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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 <[email protected]>.
18 *
19 * The scheduling policy was copied and modified from lib/rwsem.c
20 * Written by David Howells ([email protected]).
21 *
22 * This implementation incorporates the write lock stealing work of
23 * Michel Lespinasse <[email protected]>.
24 *
25 * Copyright (C) 2013 Peter Hurley <[email protected]>
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#if BITS_PER_LONG == 64
38# define LDSEM_ACTIVE_MASK 0xffffffffL
39#else
40# define LDSEM_ACTIVE_MASK 0x0000ffffL
41#endif
42
43#define LDSEM_UNLOCKED 0L
44#define LDSEM_ACTIVE_BIAS 1L
45#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
46#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
47#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
48
49struct ldsem_waiter {
50 struct list_head list;
51 struct task_struct *task;
52};
53
54/*
55 * Initialize an ldsem:
56 */
57void __init_ldsem(struct ld_semaphore *sem, const char *name,
58 struct lock_class_key *key)
59{
60#ifdef CONFIG_DEBUG_LOCK_ALLOC
61 /*
62 * Make sure we are not reinitializing a held semaphore:
63 */
64 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
65 lockdep_init_map(&sem->dep_map, name, key, 0);
66#endif
67 atomic_long_set(&sem->count, LDSEM_UNLOCKED);
68 sem->wait_readers = 0;
69 raw_spin_lock_init(&sem->wait_lock);
70 INIT_LIST_HEAD(&sem->read_wait);
71 INIT_LIST_HEAD(&sem->write_wait);
72}
73
74static void __ldsem_wake_readers(struct ld_semaphore *sem)
75{
76 struct ldsem_waiter *waiter, *next;
77 struct task_struct *tsk;
78 long adjust, count;
79
80 /*
81 * Try to grant read locks to all readers on the read wait list.
82 * Note the 'active part' of the count is incremented by
83 * the number of readers before waking any processes up.
84 */
85 adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
86 count = atomic_long_add_return(adjust, &sem->count);
87 do {
88 if (count > 0)
89 break;
90 if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust))
91 return;
92 } while (1);
93
94 list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
95 tsk = waiter->task;
96 smp_store_release(&waiter->task, NULL);
97 wake_up_process(tsk);
98 put_task_struct(tsk);
99 }
100 INIT_LIST_HEAD(&sem->read_wait);
101 sem->wait_readers = 0;
102}
103
104static inline int writer_trylock(struct ld_semaphore *sem)
105{
106 /*
107 * Only wake this writer if the active part of the count can be
108 * transitioned from 0 -> 1
109 */
110 long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count);
111 do {
112 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
113 return 1;
114 if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS))
115 return 0;
116 } while (1);
117}
118
119static void __ldsem_wake_writer(struct ld_semaphore *sem)
120{
121 struct ldsem_waiter *waiter;
122
123 waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
124 wake_up_process(waiter->task);
125}
126
127/*
128 * handle the lock release when processes blocked on it that can now run
129 * - if we come here from up_xxxx(), then:
130 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
131 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
132 * - the spinlock must be held by the caller
133 * - woken process blocks are discarded from the list after having task zeroed
134 */
135static void __ldsem_wake(struct ld_semaphore *sem)
136{
137 if (!list_empty(&sem->write_wait))
138 __ldsem_wake_writer(sem);
139 else if (!list_empty(&sem->read_wait))
140 __ldsem_wake_readers(sem);
141}
142
143static void ldsem_wake(struct ld_semaphore *sem)
144{
145 unsigned long flags;
146
147 raw_spin_lock_irqsave(&sem->wait_lock, flags);
148 __ldsem_wake(sem);
149 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
150}
151
152/*
153 * wait for the read lock to be granted
154 */
155static struct ld_semaphore __sched *
156down_read_failed(struct ld_semaphore *sem, long count, long timeout)
157{
158 struct ldsem_waiter waiter;
159 long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
160
161 /* set up my own style of waitqueue */
162 raw_spin_lock_irq(&sem->wait_lock);
163
164 /*
165 * Try to reverse the lock attempt but if the count has changed
166 * so that reversing fails, check if there are no waiters,
167 * and early-out if not
168 */
169 do {
170 if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) {
171 count += adjust;
172 break;
173 }
174 if (count > 0) {
175 raw_spin_unlock_irq(&sem->wait_lock);
176 return sem;
177 }
178 } while (1);
179
180 list_add_tail(&waiter.list, &sem->read_wait);
181 sem->wait_readers++;
182
183 waiter.task = current;
184 get_task_struct(current);
185
186 /* if there are no active locks, wake the new lock owner(s) */
187 if ((count & LDSEM_ACTIVE_MASK) == 0)
188 __ldsem_wake(sem);
189
190 raw_spin_unlock_irq(&sem->wait_lock);
191
192 /* wait to be given the lock */
193 for (;;) {
194 set_current_state(TASK_UNINTERRUPTIBLE);
195
196 if (!smp_load_acquire(&waiter.task))
197 break;
198 if (!timeout)
199 break;
200 timeout = schedule_timeout(timeout);
201 }
202
203 __set_current_state(TASK_RUNNING);
204
205 if (!timeout) {
206 /*
207 * Lock timed out but check if this task was just
208 * granted lock ownership - if so, pretend there
209 * was no timeout; otherwise, cleanup lock wait.
