blob: 52b7baef4f7a805a750a0d003f116cff7e5e4bc1 [file] [log] [blame]
Peter Hurley4898e6402013-04-16 06:15:50 -04001/*
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>
Ingo Molnarb17b0152017-02-08 18:51:35 +010035#include <linux/sched/debug.h>
Ingo Molnar0881e7b2017-02-05 15:30:50 +010036#include <linux/sched/task.h>
Peter Hurley4898e6402013-04-16 06:15:50 -040037
38
39#ifdef CONFIG_DEBUG_LOCK_ALLOC
40# define __acq(l, s, t, r, c, n, i) \
41 lock_acquire(&(l)->dep_map, s, t, r, c, n, i)
42# define __rel(l, n, i) \
43 lock_release(&(l)->dep_map, n, i)
Oleg Nesterovfb9edbe2014-01-20 19:20:06 +010044#define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
45#define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
46#define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
47#define lockdep_release(l, n, i) __rel(l, n, i)
Peter Hurley4898e6402013-04-16 06:15:50 -040048#else
49# define lockdep_acquire(l, s, t, i) do { } while (0)
50# define lockdep_acquire_nest(l, s, t, n, i) do { } while (0)
51# define lockdep_acquire_read(l, s, t, i) do { } while (0)
52# define lockdep_release(l, n, i) do { } while (0)
53#endif
54
55#ifdef CONFIG_LOCK_STAT
56# define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_)
57#else
58# define lock_stat(_lock, stat) do { } while (0)
59#endif
60
61
62#if BITS_PER_LONG == 64
63# define LDSEM_ACTIVE_MASK 0xffffffffL
64#else
65# define LDSEM_ACTIVE_MASK 0x0000ffffL
66#endif
67
68#define LDSEM_UNLOCKED 0L
69#define LDSEM_ACTIVE_BIAS 1L
70#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
71#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
72#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
73
74struct ldsem_waiter {
75 struct list_head list;
76 struct task_struct *task;
77};
78
79static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
80{
81 return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
82}
83
Peter Hurleycf872772013-12-11 21:11:58 -050084/*
85 * ldsem_cmpxchg() updates @*old with the last-known sem->count value.
86 * Returns 1 if count was successfully changed; @*old will have @new value.
87 * Returns 0 if count was not changed; @*old will have most recent sem->count
88 */
Peter Hurley4898e6402013-04-16 06:15:50 -040089static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
90{
Peter Hurleycf872772013-12-11 21:11:58 -050091 long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
92 if (tmp == *old) {
93 *old = new;
94 return 1;
95 } else {
96 *old = tmp;
97 return 0;
98 }
Peter Hurley4898e6402013-04-16 06:15:50 -040099}
100
101/*
102 * Initialize an ldsem:
103 */
104void __init_ldsem(struct ld_semaphore *sem, const char *name,
105 struct lock_class_key *key)
106{
107#ifdef CONFIG_DEBUG_LOCK_ALLOC
108 /*
109 * Make sure we are not reinitializing a held semaphore:
110 */
111 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
112 lockdep_init_map(&sem->dep_map, name, key, 0);
113#endif
114 sem->count = LDSEM_UNLOCKED;
115 sem->wait_readers = 0;
116 raw_spin_lock_init(&sem->wait_lock);
117 INIT_LIST_HEAD(&sem->read_wait);
118 INIT_LIST_HEAD(&sem->write_wait);
119}
120
121static void __ldsem_wake_readers(struct ld_semaphore *sem)
122{
123 struct ldsem_waiter *waiter, *next;
124 struct task_struct *tsk;
125 long adjust, count;
126
127 /* Try to grant read locks to all readers on the read wait list.
128 * Note the 'active part' of the count is incremented by
129 * the number of readers before waking any processes up.
