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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 | ||
20 | #if RWSEM_DEBUG | |
21 | void rwsemtrace(struct rw_semaphore *sem, const char *str) | |
22 | { | |
23 | if (sem->debug) | |
24 | printk("[%d] %s({%d,%d})\n", | |
25 | current->pid, str, sem->activity, | |
26 | list_empty(&sem->wait_list) ? 0 : 1); | |
27 | } | |
28 | #endif | |
29 | ||
30 | /* | |
31 | * initialise the semaphore | |
32 | */ | |
33 | void fastcall init_rwsem(struct rw_semaphore *sem) | |
34 | { | |
35 | sem->activity = 0; | |
36 | spin_lock_init(&sem->wait_lock); | |
37 | INIT_LIST_HEAD(&sem->wait_list); | |
38 | #if RWSEM_DEBUG | |
39 | sem->debug = 0; | |
40 | #endif | |
41 | } | |
42 | ||
43 | /* | |
44 | * handle the lock release when processes blocked on it that can now run | |
45 | * - if we come here, then: | |
46 | * - the 'active count' _reached_ zero | |
47 | * - the 'waiting count' is non-zero | |
48 | * - the spinlock must be held by the caller | |
49 | * - woken process blocks are discarded from the list after having task zeroed | |
50 | * - writers are only woken if wakewrite is non-zero | |
51 | */ | |
52 | static inline struct rw_semaphore * | |
53 | __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite) | |
54 | { | |
55 | struct rwsem_waiter *waiter; | |
56 | struct task_struct *tsk; | |
57 | int woken; | |
58 | ||
59 | rwsemtrace(sem, "Entering __rwsem_do_wake"); | |
60 | ||
61 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); | |
62 | ||
63 | if (!wakewrite) { | |
64 | if (waiter->flags & RWSEM_WAITING_FOR_WRITE) | |
65 | goto out; | |
66 | goto dont_wake_writers; | |
67 | } | |
68 | ||
69 | /* if we are allowed to wake writers try to grant a single write lock | |
70 | * if there's a writer at the front of the queue | |
71 | * - we leave the 'waiting count' incremented to signify potential | |
72 | * contention | |
73 | */ | |
74 | if (waiter->flags & RWSEM_WAITING_FOR_WRITE) { | |
75 | sem->activity = -1; | |
76 | list_del(&waiter->list); | |
77 | tsk = waiter->task; | |
78 | /* Don't touch waiter after ->task has been NULLed */ | |
79 | mb(); | |
80 | waiter->task = NULL; | |
81 | wake_up_process(tsk); | |
82 | put_task_struct(tsk); | |
83 | goto out; | |
84 | } | |
85 | ||
86 | /* grant an infinite number of read locks to the front of the queue */ | |
87 | dont_wake_writers: | |
88 | woken = 0; | |
89 | while (waiter->flags & RWSEM_WAITING_FOR_READ) { | |
90 | struct list_head *next = waiter->list.next; | |
91 | ||
92 | list_del(&waiter->list); | |
93 | tsk = waiter->task; | |
94 | mb(); | |
95 | waiter->task = NULL; | |
96 | wake_up_process(tsk); | |
97 | put_task_struct(tsk); | |
98 | woken++; | |
99 | if (list_empty(&sem->wait_list)) | |
100 | break; | |
101 | waiter = list_entry(next, struct rwsem_waiter, list); | |
102 | } | |
103 | ||
104 | sem->activity += woken; | |
105 | ||
106 | out: | |
107 | rwsemtrace(sem, "Leaving __rwsem_do_wake"); | |
108 | return sem; | |
109 | } | |
110 | ||
111 | /* | |
112 | * wake a single writer | |
113 | */ | |
114 | static inline struct rw_semaphore * | |
115 | __rwsem_wake_one_writer(struct rw_semaphore *sem) | |
116 | { | |
117 | struct rwsem_waiter *waiter; | |
118 | struct task_struct *tsk; | |
119 | ||
120 | sem->activity = -1; | |
121 | ||
