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#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 * trylock for writing -- returns 1 if successful, 0 if contention
371 */
372int ldsem_down_write_trylock(struct ld_semaphore *sem)
373{
374	long count = atomic_long_read(&sem->count);
375
376	while ((count & LDSEM_ACTIVE_MASK) == 0) {
377		if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_WRITE_BIAS)) {
378			rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
379			lock_acquired(&sem->dep_map, _RET_IP_);
380			return 1;
381		}
382	}
383	return 0;
384}
385
386/*
387 * release a read lock
388 */
389void ldsem_up_read(struct ld_semaphore *sem)
390{
391	long count;
392
393	rwsem_release(&sem->dep_map, _RET_IP_);
394
395	count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);
396	if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
397		ldsem_wake(sem);
398}
399
400/*
401 * release a write lock
402 */
403void ldsem_up_write(struct ld_semaphore *sem)
404{
405	long count;
406
407	rwsem_release(&sem->dep_map, _RET_IP_);
408
409	count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);
410	if (count < 0)
411		ldsem_wake(sem);
412}
413
414
415#ifdef CONFIG_DEBUG_LOCK_ALLOC
416
417int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
418{
419	might_sleep();
420	return __ldsem_down_read_nested(sem, subclass, timeout);
421}
422
423int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
424			    long timeout)
425{
426	might_sleep();
427	return __ldsem_down_write_nested(sem, subclass, timeout);
428}
429
430#endif
431