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1/* SPDX-License-Identifier: GPL-2.0 */
2/* rwsem.h: R/W semaphores, public interface
3 *
4 * Written by David Howells (dhowells@redhat.com).
5 * Derived from asm-i386/semaphore.h
6 */
7
8#ifndef _LINUX_RWSEM_H
9#define _LINUX_RWSEM_H
10
11#include <linux/linkage.h>
12
13#include <linux/types.h>
14#include <linux/list.h>
15#include <linux/spinlock.h>
16#include <linux/atomic.h>
17#include <linux/err.h>
18#include <linux/cleanup.h>
19
20#ifdef CONFIG_DEBUG_LOCK_ALLOC
21# define __RWSEM_DEP_MAP_INIT(lockname) \
22 .dep_map = { \
23 .name = #lockname, \
24 .wait_type_inner = LD_WAIT_SLEEP, \
25 },
26#else
27# define __RWSEM_DEP_MAP_INIT(lockname)
28#endif
29
30#ifndef CONFIG_PREEMPT_RT
31
32#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
33#include <linux/osq_lock.h>
34#endif
35
36/*
37 * For an uncontended rwsem, count and owner are the only fields a task
38 * needs to touch when acquiring the rwsem. So they are put next to each
39 * other to increase the chance that they will share the same cacheline.
40 *
41 * In a contended rwsem, the owner is likely the most frequently accessed
42 * field in the structure as the optimistic waiter that holds the osq lock
43 * will spin on owner. For an embedded rwsem, other hot fields in the
44 * containing structure should be moved further away from the rwsem to
45 * reduce the chance that they will share the same cacheline causing
46 * cacheline bouncing problem.
47 */
48struct rw_semaphore {
49 atomic_long_t count;
50 /*
51 * Write owner or one of the read owners as well flags regarding
52 * the current state of the rwsem. Can be used as a speculative
53 * check to see if the write owner is running on the cpu.
54 */
55 atomic_long_t owner;
56#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
57 struct optimistic_spin_queue osq; /* spinner MCS lock */
58#endif
59 raw_spinlock_t wait_lock;
60 struct list_head wait_list;
61#ifdef CONFIG_DEBUG_RWSEMS
62 void *magic;
63#endif
64#ifdef CONFIG_DEBUG_LOCK_ALLOC
65 struct lockdep_map dep_map;
66#endif
67};
68
69/* In all implementations count != 0 means locked */
70static inline int rwsem_is_locked(struct rw_semaphore *sem)
71{
72 return atomic_long_read(&sem->count) != 0;
73}
74
75#define RWSEM_UNLOCKED_VALUE 0L
76#define __RWSEM_COUNT_INIT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
77
78/* Common initializer macros and functions */
79
80#ifdef CONFIG_DEBUG_RWSEMS
81# define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
82#else
83# define __RWSEM_DEBUG_INIT(lockname)
84#endif
85
86#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
87#define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
88#else
89#define __RWSEM_OPT_INIT(lockname)
90#endif
91
92#define __RWSEM_INITIALIZER(name) \
93 { __RWSEM_COUNT_INIT(name), \
94 .owner = ATOMIC_LONG_INIT(0), \
95 __RWSEM_OPT_INIT(name) \
96 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
97 .wait_list = LIST_HEAD_INIT((name).wait_list), \
98 __RWSEM_DEBUG_INIT(name) \
99 __RWSEM_DEP_MAP_INIT(name) }
100
101#define DECLARE_RWSEM(name) \
102 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
103
104extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
105 struct lock_class_key *key);
106
107#define init_rwsem(sem) \
108do { \
109 static struct lock_class_key __key; \
110 \
111 __init_rwsem((sem), #sem, &__key); \
112} while (0)
113
114/*
115 * This is the same regardless of which rwsem implementation that is being used.
116 * It is just a heuristic meant to be called by somebody already holding the
117 * rwsem to see if somebody from an incompatible type is wanting access to the
118 * lock.
