Commit | Line | Data |
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4898e640 PH |
1 | /* |
2 | * Ldisc rw semaphore | |
3 | * | |
4 | * The ldisc semaphore is semantically a rw_semaphore but which enforces | |
5 | * an alternate policy, namely: | |
6 | * 1) Supports lock wait timeouts | |
7 | * 2) Write waiter has priority | |
8 | * 3) Downgrading is not supported | |
9 | * | |
10 | * Implementation notes: | |
11 | * 1) Upper half of semaphore count is a wait count (differs from rwsem | |
12 | * in that rwsem normalizes the upper half to the wait bias) | |
13 | * 2) Lacks overflow checking | |
14 | * | |
15 | * The generic counting was copied and modified from include/asm-generic/rwsem.h | |
16 | * by Paul Mackerras <paulus@samba.org>. | |
17 | * | |
18 | * The scheduling policy was copied and modified from lib/rwsem.c | |
19 | * Written by David Howells (dhowells@redhat.com). | |
20 | * | |
21 | * This implementation incorporates the write lock stealing work of | |
22 | * Michel Lespinasse <walken@google.com>. | |
23 | * | |
24 | * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com> | |
25 | * | |
26 | * This file may be redistributed under the terms of the GNU General Public | |
27 | * License v2. | |
28 | */ | |
29 | ||
30 | #include <linux/list.h> | |
31 | #include <linux/spinlock.h> | |
32 | #include <linux/atomic.h> | |
33 | #include <linux/tty.h> | |
34 | #include <linux/sched.h> | |
35 | ||
36 | ||
37 | #ifdef CONFIG_DEBUG_LOCK_ALLOC | |
38 | # define __acq(l, s, t, r, c, n, i) \ | |
39 | lock_acquire(&(l)->dep_map, s, t, r, c, n, i) | |
40 | # define __rel(l, n, i) \ | |
41 | lock_release(&(l)->dep_map, n, i) | |
42 | # ifdef CONFIG_PROVE_LOCKING | |
43 | # define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 2, NULL, i) | |
44 | # define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 2, n, i) | |
45 | # define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 2, NULL, i) | |
46 | # define lockdep_release(l, n, i) __rel(l, n, i) | |
47 | # else | |
48 | # define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i) | |
49 | # define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i) | |
50 | # define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i) | |
51 | # define lockdep_release(l, n, i) __rel(l, n, i) | |
52 | # endif | |
53 | #else | |
54 | # define lockdep_acquire(l, s, t, i) do { } while (0) | |
55 | # define lockdep_acquire_nest(l, s, t, n, i) do { } while (0) | |
56 | # define lockdep_acquire_read(l, s, t, i) do { } while (0) | |
57 | # define lockdep_release(l, n, i) do { } while (0) | |
58 | #endif | |
59 | ||
60 | #ifdef CONFIG_LOCK_STAT | |
61 | # define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_) | |
62 | #else | |
63 | # define lock_stat(_lock, stat) do { } while (0) | |
64 | #endif | |
65 | ||
66 | ||
67 | #if BITS_PER_LONG == 64 | |
68 | # define LDSEM_ACTIVE_MASK 0xffffffffL | |
69 | #else | |
70 | # define LDSEM_ACTIVE_MASK 0x0000ffffL | |
71 | #endif | |
72 | ||
73 | #define LDSEM_UNLOCKED 0L | |
74 | #define LDSEM_ACTIVE_BIAS 1L | |
75 | #define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1) | |
76 | #define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS | |
77 | #define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) | |
78 | ||
79 | struct ldsem_waiter { | |
80 | struct list_head list; | |
81 | struct task_struct *task; | |
82 | }; | |
83 | ||
84 | static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem) | |
85 | { | |
86 | return atomic_long_add_return(delta, (atomic_long_t *)&sem->count); | |
87 | } | |
88 | ||
89 | static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem) | |
90 | { | |
91 | long tmp = *old; | |
92 | *old = atomic_long_cmpxchg(&sem->count, *old, new); | |
93 | return *old == tmp; | |
94 | } | |
95 | ||
96 | /* | |
97 | * Initialize an ldsem: | |
