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1da177e4 LT |
1 | /* |
2 | * ARM semaphore implementation, taken from | |
3 | * | |
4 | * i386 semaphore implementation. | |
5 | * | |
6 | * (C) Copyright 1999 Linus Torvalds | |
7 | * (C) Copyright 2003 Ian Molton (ARM26 mods) | |
8 | * | |
9 | * Modified for ARM by Russell King | |
10 | * | |
11 | * This program is free software; you can redistribute it and/or modify | |
12 | * it under the terms of the GNU General Public License version 2 as | |
13 | * published by the Free Software Foundation. | |
14 | */ | |
15 | #include <linux/module.h> | |
16 | #include <linux/config.h> | |
17 | #include <linux/sched.h> | |
18 | #include <linux/errno.h> | |
19 | #include <linux/init.h> | |
20 | ||
21 | #include <asm/semaphore.h> | |
22 | ||
23 | /* | |
24 | * Semaphores are implemented using a two-way counter: | |
25 | * The "count" variable is decremented for each process | |
26 | * that tries to acquire the semaphore, while the "sleeping" | |
27 | * variable is a count of such acquires. | |
28 | * | |
29 | * Notably, the inline "up()" and "down()" functions can | |
30 | * efficiently test if they need to do any extra work (up | |
31 | * needs to do something only if count was negative before | |
32 | * the increment operation. | |
33 | * | |
34 | * "sleeping" and the contention routine ordering is | |
35 | * protected by the semaphore spinlock. | |
36 | * | |
37 | * Note that these functions are only called when there is | |
38 | * contention on the lock, and as such all this is the | |
39 | * "non-critical" part of the whole semaphore business. The | |
40 | * critical part is the inline stuff in <asm/semaphore.h> | |
41 | * where we want to avoid any extra jumps and calls. | |
42 | */ | |
43 | ||
44 | /* | |
45 | * Logic: | |
46 | * - only on a boundary condition do we need to care. When we go | |
47 | * from a negative count to a non-negative, we wake people up. | |
48 | * - when we go from a non-negative count to a negative do we | |
49 | * (a) synchronize with the "sleeper" count and (b) make sure | |
50 | * that we're on the wakeup list before we synchronize so that | |
51 | * we cannot lose wakeup events. | |
52 | */ | |
53 | ||
54 | void __up(struct semaphore *sem) | |
55 | { | |
56 | wake_up(&sem->wait); | |
57 | } | |
58 | ||
59 | static DEFINE_SPINLOCK(semaphore_lock); | |
60 | ||
61 | void __sched __down(struct semaphore * sem) | |
62 | { | |
63 | struct task_struct *tsk = current; | |
64 | DECLARE_WAITQUEUE(wait, tsk); | |
65 | tsk->state = TASK_UNINTERRUPTIBLE; | |
66 | add_wait_queue_exclusive(&sem->wait, &wait); | |
67 | ||
68 | spin_lock_irq(&semaphore_lock); | |
69 | sem->sleepers++; | |
70 | for (;;) { | |
71 | int sleepers = sem->sleepers; | |
72 | ||
73 | /* | |
74 | * Add "everybody else" into it. They aren't | |
75 | * playing, because we own the spinlock. | |
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_irq(&semaphore_lock); | |
83 | ||
84 | schedule(); | |
85 | tsk->state = TASK_UNINTERRUPTIBLE; | |
86 | spin_lock_irq(&semaphore_lock); | |
87 | } | |
88 | spin_unlock_irq(&semaphore_lock); | |
89 | remove_wait_queue(&sem->wait, &wait); | |
90 | tsk->state = TASK_RUNNING; | |
91 | wake_up(&sem->wait); | |
92 | } | |
93 | ||
94 | int __sched __down_interruptible(struct semaphore * sem) | |
95 | { | |
96 | int retval = 0; | |
97 | struct task_struct *tsk = current; | |
98 | DECLARE_WAITQUEUE(wait, tsk); | |
99 | tsk->state = TASK_INTERRUPTIBLE; | |
100 | add_wait_queue_exclusive(&sem->wait, &wait); | |
101 | ||
102 | spin_lock_irq(&semaphore_lock); | |
103 | sem->sleepers ++; | |
104 | for (;;) { | |
105 | int sleepers = sem->sleepers; | |
106 | ||
107 | /* | |
108 | * With signals pending, this turns into | |