119 */
120static inline int rwsem_is_contended(struct rw_semaphore *sem)
121{
122 return !list_empty(&sem->wait_list);
123}
124
125#else /* !CONFIG_PREEMPT_RT */
126
127#include <linux/rwbase_rt.h>
128
129struct rw_semaphore {
130 struct rwbase_rt rwbase;
131#ifdef CONFIG_DEBUG_LOCK_ALLOC
132 struct lockdep_map dep_map;
133#endif
134};
135
136#define __RWSEM_INITIALIZER(name) \
137 { \
138 .rwbase = __RWBASE_INITIALIZER(name), \
139 __RWSEM_DEP_MAP_INIT(name) \
140 }
141
142#define DECLARE_RWSEM(lockname) \
143 struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
144
145extern void __init_rwsem(struct rw_semaphore *rwsem, const char *name,
146 struct lock_class_key *key);
147
148#define init_rwsem(sem) \
149do { \
150 static struct lock_class_key __key; \
151 \
152 __init_rwsem((sem), #sem, &__key); \
153} while (0)
154
155static __always_inline int rwsem_is_locked(struct rw_semaphore *sem)
156{
157 return rw_base_is_locked(&sem->rwbase);
158}
159
160static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
161{
162 return rw_base_is_contended(&sem->rwbase);
163}
164
165#endif /* CONFIG_PREEMPT_RT */
166
167/*
168 * The functions below are the same for all rwsem implementations including
169 * the RT specific variant.
170 */
171
172/*
173 * lock for reading
174 */
175extern void down_read(struct rw_semaphore *sem);
176extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
177extern int __must_check down_read_killable(struct rw_semaphore *sem);
178
179/*
180 * trylock for reading -- returns 1 if successful, 0 if contention
181 */
182extern int down_read_trylock(struct rw_semaphore *sem);
183
184/*
185 * lock for writing
186 */
187extern void down_write(struct rw_semaphore *sem);
188extern int __must_check down_write_killable(struct rw_semaphore *sem);
189
190/*
191 * trylock for writing -- returns 1 if successful, 0 if contention
192 */
193extern int down_write_trylock(struct rw_semaphore *sem);
194
195/*
196 * release a read lock
197 */
198extern void up_read(struct rw_semaphore *sem);
199
200/*
201 * release a write lock
202 */
203extern void up_write(struct rw_semaphore *sem);
204
205DEFINE_GUARD(rwsem_read, struct rw_semaphore *, down_read(_T), up_read(_T))
206DEFINE_GUARD(rwsem_write, struct rw_semaphore *, down_write(_T), up_write(_T))
207
208DEFINE_FREE(up_read, struct rw_semaphore *, if (_T) up_read(_T))
209DEFINE_FREE(up_write, struct rw_semaphore *, if (_T) up_write(_T))
210
211
212/*
213 * downgrade write lock to read lock
214 */
215extern void downgrade_write(struct rw_semaphore *sem);
216
217#ifdef CONFIG_DEBUG_LOCK_ALLOC
218/*
219 * nested locking. NOTE: rwsems are not allowed to recurse
220 * (which occurs if the same task tries to acquire the same
221 * lock instance multiple times), but multiple locks of the
222 * same lock class might be taken, if the order of the locks
223 * is always the same. This ordering rule can be expressed
224 * to lockdep via the _nested() APIs, but enumerating the
225 * subclasses that are used. (If the nesting relationship is
226 * static then another method for expressing nested locking is
227 * the explicit definition of lock class keys and the use of
228 * lockdep_set_class() at lock initialization time.
229 * See Documentation/locking/lockdep-design.rst for more details.)
230 */
231extern void down_read_nested(struct rw_semaphore *sem, int subclass);
232extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
233extern void down_write_nested(struct rw_semaphore *sem, int subclass);
234extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
235extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
236
237# define down_write_nest_lock(sem, nest_lock) \
238do { \
239 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
240 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
241} while (0)
242
243/*
244 * Take/release a lock when not the owner will release it.
245 *
246 * [ This API should be avoided as much as possible - the
247 * proper abstraction for this case is completions. ]
248 */
249extern void down_read_non_owner(struct rw_semaphore *sem);
250extern void up_read_non_owner(struct rw_semaphore *sem);
251#else
252# define down_read_nested(sem, subclass) down_read(sem)
253# define down_read_killable_nested(sem, subclass) down_read_killable(sem)
254# define down_write_nest_lock(sem, nest_lock) down_write(sem)
255# define down_write_nested(sem, subclass) down_write(sem)
256# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
257# define down_read_non_owner(sem) down_read(sem)
258# define up_read_non_owner(sem) up_read(sem)
259#endif
260
261#endif /* _LINUX_RWSEM_H */