Loading...
Note: File does not exist in v3.1.
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#define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
43#define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
44#define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
45#define lockdep_release(l, n, i) __rel(l, n, i)
46#else
47# define lockdep_acquire(l, s, t, i) do { } while (0)
48# define lockdep_acquire_nest(l, s, t, n, i) do { } while (0)
49# define lockdep_acquire_read(l, s, t, i) do { } while (0)
50# define lockdep_release(l, n, i) do { } while (0)
51#endif
52
53#ifdef CONFIG_LOCK_STAT
54# define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_)
55#else
56# define lock_stat(_lock, stat) do { } while (0)
57#endif
58
59
60#if BITS_PER_LONG == 64
61# define LDSEM_ACTIVE_MASK 0xffffffffL
62#else
63# define LDSEM_ACTIVE_MASK 0x0000ffffL
64#endif
65
66#define LDSEM_UNLOCKED 0L
67#define LDSEM_ACTIVE_BIAS 1L
68#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
69#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
70#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
71
72struct ldsem_waiter {
73 struct list_head list;
74 struct task_struct *task;
75};
76
77static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
78{
79 return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
80}
81
82/*
83 * ldsem_cmpxchg() updates @*old with the last-known sem->count value.
84 * Returns 1 if count was successfully changed; @*old will have @new value.
85 * Returns 0 if count was not changed; @*old will have most recent sem->count
86 */
87static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
88{
89 long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
90 if (tmp == *old) {
91 *old = new;
92 return 1;
93 } else {
94 *old = tmp;
95 return 0;
96 }
97}
98
99/*
100 * Initialize an ldsem:
101 */
102void __init_ldsem(struct ld_semaphore *sem, const char *name,
103 struct lock_class_key *key)
104{
105#ifdef CONFIG_DEBUG_LOCK_ALLOC
106 /*
107 * Make sure we are not reinitializing a held semaphore:
108 */
109 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
110 lockdep_init_map(&sem->dep_map, name, key, 0);
111#endif
112 sem->count = LDSEM_UNLOCKED;
113 sem->wait_readers = 0;
114 raw_spin_lock_init(&sem->wait_lock);
115 INIT_LIST_HEAD(&sem->read_wait);
116 INIT_LIST_HEAD(&sem->write_wait);
117}
118
119static void __ldsem_wake_readers(struct ld_semaphore *sem)
120{
121 struct ldsem_waiter *waiter, *next;
122 struct task_struct *tsk;
123 long adjust, count;
124
125 /* Try to grant read locks to all readers on the read wait list.
126 * Note the 'active part' of the count is incremented by
127 * the number of readers before waking any processes up.
128 */
129 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
130 count = ldsem_atomic_update(adjust, sem);
131 do {
132 if (count > 0)
133 break;
134 if (ldsem_cmpxchg(&count, count - adjust, sem))
135 return;
136 } while (1);
137
138 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
139 tsk = waiter->task;
140 smp_mb();
141 waiter->task = NULL;
142 wake_up_process(tsk);
143 put_task_struct(tsk);
144 }
145 INIT_LIST_HEAD(&sem->read_wait);
146 sem->wait_readers = 0;
147}
148
149static inline int writer_trylock(struct ld_semaphore *sem)
150{
151 /* only wake this writer if the active part of the count can be
152 * transitioned from 0 -> 1
153 */
154 long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);
155 do {
156 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
157 return 1;
158 if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
159 return 0;
160 } while (1);
161}
162
163static void __ldsem_wake_writer(struct ld_semaphore *sem)
164{
165 struct ldsem_waiter *waiter;
166
167 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
168 wake_up_process(waiter->task);
169}
170
171/*
172 * handle the lock release when processes blocked on it that can now run
173 * - if we come here from up_xxxx(), then:
174 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
175 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
176 * - the spinlock must be held by the caller
177 * - woken process blocks are discarded from the list after having task zeroed
178 */
179static void __ldsem_wake(struct ld_semaphore *sem)
180{
181 if (!list_empty(&sem->write_wait))
182 __ldsem_wake_writer(sem);
183 else if (!list_empty(&sem->read_wait))
184 __ldsem_wake_readers(sem);
185}
186
187static void ldsem_wake(struct ld_semaphore *sem)
188{
189 unsigned long flags;
190
191 raw_spin_lock_irqsave(&sem->wait_lock, flags);
192 __ldsem_wake(sem);
193 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
194}
195
196/*
197 * wait for the read lock to be granted
198 */
199static struct ld_semaphore __sched *
200down_read_failed(struct ld_semaphore *sem, long count, long timeout)
201{
202 struct ldsem_waiter waiter;
203 struct task_struct *tsk = current;
204 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
205
206 /* set up my own style of waitqueue */
207 raw_spin_lock_irq(&sem->wait_lock);
208
209 /* Try to reverse the lock attempt but if the count has changed
210 * so that reversing fails, check if there are are no waiters,
211 * and early-out if not */
212 do {
213 if (ldsem_cmpxchg(&count, count + adjust, sem))
214 break;
215 if (count > 0) {
216 raw_spin_unlock_irq(&sem->wait_lock);
217 return sem;
218 }
219 } while (1);
220
221 list_add_tail(&waiter.list, &sem->read_wait);
222 sem->wait_readers++;
223
224 waiter.task = tsk;
225 get_task_struct(tsk);
226
227 /* if there are no active locks, wake the new lock owner(s) */