130 */
131 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
132 count = ldsem_atomic_update(adjust, sem);
133 do {
134 if (count > 0)
135 break;
136 if (ldsem_cmpxchg(&count, count - adjust, sem))
137 return;
138 } while (1);
139
140 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
141 tsk = waiter->task;
142 smp_mb();
143 waiter->task = NULL;
144 wake_up_process(tsk);
145 put_task_struct(tsk);
146 }
147 INIT_LIST_HEAD(&sem->read_wait);
148 sem->wait_readers = 0;
149}
150
151static inline int writer_trylock(struct ld_semaphore *sem)
152{
153 /* only wake this writer if the active part of the count can be
154 * transitioned from 0 -> 1
155 */
156 long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);
157 do {
158 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
159 return 1;
160 if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
161 return 0;
162 } while (1);
163}
164
165static void __ldsem_wake_writer(struct ld_semaphore *sem)
166{
167 struct ldsem_waiter *waiter;
168
169 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
170 wake_up_process(waiter->task);
171}
172
173/*
174 * handle the lock release when processes blocked on it that can now run
175 * - if we come here from up_xxxx(), then:
176 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
177 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
178 * - the spinlock must be held by the caller
179 * - woken process blocks are discarded from the list after having task zeroed
180 */
181static void __ldsem_wake(struct ld_semaphore *sem)
182{
183 if (!list_empty(&sem->write_wait))
184 __ldsem_wake_writer(sem);
185 else if (!list_empty(&sem->read_wait))
186 __ldsem_wake_readers(sem);
187}
188
189static void ldsem_wake(struct ld_semaphore *sem)
190{
191 unsigned long flags;
192
193 raw_spin_lock_irqsave(&sem->wait_lock, flags);
194 __ldsem_wake(sem);
195 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
196}
197
198/*
199 * wait for the read lock to be granted
200 */
201static struct ld_semaphore __sched *
202down_read_failed(struct ld_semaphore *sem, long count, long timeout)
203{
204 struct ldsem_waiter waiter;
Peter Hurley4898e6402013-04-16 06:15:50 -0400205 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
206
207 /* set up my own style of waitqueue */
208 raw_spin_lock_irq(&sem->wait_lock);
209
210 /* Try to reverse the lock attempt but if the count has changed
211 * so that reversing fails, check if there are are no waiters,
212 * and early-out if not */
213 do {
214 if (ldsem_cmpxchg(&count, count + adjust, sem))
215 break;
216 if (count > 0) {
217 raw_spin_unlock_irq(&sem->wait_lock);
218 return sem;
219 }
220 } while (1);
221
222 list_add_tail(&waiter.list, &sem->read_wait);
223 sem->wait_readers++;
224
Davidlohr Bueso5376f2e2017-01-03 13:43:12 -0800225 waiter.task = current;
226 get_task_struct(current);
Peter Hurley4898e6402013-04-16 06:15:50 -0400227
228 /* if there are no active locks, wake the new lock owner(s) */
229 if ((count & LDSEM_ACTIVE_MASK) == 0)
230 __ldsem_wake(sem);
231
232 raw_spin_unlock_irq(&sem->wait_lock);
233
234 /* wait to be given the lock */
235 for (;;) {
Davidlohr Bueso642fa442017-01-03 13:43:14 -0800236 set_current_state(TASK_UNINTERRUPTIBLE);
Peter Hurley4898e6402013-04-16 06:15:50 -0400237
238 if (!waiter.task)
239 break;
240 if (!timeout)
241 break;
242 timeout = schedule_timeout(timeout);
243 }
244
Davidlohr Bueso642fa442017-01-03 13:43:14 -0800245 __set_current_state(TASK_RUNNING);
Peter Hurley4898e6402013-04-16 06:15:50 -0400246
247 if (!timeout) {
248 /* lock timed out but check if this task was just
249 * granted lock ownership - if so, pretend there
250 * was no timeout; otherwise, cleanup lock wait */
251 raw_spin_lock_irq(&sem->wait_lock);
252 if (waiter.task) {
253 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
254 list_del(&waiter.list);
255 raw_spin_unlock_irq(&sem->wait_lock);
256 put_task_struct(waiter.task);
257 return NULL;
258 }
259 raw_spin_unlock_irq(&sem->wait_lock);
260 }
261
262 return sem;
263}
264
265/*
266 * wait for the write lock to be granted
267 */
268static struct ld_semaphore __sched *
269down_write_failed(struct ld_semaphore *sem, long count, long timeout)
270{
271 struct ldsem_waiter waiter;
Peter Hurley4898e6402013-04-16 06:15:50 -0400272 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
