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1 | /* rwsem.c: R/W semaphores: contention handling functions |
2 | * |
3 | * Written by David Howells (dhowells@redhat.com). |
4 | * Derived from arch/i386/kernel/semaphore.c |
5 | */ |
6 | #include <linux/rwsem.h> |
7 | #include <linux/sched.h> |
8 | #include <linux/init.h> |
9 | #include <linux/module.h> |
10 | |
11 | /* |
12 | * Initialize an rwsem: |
13 | */ |
14 | void __init_rwsem(struct rw_semaphore *sem, const char *name, |
15 | struct lock_class_key *key) |
16 | { |
17 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
18 | /* |
19 | * Make sure we are not reinitializing a held semaphore: |
20 | */ |
21 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); |
22 | lockdep_init_map(&sem->dep_map, name, key, 0); |
23 | #endif |
24 | sem->count = RWSEM_UNLOCKED_VALUE; |
25 | spin_lock_init(&sem->wait_lock); |
26 | INIT_LIST_HEAD(&sem->wait_list); |
27 | } |
28 | |
29 | EXPORT_SYMBOL(__init_rwsem); |
30 | |
31 | struct rwsem_waiter { |
32 | struct list_head list; |
33 | struct task_struct *task; |
34 | unsigned int flags; |
35 | #define RWSEM_WAITING_FOR_READ 0x00000001 |
36 | #define RWSEM_WAITING_FOR_WRITE 0x00000002 |
37 | }; |
38 | |
39 | /* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and |
40 | * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held |
41 | * since the rwsem value was observed. |
42 | */ |
43 | #define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ |
44 | #define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ |
45 | #define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ |
46 | |
47 | /* |
48 | * handle the lock release when processes blocked on it that can now run |
49 | * - if we come here from up_xxxx(), then: |
50 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) |
51 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) |
52 | * - there must be someone on the queue |
53 | * - the spinlock must be held by the caller |
54 | * - woken process blocks are discarded from the list after having task zeroed |
55 | * - writers are only woken if downgrading is false |
56 | */ |
57 | static struct rw_semaphore * |
58 | __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) |
59 | { |
60 | struct rwsem_waiter *waiter; |
61 | struct task_struct *tsk; |
62 | struct list_head *next; |
63 | signed long oldcount, woken, loop, adjustment; |
64 | |
65 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
66 | if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) |
67 | goto readers_only; |
68 | |
69 | if (wake_type == RWSEM_WAKE_READ_OWNED) |
70 | /* Another active reader was observed, so wakeup is not |
71 | * likely to succeed. Save the atomic op. |
72 | */ |
73 | goto out; |
74 | |
75 | /* There's a writer at the front of the queue - try to grant it the |
76 | * write lock. However, we only wake this writer if we can transition |
77 | * the active part of the count from 0 -> 1 |
78 | */ |
79 | adjustment = RWSEM_ACTIVE_WRITE_BIAS; |
80 | if (waiter->list.next == &sem->wait_list) |
81 | adjustment -= RWSEM_WAITING_BIAS; |
82 | |
83 | try_again_write: |
84 | oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; |
85 | if (oldcount & RWSEM_ACTIVE_MASK) |
86 | /* Someone grabbed the sem already */ |
87 | goto undo_write; |
88 | |
89 | /* We must be careful not to touch 'waiter' after we set ->task = NULL. |
90 | * It is an allocated on the waiter's stack and may become invalid at |
91 | * any time after that point (due to a wakeup from another source). |
92 | */ |
93 | list_del(&waiter->list); |
94 | tsk = waiter->task; |
95 | smp_mb(); |
96 | waiter->task = NULL; |
97 | wake_up_process(tsk); |
98 | put_task_struct(tsk); |
99 | goto out; |
100 | |
101 | readers_only: |
102 | /* If we come here from up_xxxx(), another thread might have reached |
103 | * rwsem_down_failed_common() before we acquired the spinlock and |
104 | * woken up a waiter, making it now active. We prefer to check for |
105 | * this first in order to not spend too much time with the spinlock |
106 | * held if we're not going to be able to wake up readers in the end. |
107 | * |
108 | * Note that we do not need to update the rwsem count: any writer |
109 | * trying to acquire rwsem will run rwsem_down_write_failed() due |
110 | * to the waiting threads and block trying to acquire the spinlock. |
111 | * |
112 | * We use a dummy atomic update in order to acquire the cache line |
113 | * exclusively since we expect to succeed and run the final rwsem |
114 | * count adjustment pretty soon. |
115 | */ |
116 | if (wake_type == RWSEM_WAKE_ANY && |
117 | rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) |
118 | /* Someone grabbed the sem for write already */ |
119 | goto out; |
120 | |
121 | /* Grant an infinite number of read locks to the readers at the front |
122 | * of the queue. Note we increment the 'active part' of the count by |
123 | * the number of readers before waking any processes up. |
124 | */ |
125 | woken = 0; |
126 | do { |