210 */
211 raw_spin_lock_irq(&sem->wait_lock);
212 if (waiter.task) {
213 atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
214 sem->wait_readers--;
215 list_del(&waiter.list);
216 raw_spin_unlock_irq(&sem->wait_lock);
217 put_task_struct(waiter.task);
218 return NULL;
219 }
220 raw_spin_unlock_irq(&sem->wait_lock);
221 }
222
223 return sem;
224}
225
226/*
227 * wait for the write lock to be granted
228 */
229static struct ld_semaphore __sched *
230down_write_failed(struct ld_semaphore *sem, long count, long timeout)
231{
232 struct ldsem_waiter waiter;
233 long adjust = -LDSEM_ACTIVE_BIAS;
234 int locked = 0;
235
236 /* set up my own style of waitqueue */
237 raw_spin_lock_irq(&sem->wait_lock);
238
239 /*
240 * Try to reverse the lock attempt but if the count has changed
241 * so that reversing fails, check if the lock is now owned,
242 * and early-out if so.
243 */
244 do {
245 if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust))
246 break;
247 if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
248 raw_spin_unlock_irq(&sem->wait_lock);
249 return sem;
250 }
251 } while (1);
252
253 list_add_tail(&waiter.list, &sem->write_wait);
254
255 waiter.task = current;
256
257 set_current_state(TASK_UNINTERRUPTIBLE);
258 for (;;) {
259 if (!timeout)
260 break;
261 raw_spin_unlock_irq(&sem->wait_lock);
262 timeout = schedule_timeout(timeout);
263 raw_spin_lock_irq(&sem->wait_lock);
264 set_current_state(TASK_UNINTERRUPTIBLE);
265 locked = writer_trylock(sem);
266 if (locked)
267 break;
268 }
269
270 if (!locked)
271 atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
272 list_del(&waiter.list);
273
274 /*
275 * In case of timeout, wake up every reader who gave the right of way
276 * to writer. Prevent separation readers into two groups:
277 * one that helds semaphore and another that sleeps.
278 * (in case of no contention with a writer)
279 */
280 if (!locked && list_empty(&sem->write_wait))
281 __ldsem_wake_readers(sem);
282
283 raw_spin_unlock_irq(&sem->wait_lock);
284
285 __set_current_state(TASK_RUNNING);
286
287 /* lock wait may have timed out */
288 if (!locked)
289 return NULL;
290 return sem;
291}
292
293
294
295static int __ldsem_down_read_nested(struct ld_semaphore *sem,
296 int subclass, long timeout)
297{
298 long count;
299
300 rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
301
302 count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count);
303 if (count <= 0) {
304 lock_contended(&sem->dep_map, _RET_IP_);
305 if (!down_read_failed(sem, count, timeout)) {
306 rwsem_release(&sem->dep_map, _RET_IP_);
307 return 0;
308 }
309 }
310 lock_acquired(&sem->dep_map, _RET_IP_);
311 return 1;
312}
313
314static int __ldsem_down_write_nested(struct ld_semaphore *sem,
315 int subclass, long timeout)
316{
317 long count;
318
319 rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
320
321 count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count);
322 if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
323 lock_contended(&sem->dep_map, _RET_IP_);
324 if (!down_write_failed(sem, count, timeout)) {
325 rwsem_release(&sem->dep_map, _RET_IP_);
326 return 0;
327 }
328 }
329 lock_acquired(&sem->dep_map, _RET_IP_);
330 return 1;
331}
332
333
334/*
335 * lock for reading -- returns 1 if successful, 0 if timed out
336 */
337int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
338{
339 might_sleep();
340 return __ldsem_down_read_nested(sem, 0, timeout);
341}
342
343/*
344 * trylock for reading -- returns 1 if successful, 0 if contention
345 */
346int ldsem_down_read_trylock(struct ld_semaphore *sem)
347{
348 long count = atomic_long_read(&sem->count);
349
350 while (count >= 0) {
351 if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) {
352 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
353 lock_acquired(&sem->dep_map, _RET_IP_);
354 return 1;
355 }
356 }
357 return 0;
358}
359
360/*
361 * lock for writing -- returns 1 if successful, 0 if timed out
362 */
363int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
364{
365 might_sleep();
366 return __ldsem_down_write_nested(sem, 0, timeout);
367}
368
369/*
370 * release a read lock
371 */
372void ldsem_up_read(struct ld_semaphore *sem)
373{
374 long count;
375
376 rwsem_release(&sem->dep_map, _RET_IP_);
377
378 count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);
379 if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
380 ldsem_wake(sem);
381}
382
383/*
384 * release a write lock
385 */
386void ldsem_up_write(struct ld_semaphore *sem)
387{
388 long count;
389
390 rwsem_release(&sem->dep_map, _RET_IP_);
391
392 count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);
393 if (count < 0)
394 ldsem_wake(sem);
395}
396
397
398#ifdef CONFIG_DEBUG_LOCK_ALLOC
399
400int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
401{
402 might_sleep();
403 return __ldsem_down_read_nested(sem, subclass, timeout);
404}
405
406int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
407 long timeout)
408{
409 might_sleep();
410 return __ldsem_down_write_nested(sem, subclass, timeout);
411}
412
413#endif