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/export.h>
11
12 enum rwsem_waiter_type {
13 RWSEM_WAITING_FOR_WRITE,
14 RWSEM_WAITING_FOR_READ
15 };
16
17 struct rwsem_waiter {
18 struct list_head list;
19 struct task_struct *task;
20 enum rwsem_waiter_type type;
21 };
22
rwsem_is_locked(struct rw_semaphore * sem)23 int rwsem_is_locked(struct rw_semaphore *sem)
24 {
25 int ret = 1;
26 unsigned long flags;
27
28 if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
29 ret = (sem->activity != 0);
30 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
31 }
32 return ret;
33 }
34 EXPORT_SYMBOL(rwsem_is_locked);
35
36 /*
37 * initialise the semaphore
38 */
__init_rwsem(struct rw_semaphore * sem,const char * name,struct lock_class_key * key)39 void __init_rwsem(struct rw_semaphore *sem, const char *name,
40 struct lock_class_key *key)
41 {
42 #ifdef CONFIG_DEBUG_LOCK_ALLOC
43 /*
44 * Make sure we are not reinitializing a held semaphore:
45 */
46 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
47 lockdep_init_map(&sem->dep_map, name, key, 0);
48 #endif
49 sem->activity = 0;
50 raw_spin_lock_init(&sem->wait_lock);
51 INIT_LIST_HEAD(&sem->wait_list);
52 }
53 EXPORT_SYMBOL(__init_rwsem);
54
55 /*
56 * handle the lock release when processes blocked on it that can now run
57 * - if we come here, then:
58 * - the 'active count' _reached_ zero
59 * - the 'waiting count' is non-zero
60 * - the spinlock must be held by the caller
61 * - woken process blocks are discarded from the list after having task zeroed
62 * - writers are only woken if wakewrite is non-zero
63 */
64 static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore * sem,int wakewrite)65 __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
66 {
67 struct rwsem_waiter *waiter;
68 struct task_struct *tsk;
69 int woken;
70
71 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
72
73 if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
74 if (wakewrite)
75 /* Wake up a writer. Note that we do not grant it the
76 * lock - it will have to acquire it when it runs. */
77 wake_up_process(waiter->task);
78 goto out;
79 }
80
81 /* grant an infinite number of read locks to the front of the queue */
82 woken = 0;
83 do {
84 struct list_head *next = waiter->list.next;
85
86 list_del(&waiter->list);
87 tsk = waiter->task;
88 smp_mb();
89 waiter->task = NULL;
90 wake_up_process(tsk);
91 put_task_struct(tsk);
92 woken++;
93 if (next == &sem->wait_list)
94 break;
95 waiter = list_entry(next, struct rwsem_waiter, list);
96 } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
97
98 sem->activity += woken;
99
100 out:
101 return sem;
102 }
103
104 /*
105 * wake a single writer
106 */
107 static inline struct rw_semaphore *
__rwsem_wake_one_writer(struct rw_semaphore * sem)108 __rwsem_wake_one_writer(struct rw_semaphore *sem)
109 {
110 struct rwsem_waiter *waiter;
111
112 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
113 wake_up_process(waiter->task);
114
115 return sem;
116 }
117
118 /*
119 * get a read lock on the semaphore
120 */
__down_read(struct rw_semaphore * sem)121 void __sched __down_read(struct rw_semaphore *sem)
122 {
123 struct rwsem_waiter waiter;
124 struct task_struct *tsk;
125 unsigned long flags;
126
127 raw_spin_lock_irqsave(&sem->wait_lock, flags);
128
129 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
130 /* granted */
131 sem->activity++;
132 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
133 goto out;
134 }
135
136 tsk = current;
137 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
138
139 /* set up my own style of waitqueue */
140 waiter.task = tsk;
141 waiter.type = RWSEM_WAITING_FOR_READ;
142 get_task_struct(tsk);
143
144 list_add_tail(&waiter.list, &sem->wait_list);
145
146 /* we don't need to touch the semaphore struct anymore */
147 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