127 | woken++; |
128 | |
129 | if (waiter->list.next == &sem->wait_list) |
130 | break; |
131 | |
132 | waiter = list_entry(waiter->list.next, |
133 | struct rwsem_waiter, list); |
134 | |
135 | } while (waiter->flags & RWSEM_WAITING_FOR_READ); |
136 | |
137 | adjustment = woken * RWSEM_ACTIVE_READ_BIAS; |
138 | if (waiter->flags & RWSEM_WAITING_FOR_READ) |
139 | /* hit end of list above */ |
140 | adjustment -= RWSEM_WAITING_BIAS; |
141 | |
142 | rwsem_atomic_add(adjustment, sem); |
143 | |
144 | next = sem->wait_list.next; |
145 | for (loop = woken; loop > 0; loop--) { |
146 | waiter = list_entry(next, struct rwsem_waiter, list); |
147 | next = waiter->list.next; |
148 | tsk = waiter->task; |
149 | smp_mb(); |
150 | waiter->task = NULL; |
151 | wake_up_process(tsk); |
152 | put_task_struct(tsk); |
153 | } |
154 | |
155 | sem->wait_list.next = next; |
156 | next->prev = &sem->wait_list; |
157 | |
158 | out: |
159 | return sem; |
160 | |
161 | /* undo the change to the active count, but check for a transition |
162 | * 1->0 */ |
163 | undo_write: |
164 | if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) |
165 | goto out; |
166 | goto try_again_write; |
167 | } |
168 | |
169 | /* |
170 | * wait for a lock to be granted |
171 | */ |
172 | static struct rw_semaphore __sched * |
173 | rwsem_down_failed_common(struct rw_semaphore *sem, |
174 | unsigned int flags, signed long adjustment) |
175 | { |
176 | struct rwsem_waiter waiter; |
177 | struct task_struct *tsk = current; |
178 | signed long count; |
179 | |
180 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
181 | |
182 | /* set up my own style of waitqueue */ |
183 | spin_lock_irq(&sem->wait_lock); |
184 | waiter.task = tsk; |
185 | waiter.flags = flags; |
186 | get_task_struct(tsk); |
187 | |
188 | if (list_empty(&sem->wait_list)) |
189 | adjustment += RWSEM_WAITING_BIAS; |
190 | list_add_tail(&waiter.list, &sem->wait_list); |
191 | |
192 | /* we're now waiting on the lock, but no longer actively locking */ |
193 | count = rwsem_atomic_update(adjustment, sem); |
194 | |
195 | /* If there are no active locks, wake the front queued process(es) up. |
196 | * |
197 | * Alternatively, if we're called from a failed down_write(), there |
198 | * were already threads queued before us and there are no active |
199 | * writers, the lock must be read owned; so we try to wake any read |
200 | * locks that were queued ahead of us. */ |
201 | if (count == RWSEM_WAITING_BIAS) |
202 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); |
203 | else if (count > RWSEM_WAITING_BIAS && |
204 | adjustment == -RWSEM_ACTIVE_WRITE_BIAS) |
205 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
206 | |
207 | spin_unlock_irq(&sem->wait_lock); |
208 | |
209 | /* wait to be given the lock */ |
210 | for (;;) { |
211 | if (!waiter.task) |
212 | break; |
213 | schedule(); |
214 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
215 | } |
216 | |
217 | tsk->state = TASK_RUNNING; |
218 | |
219 | return sem; |
220 | } |
221 | |
222 | /* |
223 | * wait for the read lock to be granted |
224 | */ |
225 | struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) |
226 | { |
227 | return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ, |
228 | -RWSEM_ACTIVE_READ_BIAS); |
229 | } |
230 | |
231 | /* |
232 | * wait for the write lock to be granted |
233 | */ |
234 | struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) |
235 | { |
236 | return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE, |
237 | -RWSEM_ACTIVE_WRITE_BIAS); |
238 | } |
239 | |
240 | /* |
241 | * handle waking up a waiter on the semaphore |
242 | * - up_read/up_write has decremented the active part of count if we come here |
243 | */ |
244 | struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
245 | { |
246 | unsigned long flags; |
247 | |
248 | spin_lock_irqsave(&sem->wait_lock, flags); |
249 | |
250 | /* do nothing if list empty */ |
251 | if (!list_empty(&sem->wait_list)) |
252 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
253 | |
254 | spin_unlock_irqrestore(&sem->wait_lock, flags); |
255 | |
256 | return sem; |
257 | } |
258 | |
259 | /* |
260 | * downgrade a write lock into a read lock |
261 | * - caller incremented waiting part of count and discovered it still negative |
262 | * - just wake up any readers at the front of the queue |
263 | */ |
264 | struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
265 | { |
266 | unsigned long flags; |
267 | |
268 | spin_lock_irqsave(&sem->wait_lock, flags); |
269 | |
270 | /* do nothing if list empty */ |
271 | if (!list_empty(&sem->wait_list)) |
272 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
273 | |
274 | spin_unlock_irqrestore(&sem->wait_lock, flags); |
275 | |
276 | return sem; |
277 | } |
278 | |
279 | EXPORT_SYMBOL(rwsem_down_read_failed); |
280 | EXPORT_SYMBOL(rwsem_down_write_failed); |
281 | EXPORT_SYMBOL(rwsem_wake); |
282 | EXPORT_SYMBOL(rwsem_downgrade_wake); |
283 |
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