Davidlohr Bueso5376f2e2017-01-03 13:43:12 -0800292 waiter.task = current;
Peter Hurley4898e6402013-04-16 06:15:50 -0400293
Davidlohr Bueso642fa442017-01-03 13:43:14 -0800294 set_current_state(TASK_UNINTERRUPTIBLE);
Peter Hurley4898e6402013-04-16 06:15:50 -0400295 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);
Davidlohr Bueso642fa442017-01-03 13:43:14 -0800301 set_current_state(TASK_UNINTERRUPTIBLE);
Greg Kroah-Hartmanf9ce5cc2015-04-30 11:22:18 +0200302 locked = writer_trylock(sem);
303 if (locked)
Peter Hurley4898e6402013-04-16 06:15:50 -0400304 break;
305 }
306
307 if (!locked)
308 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
309 list_del(&waiter.list);
310 raw_spin_unlock_irq(&sem->wait_lock);
311
Davidlohr Bueso642fa442017-01-03 13:43:14 -0800312 __set_current_state(TASK_RUNNING);
Peter Hurley4898e6402013-04-16 06:15:50 -0400313
314 /* lock wait may have timed out */
315 if (!locked)
316 return NULL;
317 return sem;
318}
319
320
321
Denys Vlasenkofc0285f2015-10-27 18:46:45 +0100322static int __ldsem_down_read_nested(struct ld_semaphore *sem,
Peter Hurley4898e6402013-04-16 06:15:50 -0400323 int subclass, long timeout)
324{
325 long count;
326
327 lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
328
329 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
330 if (count <= 0) {
331 lock_stat(sem, contended);
332 if (!down_read_failed(sem, count, timeout)) {
333 lockdep_release(sem, 1, _RET_IP_);
334 return 0;
335 }
336 }
337 lock_stat(sem, acquired);
338 return 1;
339}
340
Denys Vlasenko5ef65042015-10-27 18:46:46 +0100341static int __ldsem_down_write_nested(struct ld_semaphore *sem,
Peter Hurley4898e6402013-04-16 06:15:50 -0400342 int subclass, long timeout)
343{
344 long count;
345
346 lockdep_acquire(sem, subclass, 0, _RET_IP_);
347
348 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
349 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
350 lock_stat(sem, contended);
351 if (!down_write_failed(sem, count, timeout)) {
352 lockdep_release(sem, 1, _RET_IP_);
353 return 0;
354 }
355 }
356 lock_stat(sem, acquired);
357 return 1;
358}
359
360
361/*
362 * lock for reading -- returns 1 if successful, 0 if timed out
363 */
364int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
365{
366 might_sleep();
367 return __ldsem_down_read_nested(sem, 0, timeout);
368}
369
370/*
371 * trylock for reading -- returns 1 if successful, 0 if contention
372 */
373int ldsem_down_read_trylock(struct ld_semaphore *sem)
374{
375 long count = sem->count;
376
377 while (count >= 0) {
378 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
379 lockdep_acquire_read(sem, 0, 1, _RET_IP_);
380 lock_stat(sem, acquired);
381 return 1;
382 }
383 }
384 return 0;
385}
386
387/*
388 * lock for writing -- returns 1 if successful, 0 if timed out
389 */
390int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
391{
392 might_sleep();
393 return __ldsem_down_write_nested(sem, 0, timeout);
394}
395
396/*
397 * trylock for writing -- returns 1 if successful, 0 if contention
398 */
399int ldsem_down_write_trylock(struct ld_semaphore *sem)
400{
401 long count = sem->count;
402
403 while ((count & LDSEM_ACTIVE_MASK) == 0) {
404 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
405 lockdep_acquire(sem, 0, 1, _RET_IP_);
406 lock_stat(sem, acquired);
407 return 1;
408 }
409 }
410 return 0;
411}
412
413/*
414 * release a read lock
415 */
416void ldsem_up_read(struct ld_semaphore *sem)
417{
418 long count;
419
420 lockdep_release(sem, 1, _RET_IP_);
421
422 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
423 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
424 ldsem_wake(sem);
425}
426
427/*
428 * release a write lock
429 */
430void ldsem_up_write(struct ld_semaphore *sem)
431{
432 long count;
433
434 lockdep_release(sem, 1, _RET_IP_);
435
436 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
437 if (count < 0)
438 ldsem_wake(sem);
439}
440
441
442#ifdef CONFIG_DEBUG_LOCK_ALLOC
443
444int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
445{
446 might_sleep();
447 return __ldsem_down_read_nested(sem, subclass, timeout);
448}
449
450int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
451 long timeout)
452{
453 might_sleep();
454 return __ldsem_down_write_nested(sem, subclass, timeout);
455}
456
457#endif