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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Ldisc rw semaphore
4 *
5 * The ldisc semaphore is semantically a rw_semaphore but which enforces
6 * an alternate policy, namely:
7 * 1) Supports lock wait timeouts
8 * 2) Write waiter has priority
9 * 3) Downgrading is not supported
10 *
11 * Implementation notes:
12 * 1) Upper half of semaphore count is a wait count (differs from rwsem
13 * in that rwsem normalizes the upper half to the wait bias)
14 * 2) Lacks overflow checking
15 *
16 * The generic counting was copied and modified from include/asm-generic/rwsem.h
17 * by Paul Mackerras <paulus@samba.org>.
18 *
19 * The scheduling policy was copied and modified from lib/rwsem.c
20 * Written by David Howells (dhowells@redhat.com).
21 *
22 * This implementation incorporates the write lock stealing work of
23 * Michel Lespinasse <walken@google.com>.
24 *
25 * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com>
26 */
27
28#include <linux/list.h>
29#include <linux/spinlock.h>
30#include <linux/atomic.h>
31#include <linux/tty.h>
32#include <linux/sched.h>
33#include <linux/sched/debug.h>
34#include <linux/sched/task.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 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
204
205 /* set up my own style of waitqueue */
206 raw_spin_lock_irq(&sem->wait_lock);
207
208 /* Try to reverse the lock attempt but if the count has changed
209 * so that reversing fails, check if there are are no waiters,
210 * and early-out if not */
211 do {
212 if (ldsem_cmpxchg(&count, count + adjust, sem))
213 break;
214 if (count > 0) {
215 raw_spin_unlock_irq(&sem->wait_lock);
216 return sem;
217 }
218 } while (1);
219
220 list_add_tail(&waiter.list, &sem->read_wait);
221 sem->wait_readers++;
222
223 waiter.task = current;
224 get_task_struct(current);
225
226 /* if there are no active locks, wake the new lock owner(s) */
227 if ((count & LDSEM_ACTIVE_MASK) == 0)
228 __ldsem_wake(sem);
229
230 raw_spin_unlock_irq(&sem->wait_lock);
231
232 /* wait to be given the lock */
233 for (;;) {
234 set_current_state(TASK_UNINTERRUPTIBLE);
235
236 if (!waiter.task)
237 break;
238 if (!timeout)
239 break;
240 timeout = schedule_timeout(timeout);
241 }
242
243 __set_current_state(TASK_RUNNING);
244
245 if (!timeout) {
246 /* lock timed out but check if this task was just
247 * granted lock ownership - if so, pretend there
248 * was no timeout; otherwise, cleanup lock wait */
249 raw_spin_lock_irq(&sem->wait_lock);
250 if (waiter.task) {
251 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
252 list_del(&waiter.list);
253 raw_spin_unlock_irq(&sem->wait_lock);
254 put_task_struct(waiter.task);
255 return NULL;
256 }
257 raw_spin_unlock_irq(&sem->wait_lock);
258 }
259
260 return sem;
261}
262
263/*
264 * wait for the write lock to be granted
265 */
266static struct ld_semaphore __sched *
267down_write_failed(struct ld_semaphore *sem, long count, long timeout)
268{
269 struct ldsem_waiter waiter;
270 long adjust = -LDSEM_ACTIVE_BIAS;
271 int locked = 0;
272
273 /* set up my own style of waitqueue */
274 raw_spin_lock_irq(&sem->wait_lock);
275
276 /* Try to reverse the lock attempt but if the count has changed
277 * so that reversing fails, check if the lock is now owned,
278 * and early-out if so */
279 do {
280 if (ldsem_cmpxchg(&count, count + adjust, sem))
281 break;
282 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
283 raw_spin_unlock_irq(&sem->wait_lock);
284 return sem;
285 }
286 } while (1);
287
288 list_add_tail(&waiter.list, &sem->write_wait);
289
290 waiter.task = current;
291
292 set_current_state(TASK_UNINTERRUPTIBLE);
293 for (;;) {
294 if (!timeout)