98 | */ | |
99 | void __init_ldsem(struct ld_semaphore *sem, const char *name, | |
100 | struct lock_class_key *key) | |
101 | { | |
102 | #ifdef CONFIG_DEBUG_LOCK_ALLOC | |
103 | /* | |
104 | * Make sure we are not reinitializing a held semaphore: | |
105 | */ | |
106 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); | |
107 | lockdep_init_map(&sem->dep_map, name, key, 0); | |
108 | #endif | |
109 | sem->count = LDSEM_UNLOCKED; | |
110 | sem->wait_readers = 0; | |
111 | raw_spin_lock_init(&sem->wait_lock); | |
112 | INIT_LIST_HEAD(&sem->read_wait); | |
113 | INIT_LIST_HEAD(&sem->write_wait); | |
114 | } | |
115 | ||
116 | static void __ldsem_wake_readers(struct ld_semaphore *sem) | |
117 | { | |
118 | struct ldsem_waiter *waiter, *next; | |
119 | struct task_struct *tsk; | |
120 | long adjust, count; | |
121 | ||
122 | /* Try to grant read locks to all readers on the read wait list. | |
123 | * Note the 'active part' of the count is incremented by | |
124 | * the number of readers before waking any processes up. | |
125 | */ | |
126 | adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); | |
127 | count = ldsem_atomic_update(adjust, sem); | |
128 | do { | |
129 | if (count > 0) | |
130 | break; | |
131 | if (ldsem_cmpxchg(&count, count - adjust, sem)) | |
132 | return; | |
133 | } while (1); | |
134 | ||
135 | list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { | |
136 | tsk = waiter->task; | |
137 | smp_mb(); | |
138 | waiter->task = NULL; | |
139 | wake_up_process(tsk); | |
140 | put_task_struct(tsk); | |
141 | } | |
142 | INIT_LIST_HEAD(&sem->read_wait); | |
143 | sem->wait_readers = 0; | |
144 | } | |
145 | ||
146 | static inline int writer_trylock(struct ld_semaphore *sem) | |
147 | { | |
148 | /* only wake this writer if the active part of the count can be | |
149 | * transitioned from 0 -> 1 | |
150 | */ | |
151 | long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem); | |
152 | do { | |
153 | if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) | |
154 | return 1; | |
155 | if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem)) | |
156 | return 0; | |
157 | } while (1); | |
158 | } | |
159 | ||
160 | static void __ldsem_wake_writer(struct ld_semaphore *sem) | |
161 | { | |
162 | struct ldsem_waiter *waiter; | |
163 | ||
164 | waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); | |
165 | wake_up_process(waiter->task); | |
166 | } | |
167 | ||
168 | /* | |
169 | * handle the lock release when processes blocked on it that can now run | |
170 | * - if we come here from up_xxxx(), then: | |
171 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) | |
172 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) | |
173 | * - the spinlock must be held by the caller | |
174 | * - woken process blocks are discarded from the list after having task zeroed | |
175 | */ | |
176 | static void __ldsem_wake(struct ld_semaphore *sem) | |
177 | { | |
178 | if (!list_empty(&sem->write_wait)) | |
179 | __ldsem_wake_writer(sem); | |
180 | else if (!list_empty(&sem->read_wait)) | |
181 | __ldsem_wake_readers(sem); | |
182 | } | |
183 | ||
184 | static void ldsem_wake(struct ld_semaphore *sem) | |
185 | { | |
186 | unsigned long flags; | |
187 | ||
188 | raw_spin_lock_irqsave(&sem->wait_lock, flags); | |
189 | __ldsem_wake(sem); | |
190 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); | |
191 | } | |
192 | ||
193 | /* | |
194 | * wait for the read lock to be granted | |
195 | */ | |
196 | static struct ld_semaphore __sched * | |
197 | down_read_failed(struct ld_semaphore *sem, long count, long timeout) | |
198 | { | |
199 | struct ldsem_waiter waiter; | |
200 | struct task_struct *tsk = current; | |
201 | long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; | |
202 | ||
203 | /* set up my own style of waitqueue */ | |
