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1da177e4 LT |
1 | /* rwsem-spinlock.c: R/W semaphores: contention handling functions for |
2 | * generic spinlock implementation | |
3 | * | |
4 | * Copyright (c) 2001 David Howells (dhowells@redhat.com). | |
5 | * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de> | |
6 | * - Derived also from comments by Linus | |
7 | */ | |
8 | #include <linux/rwsem.h> | |
9 | #include <linux/sched.h> | |
10 | #include <linux/module.h> | |
11 | ||
12 | struct rwsem_waiter { | |
13 | struct list_head list; | |
14 | struct task_struct *task; | |
15 | unsigned int flags; | |
16 | #define RWSEM_WAITING_FOR_READ 0x00000001 | |
17 | #define RWSEM_WAITING_FOR_WRITE 0x00000002 | |
18 | }; | |
19 | ||
1da177e4 LT |
20 | /* |
21 | * initialise the semaphore | |
22 | */ | |
23 | void fastcall init_rwsem(struct rw_semaphore *sem) | |
24 | { | |
25 | sem->activity = 0; | |
26 | spin_lock_init(&sem->wait_lock); | |
27 | INIT_LIST_HEAD(&sem->wait_list); | |
1da177e4 LT |
28 | } |
29 | ||
30 | /* | |
31 | * handle the lock release when processes blocked on it that can now run | |
32 | * - if we come here, then: | |
33 | * - the 'active count' _reached_ zero | |
34 | * - the 'waiting count' is non-zero | |
35 | * - the spinlock must be held by the caller | |
36 | * - woken process blocks are discarded from the list after having task zeroed | |
37 | * - writers are only woken if wakewrite is non-zero | |
38 | */ | |
39 | static inline struct rw_semaphore * | |
40 | __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite) | |
41 | { | |
42 | struct rwsem_waiter *waiter; | |
43 | struct task_struct *tsk; | |
44 | int woken; | |
45 | ||
1da177e4 LT |
46 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
47 | ||
48 | if (!wakewrite) { | |
49 | if (waiter->flags & RWSEM_WAITING_FOR_WRITE) | |
50 | goto out; | |
51 | goto dont_wake_writers; | |
52 | } | |
53 | ||
54 | /* if we are allowed to wake writers try to grant a single write lock | |
55 | * if there's a writer at the front of the queue | |
56 | * - we leave the 'waiting count' incremented to signify potential | |
57 | * contention | |
58 | */ | |
59 | if (waiter->flags & RWSEM_WAITING_FOR_WRITE) { | |
60 | sem->activity = -1; | |
61 | list_del(&waiter->list); | |
62 | tsk = waiter->task; | |
63 | /* Don't touch waiter after ->task has been NULLed */ | |
d59dd462 | 64 | smp_mb(); |
1da177e4 LT |
65 | waiter->task = NULL; |
66 | wake_up_process(tsk); | |
67 | put_task_struct(tsk); | |
68 | goto out; | |
69 | } | |
70 | ||
71 | /* grant an infinite number of read locks to the front of the queue */ | |
72 | dont_wake_writers: | |
73 | woken = 0; | |
74 | while (waiter->flags & RWSEM_WAITING_FOR_READ) { | |
75 | struct list_head *next = waiter->list.next; | |
76 | ||
77 | list_del(&waiter->list); | |
78 | tsk = waiter->task; | |
d59dd462 | 79 | smp_mb(); |
1da177e4 LT |
80 | waiter->task = NULL; |
81 | wake_up_process(tsk); | |
82 | put_task_struct(tsk); | |
83 | woken++; | |
84 | if (list_empty(&sem->wait_list)) | |
85 | break; | |
86 | waiter = list_entry(next, struct rwsem_waiter, list); | |
87 | } | |
88 | ||
89 | sem->activity += woken; | |