295 break;
296 raw_spin_unlock_irq(&sem->wait_lock);
297 timeout = schedule_timeout(timeout);
298 raw_spin_lock_irq(&sem->wait_lock);
299 set_current_state(TASK_UNINTERRUPTIBLE);
300 locked = writer_trylock(sem);
301 if (locked)
302 break;
303 }
304
305 if (!locked)
306 ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
307 list_del(&waiter.list);
308 raw_spin_unlock_irq(&sem->wait_lock);
309
310 __set_current_state(TASK_RUNNING);
311
312 /* lock wait may have timed out */
313 if (!locked)
314 return NULL;
315 return sem;
316}
317
318
319
320static int __ldsem_down_read_nested(struct ld_semaphore *sem,
321 int subclass, long timeout)
322{
323 long count;
324
325 lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
326
327 count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
328 if (count <= 0) {
329 lock_stat(sem, contended);
330 if (!down_read_failed(sem, count, timeout)) {
331 lockdep_release(sem, 1, _RET_IP_);
332 return 0;
333 }
334 }
335 lock_stat(sem, acquired);
336 return 1;
337}
338
339static int __ldsem_down_write_nested(struct ld_semaphore *sem,
340 int subclass, long timeout)
341{
342 long count;
343
344 lockdep_acquire(sem, subclass, 0, _RET_IP_);
345
346 count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
347 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
348 lock_stat(sem, contended);
349 if (!down_write_failed(sem, count, timeout)) {
350 lockdep_release(sem, 1, _RET_IP_);
351 return 0;
352 }
353 }
354 lock_stat(sem, acquired);
355 return 1;
356}
357
358
359/*
360 * lock for reading -- returns 1 if successful, 0 if timed out
361 */
362int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
363{
364 might_sleep();
365 return __ldsem_down_read_nested(sem, 0, timeout);
366}
367
368/*
369 * trylock for reading -- returns 1 if successful, 0 if contention
370 */
371int ldsem_down_read_trylock(struct ld_semaphore *sem)
372{
373 long count = sem->count;
374
375 while (count >= 0) {
376 if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
377 lockdep_acquire_read(sem, 0, 1, _RET_IP_);
378 lock_stat(sem, acquired);
379 return 1;
380 }
381 }
382 return 0;
383}
384
385/*
386 * lock for writing -- returns 1 if successful, 0 if timed out
387 */
388int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
389{
390 might_sleep();
391 return __ldsem_down_write_nested(sem, 0, timeout);
392}
393
394/*
395 * trylock for writing -- returns 1 if successful, 0 if contention
396 */
397int ldsem_down_write_trylock(struct ld_semaphore *sem)
398{
399 long count = sem->count;
400
401 while ((count & LDSEM_ACTIVE_MASK) == 0) {
402 if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
403 lockdep_acquire(sem, 0, 1, _RET_IP_);
404 lock_stat(sem, acquired);
405 return 1;
406 }
407 }
408 return 0;
409}
410
411/*
412 * release a read lock
413 */
414void ldsem_up_read(struct ld_semaphore *sem)
415{
416 long count;
417
418 lockdep_release(sem, 1, _RET_IP_);
419
420 count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
421 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
422 ldsem_wake(sem);
423}
424
425/*
426 * release a write lock
427 */
428void ldsem_up_write(struct ld_semaphore *sem)
429{
430 long count;
431
432 lockdep_release(sem, 1, _RET_IP_);
433
434 count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
435 if (count < 0)
436 ldsem_wake(sem);
437}
438
439
440#ifdef CONFIG_DEBUG_LOCK_ALLOC
441
442int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
443{
444 might_sleep();
445 return __ldsem_down_read_nested(sem, subclass, timeout);
446}
447
448int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
449 long timeout)
450{
451 might_sleep();
452 return __ldsem_down_write_nested(sem, subclass, timeout);
453}
454
455#endif