Root/
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 | * Writer lock-stealing by Alex Shi <alex.shi@intel.com> |
7 | * and Michel Lespinasse <walken@google.com> |
8 | */ |
9 | #include <linux/rwsem.h> |
10 | #include <linux/sched.h> |
11 | #include <linux/init.h> |
12 | #include <linux/export.h> |
13 | |
14 | /* |
15 | * Initialize an rwsem: |
16 | */ |
17 | void __init_rwsem(struct rw_semaphore *sem, const char *name, |
18 | struct lock_class_key *key) |
19 | { |
20 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
21 | /* |
22 | * Make sure we are not reinitializing a held semaphore: |
23 | */ |
24 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); |
25 | lockdep_init_map(&sem->dep_map, name, key, 0); |
26 | #endif |
27 | sem->count = RWSEM_UNLOCKED_VALUE; |
28 | raw_spin_lock_init(&sem->wait_lock); |
29 | INIT_LIST_HEAD(&sem->wait_list); |
30 | } |
31 | |
32 | EXPORT_SYMBOL(__init_rwsem); |
33 | |
34 | enum rwsem_waiter_type { |
35 | RWSEM_WAITING_FOR_WRITE, |
36 | RWSEM_WAITING_FOR_READ |
37 | }; |
38 | |
39 | struct rwsem_waiter { |
40 | struct list_head list; |
41 | struct task_struct *task; |
42 | enum rwsem_waiter_type type; |
43 | }; |
44 | |
45 | enum rwsem_wake_type { |
46 | RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */ |
47 | RWSEM_WAKE_READERS, /* Wake readers only */ |
48 | RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ |
49 | }; |
50 | |
51 | /* |
52 | * handle the lock release when processes blocked on it that can now run |
53 | * - if we come here from up_xxxx(), then: |
54 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) |
55 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) |
56 | * - there must be someone on the queue |
57 | * - the spinlock must be held by the caller |
58 | * - woken process blocks are discarded from the list after having task zeroed |
59 | * - writers are only woken if downgrading is false |
60 | */ |
61 | static struct rw_semaphore * |
62 | __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) |
63 | { |
64 | struct rwsem_waiter *waiter; |
65 | struct task_struct *tsk; |
66 | struct list_head *next; |
67 | long oldcount, woken, loop, adjustment; |
68 | |
69 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
70 | if (waiter->type == RWSEM_WAITING_FOR_WRITE) { |
71 | if (wake_type == RWSEM_WAKE_ANY) |
72 | /* Wake writer at the front of the queue, but do not |
73 | * grant it the lock yet as we want other writers |
74 | * to be able to steal it. Readers, on the other hand, |
75 | * will block as they will notice the queued writer. |
76 | */ |
77 | wake_up_process(waiter->task); |
78 | goto out; |
79 | } |
80 | |
81 | /* Writers might steal the lock before we grant it to the next reader. |
82 | * We prefer to do the first reader grant before counting readers |
83 | * so we can bail out early if a writer stole the lock. |
84 | */ |
85 | adjustment = 0; |
86 | if (wake_type != RWSEM_WAKE_READ_OWNED) { |
87 | adjustment = RWSEM_ACTIVE_READ_BIAS; |
88 | try_reader_grant: |
89 | oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; |
90 | if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { |
91 | /* A writer stole the lock. Undo our reader grant. */ |
92 | if (rwsem_atomic_update(-adjustment, sem) & |
93 | RWSEM_ACTIVE_MASK) |
94 | goto out; |
95 | /* Last active locker left. Retry waking readers. */ |
96 | goto try_reader_grant; |
97 | } |
98 | } |
99 | |
100 | /* Grant an infinite number of read locks to the readers at the front |
101 | * of the queue. Note we increment the 'active part' of the count by |
102 | * the number of readers before waking any processes up. |
103 | */ |
104 | woken = 0; |
105 | do { |
106 | woken++; |
107 | |
108 | if (waiter->list.next == &sem->wait_list) |
109 | break; |
110 | |
111 | waiter = list_entry(waiter->list.next, |
112 | struct rwsem_waiter, list); |
113 | |
114 | } while (waiter->type != RWSEM_WAITING_FOR_WRITE); |
115 | |
116 | adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; |
117 | if (waiter->type != RWSEM_WAITING_FOR_WRITE) |
118 | /* hit end of list above */ |
119 | adjustment -= RWSEM_WAITING_BIAS; |
120 | |
121 | if (adjustment) |
122 | rwsem_atomic_add(adjustment, sem); |
123 | |
124 | next = sem->wait_list.next; |
125 | loop = woken; |
126 | do { |
127 | waiter = list_entry(next, struct rwsem_waiter, list); |
128 | next = waiter->list.next; |
129 | tsk = waiter->task; |
130 | smp_mb(); |
131 | waiter->task = NULL; |
132 | wake_up_process(tsk); |
133 | put_task_struct(tsk); |
134 | } while (--loop); |
135 | |
136 | sem->wait_list.next = next; |
137 | next->prev = &sem->wait_list; |
138 | |
139 | out: |
140 | return sem; |
141 | } |
142 | |
143 | /* |
144 | * wait for the read lock to be granted |
145 | */ |
146 | struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) |
147 | { |
148 | long count, adjustment = -RWSEM_ACTIVE_READ_BIAS; |
149 | struct rwsem_waiter waiter; |
150 | struct task_struct *tsk = current; |
151 | |
152 | /* set up my own style of waitqueue */ |
153 | waiter.task = tsk; |
154 | waiter.type = RWSEM_WAITING_FOR_READ; |