90 | ||
91 | out: | |
1da177e4 LT |
92 | return sem; |
93 | } | |
94 | ||
95 | /* | |
96 | * wake a single writer | |
97 | */ | |
98 | static inline struct rw_semaphore * | |
99 | __rwsem_wake_one_writer(struct rw_semaphore *sem) | |
100 | { | |
101 | struct rwsem_waiter *waiter; | |
102 | struct task_struct *tsk; | |
103 | ||
104 | sem->activity = -1; | |
105 | ||
106 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); | |
107 | list_del(&waiter->list); | |
108 | ||
109 | tsk = waiter->task; | |
d59dd462 | 110 | smp_mb(); |
1da177e4 LT |
111 | waiter->task = NULL; |
112 | wake_up_process(tsk); | |
113 | put_task_struct(tsk); | |
114 | return sem; | |
115 | } | |
116 | ||
117 | /* | |
118 | * get a read lock on the semaphore | |
119 | */ | |
120 | void fastcall __sched __down_read(struct rw_semaphore *sem) | |
121 | { | |
122 | struct rwsem_waiter waiter; | |
123 | struct task_struct *tsk; | |
124 | ||
1da177e4 LT |
125 | spin_lock_irq(&sem->wait_lock); |
126 | ||
127 | if (sem->activity >= 0 && list_empty(&sem->wait_list)) { | |
128 | /* granted */ | |
129 | sem->activity++; | |
130 | spin_unlock_irq(&sem->wait_lock); | |
131 | goto out; | |
132 | } | |
133 | ||
134 | tsk = current; | |
135 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
136 | ||
137 | /* set up my own style of waitqueue */ | |
138 | waiter.task = tsk; | |
139 | waiter.flags = RWSEM_WAITING_FOR_READ; | |
140 | get_task_struct(tsk); | |
141 | ||
142 | list_add_tail(&waiter.list, &sem->wait_list); | |
143 | ||
144 | /* we don't need to touch the semaphore struct anymore */ | |
145 | spin_unlock_irq(&sem->wait_lock); | |
146 | ||
147 | /* wait to be given the lock */ | |
148 | for (;;) { | |
149 | if (!waiter.task) | |
150 | break; | |
151 | schedule(); | |
152 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
153 | } | |
154 | ||
155 | tsk->state = TASK_RUNNING; | |
1da177e4 | 156 | out: |
c4e05116 | 157 | ; |
1da177e4 LT |
158 | } |
159 | ||
160 | /* | |
161 | * trylock for reading -- returns 1 if successful, 0 if contention | |
162 | */ | |
163 | int fastcall __down_read_trylock(struct rw_semaphore *sem) | |
164 | { | |
165 | unsigned long flags; | |
166 | int ret = 0; | |
167 | ||
1da177e4 LT |
168 | |
169 | spin_lock_irqsave(&sem->wait_lock, flags); | |
170 | ||
171 | if (sem->activity >= 0 && list_empty(&sem->wait_list)) { | |
172 | /* granted */ | |
173 | sem->activity++; | |
174 | ret = 1; | |
175 | } | |
176 | ||
177 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
178 | ||
1da177e4 LT |
179 | return ret; |
180 | } | |
181 | ||
182 | /* | |
183 | * get a write lock on the semaphore | |
184 | * - we increment the waiting count anyway to indicate an exclusive lock | |
185 | */ | |
186 | void fastcall __sched __down_write(struct rw_semaphore *sem) | |
187 | { | |
188 | struct rwsem_waiter waiter; | |
189 | struct task_struct *tsk; | |
190 | ||
1da177e4 LT |
191 | spin_lock_irq(&sem->wait_lock); |
192 | ||
193 | if (sem->activity == 0 && list_empty(&sem->wait_list)) { | |
194 | /* granted */ | |
195 | sem->activity = -1; | |
196 | spin_unlock_irq(&sem->wait_lock); | |
197 | goto out; | |
198 | } | |
199 | ||
200 | tsk = current; | |