228 if ((count & LDSEM_ACTIVE_MASK) == 0)
229 __ldsem_wake(sem);
230
231 raw_spin_unlock_irq(&sem->wait_lock);
232
233 /* wait to be given the lock */
234 for (;;) {
235 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
236
237 if (!waiter.task)
238 break;
239 if (!timeout)
240 break;
241 timeout = schedule_timeout(timeout);
242 }
243
244 __set_task_state(tsk, TASK_RUNNING);
245
246 if (!timeout) {
247 /* lock timed out but check if this task was just
248 * granted lock ownership - if so, pretend there
249 * was no timeout; otherwise, cleanup lock wait */
250 raw_spin_lock_irq(&sem->wait_lock);
251 if (waiter.task) {
252 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
253 list_del(&waiter.list);
254 raw_spin_unlock_irq(&sem->wait_lock);
255 put_task_struct(waiter.task);
256 return NULL;
257 }
258 raw_spin_unlock_irq(&sem->wait_lock);
259 }
260
261 return sem;
262}
263
264/*
265 * wait for the write lock to be granted
266 */
267static struct ld_semaphore __sched *
268down_write_failed(struct ld_semaphore *sem, long count, long timeout)
269{
270 struct ldsem_waiter waiter;
271 struct task_struct *tsk = current;
272 long adjust = -LDSEM_ACTIVE_BIAS;
273 int locked = 0;
274
275 /* set up my own style of waitqueue */
276 raw_spin_lock_irq(&sem->wait_lock);
277
278 /* Try to reverse the lock attempt but if the count has changed
279 * so that reversing fails, check if the lock is now owned,
280 * and early-out if so */
281 do {
282 if (ldsem_cmpxchg(&count, count + adjust, sem))
283 break;
284 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
285 raw_spin_unlock_irq(&sem->wait_lock);
286 return sem;
287 }
288 } while (1);
289
290 list_add_tail(&waiter.list, &sem->write_wait);
291
292 waiter.task = tsk;
293
294 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
295 for (;;) {
296 if (!timeout)
297 break;
298 raw_spin_unlock_irq(&sem->wait_lock);
299 timeout = schedule_timeout(timeout);
300 raw_spin_lock_irq(&sem->wait_lock);
301 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
302 if ((locked = writer_trylock(sem)))
303 break;
304 }
305
306 if (!locked)
307 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
308 list_del(&waiter.list);
309 raw_spin_unlock_irq(&sem->wait_lock);
310
311 __set_task_state(tsk, TASK_RUNNING);
312
313 /* lock wait may have timed out */
314 if (!locked)
315 return NULL;
316 return sem;
317}
318
319
320
321static inline int __ldsem_down_read_nested(struct ld_semaphore *sem,
322 int subclass, long timeout)
323{
324 long count;
325
326 lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
327
328 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
329 if (count <= 0) {
330 lock_stat(sem, contended);
331 if (!down_read_failed(sem, count, timeout)) {
332 lockdep_release(sem, 1, _RET_IP_);
333 return 0;
334 }
335 }
336 lock_stat(sem, acquired);
337 return 1;
338}
339
340static inline int __ldsem_down_write_nested(struct ld_semaphore *sem,
341 int subclass, long timeout)
342{
343 long count;
344
345 lockdep_acquire(sem, subclass, 0, _RET_IP_);
346
347 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
348 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
349 lock_stat(sem, contended);
350 if (!down_write_failed(sem, count, timeout)) {
351 lockdep_release(sem, 1, _RET_IP_);
352 return 0;
353 }
354 }
355 lock_stat(sem, acquired);
356 return 1;
357}
358
359
360/*
361 * lock for reading -- returns 1 if successful, 0 if timed out
362 */
363int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
364{
365 might_sleep();
366 return __ldsem_down_read_nested(sem, 0, timeout);
367}
368
369/*
370 * trylock for reading -- returns 1 if successful, 0 if contention
371 */
372int ldsem_down_read_trylock(struct ld_semaphore *sem)
373{
374 long count = sem->count;
375
376 while (count >= 0) {
377 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
378 lockdep_acquire_read(sem, 0, 1, _RET_IP_);
379 lock_stat(sem, acquired);
380 return 1;
381 }
382 }
383 return 0;
384}
385
386/*
387 * lock for writing -- returns 1 if successful, 0 if timed out
388 */
389int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
390{
391 might_sleep();
392 return __ldsem_down_write_nested(sem, 0, timeout);
393}
394
395/*
396 * trylock for writing -- returns 1 if successful, 0 if contention
397 */
398int ldsem_down_write_trylock(struct ld_semaphore *sem)
399{
400 long count = sem->count;
401
402 while ((count & LDSEM_ACTIVE_MASK) == 0) {
403 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
404 lockdep_acquire(sem, 0, 1, _RET_IP_);
405 lock_stat(sem, acquired);
406 return 1;
407 }
408 }
409 return 0;
410}
411
412/*
413 * release a read lock
414 */
415void ldsem_up_read(struct ld_semaphore *sem)
416{
417 long count;
418
419 lockdep_release(sem, 1, _RET_IP_);
420
421 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
422 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
423 ldsem_wake(sem);
424}
425
426/*
427 * release a write lock
428 */
429void ldsem_up_write(struct ld_semaphore *sem)
430{
431 long count;
432
433 lockdep_release(sem, 1, _RET_IP_);
434
435 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
436 if (count < 0)
437 ldsem_wake(sem);
438}
439
440
441#ifdef CONFIG_DEBUG_LOCK_ALLOC
442
443int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
444{
445 might_sleep();
446 return __ldsem_down_read_nested(sem, subclass, timeout);
447}
448
449int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
450 long timeout)
451{
452 might_sleep();
453 return __ldsem_down_write_nested(sem, subclass, timeout);
454}
455
456#endif