148
149 /* wait to be given the lock */
150 for (;;) {
151 if (!waiter.task)
152 break;
153 schedule();
154 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
155 }
156
157 tsk->state = TASK_RUNNING;
158 out:
159 ;
160 }
161
162 /*
163 * trylock for reading -- returns 1 if successful, 0 if contention
164 */
__down_read_trylock(struct rw_semaphore * sem)165 int __down_read_trylock(struct rw_semaphore *sem)
166 {
167 unsigned long flags;
168 int ret = 0;
169
170
171 raw_spin_lock_irqsave(&sem->wait_lock, flags);
172
173 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
174 /* granted */
175 sem->activity++;
176 ret = 1;
177 }
178
179 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
180
181 return ret;
182 }
183
184 /*
185 * get a write lock on the semaphore
186 */
__down_write_nested(struct rw_semaphore * sem,int subclass)187 void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
188 {
189 struct rwsem_waiter waiter;
190 struct task_struct *tsk;
191 unsigned long flags;
192
193 raw_spin_lock_irqsave(&sem->wait_lock, flags);
194
195 /* set up my own style of waitqueue */
196 tsk = current;
197 waiter.task = tsk;
198 waiter.type = RWSEM_WAITING_FOR_WRITE;
199 list_add_tail(&waiter.list, &sem->wait_list);
200
201 /* wait for someone to release the lock */
202 for (;;) {
203 /*
204 * That is the key to support write lock stealing: allows the
205 * task already on CPU to get the lock soon rather than put
206 * itself into sleep and waiting for system woke it or someone
207 * else in the head of the wait list up.
208 */
209 if (sem->activity == 0)
210 break;
211 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
212 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
213 schedule();
214 raw_spin_lock_irqsave(&sem->wait_lock, flags);
215 }
216 /* got the lock */
217 sem->activity = -1;
218 list_del(&waiter.list);
219
220 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
221 }
222
__down_write(struct rw_semaphore * sem)223 void __sched __down_write(struct rw_semaphore *sem)
224 {
225 __down_write_nested(sem, 0);
226 }
227
228 /*
229 * trylock for writing -- returns 1 if successful, 0 if contention
230 */
__down_write_trylock(struct rw_semaphore * sem)231 int __down_write_trylock(struct rw_semaphore *sem)
232 {
233 unsigned long flags;
234 int ret = 0;
235
236 raw_spin_lock_irqsave(&sem->wait_lock, flags);
237
238 if (sem->activity == 0) {
239 /* got the lock */
240 sem->activity = -1;
241 ret = 1;
242 }
243
244 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
245
246 return ret;
247 }
248
249 /*
250 * release a read lock on the semaphore
251 */
__up_read(struct rw_semaphore * sem)252 void __up_read(struct rw_semaphore *sem)
253 {
254 unsigned long flags;
255
256 raw_spin_lock_irqsave(&sem->wait_lock, flags);
257
258 if (--sem->activity == 0 && !list_empty(&sem->wait_list))
259 sem = __rwsem_wake_one_writer(sem);
260
261 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
262 }
263
264 /*
265 * release a write lock on the semaphore
266 */
__up_write(struct rw_semaphore * sem)267 void __up_write(struct rw_semaphore *sem)
268 {
269 unsigned long flags;
270
271 raw_spin_lock_irqsave(&sem->wait_lock, flags);
272
273 sem->activity = 0;
274 if (!list_empty(&sem->wait_list))
275 sem = __rwsem_do_wake(sem, 1);
276
277 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
278 }
279
280 /*
281 * downgrade a write lock into a read lock
282 * - just wake up any readers at the front of the queue
283 */
__downgrade_write(struct rw_semaphore * sem)284 void __downgrade_write(struct rw_semaphore *sem)
285 {
286 unsigned long flags;
287
288 raw_spin_lock_irqsave(&sem->wait_lock, flags);
289
290 sem->activity = 1;
291 if (!list_empty(&sem->wait_list))
292 sem = __rwsem_do_wake(sem, 0);
293
294 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
295 }
296
297