204 | raw_spin_lock_irq(&sem->wait_lock); | |
205 | ||
206 | /* Try to reverse the lock attempt but if the count has changed | |
207 | * so that reversing fails, check if there are are no waiters, | |
208 | * and early-out if not */ | |
209 | do { | |
210 | if (ldsem_cmpxchg(&count, count + adjust, sem)) | |
211 | break; | |
212 | if (count > 0) { | |
213 | raw_spin_unlock_irq(&sem->wait_lock); | |
214 | return sem; | |
215 | } | |
216 | } while (1); | |
217 | ||
218 | list_add_tail(&waiter.list, &sem->read_wait); | |
219 | sem->wait_readers++; | |
220 | ||
221 | waiter.task = tsk; | |
222 | get_task_struct(tsk); | |
223 | ||
224 | /* if there are no active locks, wake the new lock owner(s) */ | |
225 | if ((count & LDSEM_ACTIVE_MASK) == 0) | |
226 | __ldsem_wake(sem); | |
227 | ||
228 | raw_spin_unlock_irq(&sem->wait_lock); | |
229 | ||
230 | /* wait to be given the lock */ | |
231 | for (;;) { | |
232 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
233 | ||
234 | if (!waiter.task) | |
235 | break; | |
236 | if (!timeout) | |
237 | break; | |
238 | timeout = schedule_timeout(timeout); | |
239 | } | |
240 | ||
241 | __set_task_state(tsk, TASK_RUNNING); | |
242 | ||
243 | if (!timeout) { | |
244 | /* lock timed out but check if this task was just | |
245 | * granted lock ownership - if so, pretend there | |
246 | * was no timeout; otherwise, cleanup lock wait */ | |
247 | raw_spin_lock_irq(&sem->wait_lock); | |
248 | if (waiter.task) { | |
249 | ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); | |
250 | list_del(&waiter.list); | |
251 | raw_spin_unlock_irq(&sem->wait_lock); | |
252 | put_task_struct(waiter.task); | |
253 | return NULL; | |
254 | } | |
255 | raw_spin_unlock_irq(&sem->wait_lock); | |
256 | } | |
257 | ||
258 | return sem; | |
259 | } | |
260 | ||
261 | /* | |
262 | * wait for the write lock to be granted | |
263 | */ | |
264 | static struct ld_semaphore __sched * | |
265 | down_write_failed(struct ld_semaphore *sem, long count, long timeout) | |
266 | { | |
267 | struct ldsem_waiter waiter; | |
268 | struct task_struct *tsk = current; | |
269 | long adjust = -LDSEM_ACTIVE_BIAS; | |
270 | int locked = 0; | |
271 | ||
272 | /* set up my own style of waitqueue */ | |
273 | raw_spin_lock_irq(&sem->wait_lock); | |
274 | ||
275 | /* Try to reverse the lock attempt but if the count has changed | |
276 | * so that reversing fails, check if the lock is now owned, | |
277 | * and early-out if so */ | |
278 | do { | |
279 | if (ldsem_cmpxchg(&count, count + adjust, sem)) | |
280 | break; | |
281 | if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { | |
282 | raw_spin_unlock_irq(&sem->wait_lock); | |
283 | return sem; | |
284 | } | |
285 | } while (1); | |
286 | ||
287 | list_add_tail(&waiter.list, &sem->write_wait); | |
288 | ||
289 | waiter.task = tsk; | |
290 | ||
291 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
292 | for (;;) { | |
293 | if (!timeout) | |
294 | break; | |
295 | raw_spin_unlock_irq(&sem->wait_lock); | |
296 | timeout = schedule_timeout(timeout); | |
297 | raw_spin_lock_irq(&sem->wait_lock); | |
298 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
299 | if ((locked = writer_trylock(sem))) | |
300 | break; | |
301 | } | |
302 | ||
303 | if (!locked) | |
304 | ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); | |
305 | list_del(&waiter.list); | |
306 | raw_spin_unlock_irq(&sem->wait_lock); | |
307 | ||
308 | __set_task_state(tsk, TASK_RUNNING); | |
309 | ||
310 | /* lock wait may have timed out */ | |
311 | if (!locked) | |
312 | return NULL; | |
313 | return sem; | |
314 | } | |
315 | ||
316 | ||
317 | ||
318 | static inline int __ldsem_down_read_nested(struct ld_semaphore *sem, | |
319 | int subclass, long timeout) | |
320 | { | |
321 | long count; | |
322 | ||
323 | lockdep_acquire_read(sem, subclass, 0, _RET_IP_); | |