201 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
202 | ||
203 | /* set up my own style of waitqueue */ | |
204 | waiter.task = tsk; | |
205 | waiter.flags = RWSEM_WAITING_FOR_WRITE; | |
206 | get_task_struct(tsk); | |
207 | ||
208 | list_add_tail(&waiter.list, &sem->wait_list); | |
209 | ||
210 | /* we don't need to touch the semaphore struct anymore */ | |
211 | spin_unlock_irq(&sem->wait_lock); | |
212 | ||
213 | /* wait to be given the lock */ | |
214 | for (;;) { | |
215 | if (!waiter.task) | |
216 | break; | |
217 | schedule(); | |
218 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
219 | } | |
220 | ||
221 | tsk->state = TASK_RUNNING; | |
1da177e4 | 222 | out: |
c4e05116 | 223 | ; |
1da177e4 LT |
224 | } |
225 | ||
226 | /* | |
227 | * trylock for writing -- returns 1 if successful, 0 if contention | |
228 | */ | |
229 | int fastcall __down_write_trylock(struct rw_semaphore *sem) | |
230 | { | |
231 | unsigned long flags; | |
232 | int ret = 0; | |
233 | ||
1da177e4 LT |
234 | spin_lock_irqsave(&sem->wait_lock, flags); |
235 | ||
236 | if (sem->activity == 0 && list_empty(&sem->wait_list)) { | |
237 | /* granted */ | |
238 | sem->activity = -1; | |
239 | ret = 1; | |
240 | } | |
241 | ||
242 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
243 | ||
1da177e4 LT |
244 | return ret; |
245 | } | |
246 | ||
247 | /* | |
248 | * release a read lock on the semaphore | |
249 | */ | |
250 | void fastcall __up_read(struct rw_semaphore *sem) | |
251 | { | |
252 | unsigned long flags; | |
253 | ||
1da177e4 LT |
254 | spin_lock_irqsave(&sem->wait_lock, flags); |
255 | ||
256 | if (--sem->activity == 0 && !list_empty(&sem->wait_list)) | |
257 | sem = __rwsem_wake_one_writer(sem); | |
258 | ||
259 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
1da177e4 LT |
260 | } |
261 | ||
262 | /* | |
263 | * release a write lock on the semaphore | |
264 | */ | |
265 | void fastcall __up_write(struct rw_semaphore *sem) | |
266 | { | |
267 | unsigned long flags; | |
268 | ||
1da177e4 LT |
269 | spin_lock_irqsave(&sem->wait_lock, flags); |
270 | ||
271 | sem->activity = 0; | |
272 | if (!list_empty(&sem->wait_list)) | |
273 | sem = __rwsem_do_wake(sem, 1); | |
274 | ||
275 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
1da177e4 LT |
276 | } |
277 | ||
278 | /* | |
279 | * downgrade a write lock into a read lock | |
280 | * - just wake up any readers at the front of the queue | |
281 | */ | |
282 | void fastcall __downgrade_write(struct rw_semaphore *sem) | |
283 | { | |
284 | unsigned long flags; | |
285 | ||
1da177e4 LT |
286 | spin_lock_irqsave(&sem->wait_lock, flags); |
287 | ||
288 | sem->activity = 1; | |
289 | if (!list_empty(&sem->wait_list)) | |
290 | sem = __rwsem_do_wake(sem, 0); | |
291 | ||
292 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
1da177e4 LT |
293 | } |
294 | ||
295 | EXPORT_SYMBOL(init_rwsem); | |
296 | EXPORT_SYMBOL(__down_read); | |
297 | EXPORT_SYMBOL(__down_read_trylock); | |
298 | EXPORT_SYMBOL(__down_write); | |
299 | EXPORT_SYMBOL(__down_write_trylock); | |
300 | EXPORT_SYMBOL(__up_read); | |
301 | EXPORT_SYMBOL(__up_write); | |
302 | EXPORT_SYMBOL(__downgrade_write); |