109 | * the trylock failure case - we won't be | |
110 | * sleeping, and we* can't get the lock as | |
111 | * it has contention. Just correct the count | |
112 | * and exit. | |
113 | */ | |
114 | if (signal_pending(current)) { | |
115 | retval = -EINTR; | |
116 | sem->sleepers = 0; | |
117 | atomic_add(sleepers, &sem->count); | |
118 | break; | |
119 | } | |
120 | ||
121 | /* | |
122 | * Add "everybody else" into it. They aren't | |
123 | * playing, because we own the spinlock. The | |
124 | * "-1" is because we're still hoping to get | |
125 | * the lock. | |
126 | */ | |
127 | if (!atomic_add_negative(sleepers - 1, &sem->count)) { | |
128 | sem->sleepers = 0; | |
129 | break; | |
130 | } | |
131 | sem->sleepers = 1; /* us - see -1 above */ | |
132 | spin_unlock_irq(&semaphore_lock); | |
133 | ||
134 | schedule(); | |
135 | tsk->state = TASK_INTERRUPTIBLE; | |
136 | spin_lock_irq(&semaphore_lock); | |
137 | } | |
138 | spin_unlock_irq(&semaphore_lock); | |
139 | tsk->state = TASK_RUNNING; | |
140 | remove_wait_queue(&sem->wait, &wait); | |
141 | wake_up(&sem->wait); | |
142 | return retval; | |
143 | } | |
144 | ||
145 | /* | |
146 | * Trylock failed - make sure we correct for | |
147 | * having decremented the count. | |
148 | * | |
149 | * We could have done the trylock with a | |
150 | * single "cmpxchg" without failure cases, | |
151 | * but then it wouldn't work on a 386. | |
152 | */ | |
153 | int __down_trylock(struct semaphore * sem) | |
154 | { | |
155 | int sleepers; | |
156 | unsigned long flags; | |
157 | ||
158 | spin_lock_irqsave(&semaphore_lock, flags); | |
159 | sleepers = sem->sleepers + 1; | |
160 | sem->sleepers = 0; | |
161 | ||
162 | /* | |
163 | * Add "everybody else" and us into it. They aren't | |
164 | * playing, because we own the spinlock. | |
165 | */ | |
166 | if (!atomic_add_negative(sleepers, &sem->count)) | |
167 | wake_up(&sem->wait); | |
168 | ||
169 | spin_unlock_irqrestore(&semaphore_lock, flags); | |
170 | return 1; | |
171 | } | |
172 | ||
173 | /* | |
174 | * The semaphore operations have a special calling sequence that | |
175 | * allow us to do a simpler in-line version of them. These routines | |
176 | * need to convert that sequence back into the C sequence when | |
177 | * there is contention on the semaphore. | |
178 | * | |
179 | * ip contains the semaphore pointer on entry. Save the C-clobbered | |
180 | * registers (r0 to r3 and lr), but not ip, as we use it as a return | |
181 | * value in some cases.. | |
182 | */ | |
183 | asm(" .section .sched.text , #alloc, #execinstr \n\ | |
184 | .align 5 \n\ | |
185 | .globl __down_failed \n\ | |
186 | __down_failed: \n\ | |
187 | stmfd sp!, {r0 - r3, lr} \n\ | |
188 | mov r0, ip \n\ | |
189 | bl __down \n\ | |
190 | ldmfd sp!, {r0 - r3, pc}^ \n\ | |
191 | \n\ | |
192 | .align 5 \n\ | |
193 | .globl __down_interruptible_failed \n\ | |
194 | __down_interruptible_failed: \n\ | |
195 | stmfd sp!, {r0 - r3, lr} \n\ | |
196 | mov r0, ip \n\ | |
197 | bl __down_interruptible \n\ | |
198 | mov ip, r0 \n\ | |
199 | ldmfd sp!, {r0 - r3, pc}^ \n\ | |
200 | \n\ | |
201 | .align 5 \n\ | |
202 | .globl __down_trylock_failed \n\ | |
203 | __down_trylock_failed: \n\ | |
204 | stmfd sp!, {r0 - r3, lr} \n\ | |
205 | mov r0, ip \n\ | |
206 | bl __down_trylock \n\ | |
207 | mov ip, r0 \n\ | |
208 | ldmfd sp!, {r0 - r3, pc}^ \n\ | |
209 | \n\ | |
210 | .align 5 \n\ | |
211 | .globl __up_wakeup \n\ | |
212 | __up_wakeup: \n\ | |
213 | stmfd sp!, {r0 - r3, lr} \n\ | |
214 | mov r0, ip \n\ | |
215 | bl __up \n\ | |
216 | ldmfd sp!, {r0 - r3, pc}^ \n\ | |
217 | "); | |
218 | ||
219 | EXPORT_SYMBOL(__down_failed); | |
220 | EXPORT_SYMBOL(__down_interruptible_failed); | |
221 | EXPORT_SYMBOL(__down_trylock_failed); | |
222 | EXPORT_SYMBOL(__up_wakeup); | |
223 |