324 | ||
325 | count = ldsem_atomic_update(LDSEM_READ_BIAS, sem); | |
326 | if (count <= 0) { | |
327 | lock_stat(sem, contended); | |
328 | if (!down_read_failed(sem, count, timeout)) { | |
329 | lockdep_release(sem, 1, _RET_IP_); | |
330 | return 0; | |
331 | } | |
332 | } | |
333 | lock_stat(sem, acquired); | |
334 | return 1; | |
335 | } | |
336 | ||
337 | static inline int __ldsem_down_write_nested(struct ld_semaphore *sem, | |
338 | int subclass, long timeout) | |
339 | { | |
340 | long count; | |
341 | ||
342 | lockdep_acquire(sem, subclass, 0, _RET_IP_); | |
343 | ||
344 | count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem); | |
345 | if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { | |
346 | lock_stat(sem, contended); | |
347 | if (!down_write_failed(sem, count, timeout)) { | |
348 | lockdep_release(sem, 1, _RET_IP_); | |
349 | return 0; | |
350 | } | |
351 | } | |
352 | lock_stat(sem, acquired); | |
353 | return 1; | |
354 | } | |
355 | ||
356 | ||
357 | /* | |
358 | * lock for reading -- returns 1 if successful, 0 if timed out | |
359 | */ | |
360 | int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) | |
361 | { | |
362 | might_sleep(); | |
363 | return __ldsem_down_read_nested(sem, 0, timeout); | |
364 | } | |
365 | ||
366 | /* | |
367 | * trylock for reading -- returns 1 if successful, 0 if contention | |
368 | */ | |
369 | int ldsem_down_read_trylock(struct ld_semaphore *sem) | |
370 | { | |
371 | long count = sem->count; | |
372 | ||
373 | while (count >= 0) { | |
374 | if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) { | |
375 | lockdep_acquire_read(sem, 0, 1, _RET_IP_); | |
376 | lock_stat(sem, acquired); | |
377 | return 1; | |
378 | } | |
379 | } | |
380 | return 0; | |
381 | } | |
382 | ||
383 | /* | |
384 | * lock for writing -- returns 1 if successful, 0 if timed out | |
385 | */ | |
386 | int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) | |
387 | { | |
388 | might_sleep(); | |
389 | return __ldsem_down_write_nested(sem, 0, timeout); | |
390 | } | |
391 | ||
392 | /* | |
393 | * trylock for writing -- returns 1 if successful, 0 if contention | |
394 | */ | |
395 | int ldsem_down_write_trylock(struct ld_semaphore *sem) | |
396 | { | |
397 | long count = sem->count; | |
398 | ||
399 | while ((count & LDSEM_ACTIVE_MASK) == 0) { | |
400 | if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) { | |
401 | lockdep_acquire(sem, 0, 1, _RET_IP_); | |
402 | lock_stat(sem, acquired); | |
403 | return 1; | |
404 | } | |
405 | } | |
406 | return 0; | |
407 | } | |
408 | ||
409 | /* | |
410 | * release a read lock | |
411 | */ | |
412 | void ldsem_up_read(struct ld_semaphore *sem) | |
413 | { | |
414 | long count; | |
415 | ||
416 | lockdep_release(sem, 1, _RET_IP_); | |
417 | ||
418 | count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem); | |
419 | if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) | |
420 | ldsem_wake(sem); | |
421 | } | |
422 | ||
423 | /* | |
424 | * release a write lock | |
425 | */ | |
426 | void ldsem_up_write(struct ld_semaphore *sem) | |
427 | { | |
428 | long count; | |
429 | ||
430 | lockdep_release(sem, 1, _RET_IP_); | |
431 | ||
432 | count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem); | |
433 | if (count < 0) | |
434 | ldsem_wake(sem); | |
435 | } | |
436 | ||
437 | ||
438 | #ifdef CONFIG_DEBUG_LOCK_ALLOC | |
439 | ||
440 | int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) | |
441 | { | |
442 | might_sleep(); | |
443 | return __ldsem_down_read_nested(sem, subclass, timeout); | |
444 | } | |
445 | ||
446 | int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, | |
447 | long timeout) | |
448 | { | |
449 | might_sleep(); | |
450 | return __ldsem_down_write_nested(sem, subclass, timeout); | |
451 | } | |
452 | ||
453 | #endif |