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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 locked = writer_trylock(sem);
303 if (locked)
304 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
312 __set_task_state(tsk, TASK_RUNNING);
313
314 /* lock wait may have timed out */
315 if (!locked)
316 return NULL;
317 return sem;
318}
319
320
321
322static int __ldsem_down_read_nested(struct ld_semaphore *sem,
323 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
341static int __ldsem_down_write_nested(struct ld_semaphore *sem,
342 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