |
99 | DECLARE_WAITQUEUE(wait, tsk); | |
100 | unsigned long flags; | |
101 | ||
102 | tsk->state = TASK_INTERRUPTIBLE; | |
103 | spin_lock_irqsave(&sem->wait.lock, flags); | |
104 | add_wait_queue_exclusive_locked(&sem->wait, &wait); | |
105 | ||
106 | sem->sleepers++; | |
107 | for (;;) { | |
108 | int sleepers = sem->sleepers; | |
109 | ||
110 | /* | |
111 | * With signals pending, this turns into | |
112 | * the trylock failure case - we won't be | |
113 | * sleeping, and we* can't get the lock as | |
114 | * it has contention. Just correct the count | |
115 | * and exit. | |
116 | */ | |
117 | if (signal_pending(current)) { | |
118 | retval = -EINTR; | |
119 | sem->sleepers = 0; | |
120 | atomic_add(sleepers, &sem->count); | |
121 | break; | |
122 | } | |
123 | ||
124 | /* | |
125 | * Add "everybody else" into it. They aren't | |
126 | * playing, because we own the spinlock in | |
127 | * wait_queue_head. The "-1" is because we're | |
128 | * still hoping to get the semaphore. | |
129 | */ | |
130 | if (!atomic_add_negative(sleepers - 1, &sem->count)) { | |
131 | sem->sleepers = 0; | |
132 | break; | |
133 | } | |
134 | sem->sleepers = 1; /* us - see -1 above */ | |
135 | spin_unlock_irqrestore(&sem->wait.lock, flags); | |
136 | ||
137 | schedule(); | |
138 | ||
139 | spin_lock_irqsave(&sem->wait.lock, flags); | |
140 | tsk->state = TASK_INTERRUPTIBLE; | |
141 | } | |
142 | remove_wait_queue_locked(&sem->wait, &wait); | |
143 | wake_up_locked(&sem->wait); | |
144 | spin_unlock_irqrestore(&sem->wait.lock, flags); | |
145 | ||
146 | tsk->state = TASK_RUNNING; | |
147 | return retval; | |
148 | } | |
149 | ||
150 | /* | |
151 | * Trylock failed - make sure we correct for | |
152 | * having decremented the count. | |
153 | * | |
154 | * We could have done the trylock with a | |
155 | * single "cmpxchg" without failure cases, | |
156 | * but then it wouldn't work on a 386. | |
157 | */ | |
158 | int __down_trylock(struct semaphore * sem) | |
159 | { | |
160 | int sleepers; | |
161 | unsigned long flags; | |
162 | ||
163 | spin_lock_irqsave(&sem->wait.lock, flags); | |
164 | sleepers = sem->sleepers + 1; | |
165 | sem->sleepers = 0; | |
166 | ||
167 | /* | |
168 | * Add "everybody else" and us into it. They aren't | |
169 | * playing, because we own the spinlock in the | |
170 | * wait_queue_head. | |
171 | */ | |
172 | if (!atomic_add_negative(sleepers, &sem->count)) { | |
173 | wake_up_locked(&sem->wait); | |
174 | } | |
175 | ||
176 | spin_unlock_irqrestore(&sem->wait.lock, flags); | |
177 | return 1; | |
178 | } | |
179 | ||
180 |