155 | get_task_struct(tsk); |
156 | |
157 | raw_spin_lock_irq(&sem->wait_lock); |
158 | if (list_empty(&sem->wait_list)) |
159 | adjustment += RWSEM_WAITING_BIAS; |
160 | list_add_tail(&waiter.list, &sem->wait_list); |
161 | |
162 | /* we're now waiting on the lock, but no longer actively locking */ |
163 | count = rwsem_atomic_update(adjustment, sem); |
164 | |
165 | /* If there are no active locks, wake the front queued process(es). |
166 | * |
167 | * If there are no writers and we are first in the queue, |
168 | * wake our own waiter to join the existing active readers ! |
169 | */ |
170 | if (count == RWSEM_WAITING_BIAS || |
171 | (count > RWSEM_WAITING_BIAS && |
172 | adjustment != -RWSEM_ACTIVE_READ_BIAS)) |
173 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
174 | |
175 | raw_spin_unlock_irq(&sem->wait_lock); |
176 | |
177 | /* wait to be given the lock */ |
178 | while (true) { |
179 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
180 | if (!waiter.task) |
181 | break; |
182 | schedule(); |
183 | } |
184 | |
185 | tsk->state = TASK_RUNNING; |
186 | |
187 | return sem; |
188 | } |
189 | |
190 | /* |
191 | * wait until we successfully acquire the write lock |
192 | */ |
193 | struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) |
194 | { |
195 | long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS; |
196 | struct rwsem_waiter waiter; |
197 | struct task_struct *tsk = current; |
198 | |
199 | /* set up my own style of waitqueue */ |
200 | waiter.task = tsk; |
201 | waiter.type = RWSEM_WAITING_FOR_WRITE; |
202 | |
203 | raw_spin_lock_irq(&sem->wait_lock); |
204 | if (list_empty(&sem->wait_list)) |
205 | adjustment += RWSEM_WAITING_BIAS; |
206 | list_add_tail(&waiter.list, &sem->wait_list); |
207 | |
208 | /* we're now waiting on the lock, but no longer actively locking */ |
209 | count = rwsem_atomic_update(adjustment, sem); |
210 | |
211 | /* If there were already threads queued before us and there are no |
212 | * active writers, the lock must be read owned; so we try to wake |
213 | * any read locks that were queued ahead of us. */ |
214 | if (count > RWSEM_WAITING_BIAS && |
215 | adjustment == -RWSEM_ACTIVE_WRITE_BIAS) |
216 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS); |
217 | |
218 | /* wait until we successfully acquire the lock */ |
219 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
220 | while (true) { |
221 | if (!(count & RWSEM_ACTIVE_MASK)) { |
222 | /* Try acquiring the write lock. */ |
223 | count = RWSEM_ACTIVE_WRITE_BIAS; |
224 | if (!list_is_singular(&sem->wait_list)) |
225 | count += RWSEM_WAITING_BIAS; |
226 | |
227 | if (sem->count == RWSEM_WAITING_BIAS && |
228 | cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == |
229 | RWSEM_WAITING_BIAS) |
230 | break; |
231 | } |
232 | |
233 | raw_spin_unlock_irq(&sem->wait_lock); |
234 | |
235 | /* Block until there are no active lockers. */ |
236 | do { |
237 | schedule(); |
238 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
239 | } while ((count = sem->count) & RWSEM_ACTIVE_MASK); |
240 | |
241 | raw_spin_lock_irq(&sem->wait_lock); |
242 | } |
243 | |
244 | list_del(&waiter.list); |
245 | raw_spin_unlock_irq(&sem->wait_lock); |
246 | tsk->state = TASK_RUNNING; |
247 | |
248 | return sem; |
249 | } |
250 | |
251 | /* |
252 | * handle waking up a waiter on the semaphore |
253 | * - up_read/up_write has decremented the active part of count if we come here |
254 | */ |
255 | struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
256 | { |
257 | unsigned long flags; |
258 | |
259 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
260 | |
261 | /* do nothing if list empty */ |
262 | if (!list_empty(&sem->wait_list)) |
263 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
264 | |
265 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
266 | |
267 | return sem; |
268 | } |
269 | |
270 | /* |
271 | * downgrade a write lock into a read lock |
272 | * - caller incremented waiting part of count and discovered it still negative |
273 | * - just wake up any readers at the front of the queue |
274 | */ |
275 | struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
276 | { |
277 | unsigned long flags; |
278 | |
279 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
280 | |
281 | /* do nothing if list empty */ |
282 | if (!list_empty(&sem->wait_list)) |
283 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
284 | |
285 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
286 | |
287 | return sem; |
288 | } |
289 | |
290 | EXPORT_SYMBOL(rwsem_down_read_failed); |
291 | EXPORT_SYMBOL(rwsem_down_write_failed); |
292 | EXPORT_SYMBOL(rwsem_wake); |
293 | EXPORT_SYMBOL(rwsem_downgrade_wake); |
294 |
Branches:
ben-wpan
ben-wpan-stefan
javiroman/ks7010
jz-2.6.34
jz-2.6.34-rc5
jz-2.6.34-rc6
jz-2.6.34-rc7
jz-2.6.35
jz-2.6.36
jz-2.6.37
jz-2.6.38
jz-2.6.39
jz-3.0
jz-3.1
jz-3.11
jz-3.12
jz-3.13
jz-3.15
jz-3.16
jz-3.18-dt
jz-3.2
jz-3.3
jz-3.4
jz-3.5
jz-3.6
jz-3.6-rc2-pwm
jz-3.9
jz-3.9-clk
jz-3.9-rc8
jz47xx
jz47xx-2.6.38
master
Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
v2.6.34-rc6
v2.6.34-rc7
v3.9