122 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); | |
123 | list_del(&waiter->list); | |
124 | ||
125 | tsk = waiter->task; | |
126 | mb(); | |
127 | waiter->task = NULL; | |
128 | wake_up_process(tsk); | |
129 | put_task_struct(tsk); | |
130 | return sem; | |
131 | } | |
132 | ||
133 | /* | |
134 | * get a read lock on the semaphore | |
135 | */ | |
136 | void fastcall __sched __down_read(struct rw_semaphore *sem) | |
137 | { | |
138 | struct rwsem_waiter waiter; | |
139 | struct task_struct *tsk; | |
140 | ||
141 | rwsemtrace(sem, "Entering __down_read"); | |
142 | ||
143 | spin_lock_irq(&sem->wait_lock); | |
144 | ||
145 | if (sem->activity >= 0 && list_empty(&sem->wait_list)) { | |
146 | /* granted */ | |
147 | sem->activity++; | |
148 | spin_unlock_irq(&sem->wait_lock); | |
149 | goto out; | |
150 | } | |
151 | ||
152 | tsk = current; | |
153 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
154 | ||
155 | /* set up my own style of waitqueue */ | |
156 | waiter.task = tsk; | |
157 | waiter.flags = RWSEM_WAITING_FOR_READ; | |
158 | get_task_struct(tsk); | |
159 | ||
160 | list_add_tail(&waiter.list, &sem->wait_list); | |
161 | ||
162 | /* we don't need to touch the semaphore struct anymore */ | |
163 | spin_unlock_irq(&sem->wait_lock); | |
164 | ||
165 | /* wait to be given the lock */ | |
166 | for (;;) { | |
167 | if (!waiter.task) | |
168 | break; | |
169 | schedule(); | |
170 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
171 | } | |
172 | ||
173 | tsk->state = TASK_RUNNING; | |
174 | ||
175 | out: | |
176 | rwsemtrace(sem, "Leaving __down_read"); | |
177 | } | |
178 | ||
179 | /* | |
180 | * trylock for reading -- returns 1 if successful, 0 if contention | |
181 | */ | |
182 | int fastcall __down_read_trylock(struct rw_semaphore *sem) | |
183 | { | |
184 | unsigned long flags; | |
185 | int ret = 0; | |
186 | ||
187 | rwsemtrace(sem, "Entering __down_read_trylock"); | |
188 | ||
189 | spin_lock_irqsave(&sem->wait_lock, flags); | |
190 | ||
191 | if (sem->activity >= 0 && list_empty(&sem->wait_list)) { | |
192 | /* granted */ | |
193 | sem->activity++; | |
194 | ret = 1; | |
195 | } | |
196 | ||
197 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
198 | ||
199 | rwsemtrace(sem, "Leaving __down_read_trylock"); | |
200 | return ret; | |
201 | } | |
202 | ||
203 | /* | |
204 | * get a write lock on the semaphore | |
205 | * - we increment the waiting count anyway to indicate an exclusive lock | |
206 | */ | |
207 | void fastcall __sched __down_write(struct rw_semaphore *sem) | |
208 | { | |
209 | struct rwsem_waiter waiter; | |
210 | struct task_struct *tsk; | |
211 | ||
212 | rwsemtrace(sem, "Entering __down_write"); | |
213 | ||
214 | spin_lock_irq(&sem->wait_lock); | |
215 | ||
216 | if (sem->activity == 0 && list_empty(&sem->wait_list)) { | |
217 | /* granted */ | |
218 | sem->activity = -1; | |
219 | spin_unlock_irq(&sem->wait_lock); | |
220 | goto out; | |
221 | } | |
222 | ||
223 | tsk = current; | |
224 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
225 | ||
226 | /* set up my own style of waitqueue */ | |
227 | waiter.task = tsk; | |
228 | waiter.flags = RWSEM_WAITING_FOR_WRITE; | |
229 | get_task_struct(tsk); | |
230 | ||
231 | list_add_tail(&waiter.list, &sem->wait_list); | |
232 | ||
233 | /* we don't need to touch the semaphore struct anymore */ | |
