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