234 | spin_unlock_irq(&sem->wait_lock); | |
235 | ||
236 | /* wait to be given the lock */ | |
237 | for (;;) { | |
238 | if (!waiter.task) | |
239 | break; | |
240 | schedule(); | |
241 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
242 | } | |
243 | ||
244 | tsk->state = TASK_RUNNING; | |
245 | ||
246 | out: | |
247 | rwsemtrace(sem, "Leaving __down_write"); | |
248 | } | |
249 | ||
250 | /* | |
251 | * trylock for writing -- returns 1 if successful, 0 if contention | |
252 | */ | |
253 | int fastcall __down_write_trylock(struct rw_semaphore *sem) | |
254 | { | |
255 | unsigned long flags; | |
256 | int ret = 0; | |
257 | ||
258 | rwsemtrace(sem, "Entering __down_write_trylock"); | |
259 | ||
260 | spin_lock_irqsave(&sem->wait_lock, flags); | |
261 | ||
262 | if (sem->activity == 0 && list_empty(&sem->wait_list)) { | |
263 | /* granted */ | |
264 | sem->activity = -1; | |
265 | ret = 1; | |
266 | } | |
267 | ||
268 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
269 | ||
270 | rwsemtrace(sem, "Leaving __down_write_trylock"); | |
271 | return ret; | |
272 | } | |
273 | ||
274 | /* | |
275 | * release a read lock on the semaphore | |
276 | */ | |
277 | void fastcall __up_read(struct rw_semaphore *sem) | |
278 | { | |
279 | unsigned long flags; | |
280 | ||
281 | rwsemtrace(sem, "Entering __up_read"); | |
282 | ||
283 | spin_lock_irqsave(&sem->wait_lock, flags); | |
284 | ||
285 | if (--sem->activity == 0 && !list_empty(&sem->wait_list)) | |
286 | sem = __rwsem_wake_one_writer(sem); | |
287 | ||
288 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
289 | ||
290 | rwsemtrace(sem, "Leaving __up_read"); | |
291 | } | |
292 | ||
293 | /* | |
294 | * release a write lock on the semaphore | |
295 | */ | |
296 | void fastcall __up_write(struct rw_semaphore *sem) | |
297 | { | |
298 | unsigned long flags; | |
299 | ||
300 | rwsemtrace(sem, "Entering __up_write"); | |
301 | ||
302 | spin_lock_irqsave(&sem->wait_lock, flags); | |
303 | ||
304 | sem->activity = 0; | |
305 | if (!list_empty(&sem->wait_list)) | |
306 | sem = __rwsem_do_wake(sem, 1); | |
307 | ||
308 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
309 | ||
310 | rwsemtrace(sem, "Leaving __up_write"); | |
311 | } | |
312 | ||
313 | /* | |
314 | * downgrade a write lock into a read lock | |
315 | * - just wake up any readers at the front of the queue | |
316 | */ | |
317 | void fastcall __downgrade_write(struct rw_semaphore *sem) | |
318 | { | |
319 | unsigned long flags; | |
320 | ||
321 | rwsemtrace(sem, "Entering __downgrade_write"); | |
322 | ||
323 | spin_lock_irqsave(&sem->wait_lock, flags); | |
324 | ||
325 | sem->activity = 1; | |
326 | if (!list_empty(&sem->wait_list)) | |
327 | sem = __rwsem_do_wake(sem, 0); | |
328 | ||
329 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
330 | ||
331 | rwsemtrace(sem, "Leaving __downgrade_write"); | |
332 | } | |
333 | ||
334 | EXPORT_SYMBOL(init_rwsem); | |
335 | EXPORT_SYMBOL(__down_read); | |
336 | EXPORT_SYMBOL(__down_read_trylock); | |
337 | EXPORT_SYMBOL(__down_write); | |
338 | EXPORT_SYMBOL(__down_write_trylock); | |
339 | EXPORT_SYMBOL(__up_read); | |
340 | EXPORT_SYMBOL(__up_write); | |
341 | EXPORT_SYMBOL(__downgrade_write); | |
342 | #if RWSEM_DEBUG | |
343 | EXPORT_SYMBOL(rwsemtrace); | |
344 | #endif |