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 */
48 struct 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 #define RWSEM_UNLOCKED_VALUE 0UL
70 #define RWSEM_WRITER_LOCKED (1UL << 0)
71 #define __RWSEM_COUNT_INIT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
72
73 static inline int rwsem_is_locked(struct rw_semaphore *sem)
74 {
75 return atomic_long_read(&sem->count) != RWSEM_UNLOCKED_VALUE;
76 }
77
78 static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
79 {
80 WARN_ON(atomic_long_read(&sem->count) == RWSEM_UNLOCKED_VALUE);
81 }
82
83 static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
84 {
85 WARN_ON(!(atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED));
86 }
87
88 /* Common initializer macros and functions */
89
90 #ifdef CONFIG_DEBUG_RWSEMS
91 # define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
92 #else
93 # define __RWSEM_DEBUG_INIT(lockname)
94 #endif
95
96 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
97 #define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
98 #else
99 #define __RWSEM_OPT_INIT(lockname)
100 #endif
101
102 #define __RWSEM_INITIALIZER(name) \
103 { __RWSEM_COUNT_INIT(name), \
104 .owner = ATOMIC_LONG_INIT(0), \
105 __RWSEM_OPT_INIT(name) \
106 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
107 .wait_list = LIST_HEAD_INIT((name).wait_list), \
108 __RWSEM_DEBUG_INIT(name) \
109 __RWSEM_DEP_MAP_INIT(name) }
110
111 #define DECLARE_RWSEM(name) \
112 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
113
114 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
115 struct lock_class_key *key);
116
117 #define init_rwsem(sem) \
118 do { \
119 static struct lock_class_key __key; \
120 \
121 __init_rwsem((sem), #sem, &__key); \
122 } while (0)
123
124 /*
125 * This is the same regardless of which rwsem implementation that is being used.
126 * It is just a heuristic meant to be called by somebody already holding the
127 * rwsem to see if somebody from an incompatible type is wanting access to the
128 * lock.
129 */
130 static inline int rwsem_is_contended(struct rw_semaphore *sem)
131 {
132 return !list_empty(&sem->wait_list);
133 }
134
135 #else /* !CONFIG_PREEMPT_RT */
136
137 #include <linux/rwbase_rt.h>
138
139 struct rw_semaphore {
140 struct rwbase_rt rwbase;
141 #ifdef CONFIG_DEBUG_LOCK_ALLOC
142 struct lockdep_map dep_map;
143 #endif
144 };
145
146 #define __RWSEM_INITIALIZER(name) \
147 { \
148 .rwbase = __RWBASE_INITIALIZER(name), \
149 __RWSEM_DEP_MAP_INIT(name) \
150 }
151
152 #define DECLARE_RWSEM(lockname) \
153 struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
154
155 extern void __init_rwsem(struct rw_semaphore *rwsem, const char *name,
156 struct lock_class_key *key);
157
158 #define init_rwsem(sem) \
159 do { \
160 static struct lock_class_key __key; \
161 \
162 __init_rwsem((sem), #sem, &__key); \
163 } while (0)
164
165 static __always_inline int rwsem_is_locked(const struct rw_semaphore *sem)
166 {
167 return rw_base_is_locked(&sem->rwbase);
168 }
169
170 static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
171 {
172 WARN_ON(!rwsem_is_locked(sem));
173 }
174
175 static __always_inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
176 {
177 WARN_ON(!rw_base_is_write_locked(&sem->rwbase));
178 }
179
180 static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
181 {
182 return rw_base_is_contended(&sem->rwbase);
183 }
184
185 #endif /* CONFIG_PREEMPT_RT */
186
187 /*
188 * The functions below are the same for all rwsem implementations including
189 * the RT specific variant.
190 */
191
192 static inline void rwsem_assert_held(const struct rw_semaphore *sem)
193 {
194 if (IS_ENABLED(CONFIG_LOCKDEP))
195 lockdep_assert_held(sem);
196 else
197 rwsem_assert_held_nolockdep(sem);
198 }
199
200 static inline void rwsem_assert_held_write(const struct rw_semaphore *sem)
201 {
202 if (IS_ENABLED(CONFIG_LOCKDEP))
203 lockdep_assert_held_write(sem);
204 else
205 rwsem_assert_held_write_nolockdep(sem);
206 }
207
208 /*
209 * lock for reading
210 */
211 extern void down_read(struct rw_semaphore *sem);
212 extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
213 extern int __must_check down_read_killable(struct rw_semaphore *sem);
214
215 /*
216 * trylock for reading -- returns 1 if successful, 0 if contention
217 */
218 extern int down_read_trylock(struct rw_semaphore *sem);
219
220 /*
221 * lock for writing
222 */
223 extern void down_write(struct rw_semaphore *sem);
224 extern int __must_check down_write_killable(struct rw_semaphore *sem);
225
226 /*
227 * trylock for writing -- returns 1 if successful, 0 if contention
228 */
229 extern int down_write_trylock(struct rw_semaphore *sem);
230
231 /*
232 * release a read lock
233 */
234 extern void up_read(struct rw_semaphore *sem);
235
236 /*
237 * release a write lock
238 */
239 extern void up_write(struct rw_semaphore *sem);
240
241 DEFINE_GUARD(rwsem_read, struct rw_semaphore *, down_read(_T), up_read(_T))
242 DEFINE_GUARD_COND(rwsem_read, _try, down_read_trylock(_T))
243 DEFINE_GUARD_COND(rwsem_read, _intr, down_read_interruptible(_T) == 0)
244
245 DEFINE_GUARD(rwsem_write, struct rw_semaphore *, down_write(_T), up_write(_T))
246 DEFINE_GUARD_COND(rwsem_write, _try, down_write_trylock(_T))
247
248 /*
249 * downgrade write lock to read lock
250 */
251 extern void downgrade_write(struct rw_semaphore *sem);
252
253 #ifdef CONFIG_DEBUG_LOCK_ALLOC
254 /*
255 * nested locking. NOTE: rwsems are not allowed to recurse
256 * (which occurs if the same task tries to acquire the same
257 * lock instance multiple times), but multiple locks of the
258 * same lock class might be taken, if the order of the locks
259 * is always the same. This ordering rule can be expressed
260 * to lockdep via the _nested() APIs, but enumerating the
261 * subclasses that are used. (If the nesting relationship is
262 * static then another method for expressing nested locking is
263 * the explicit definition of lock class keys and the use of
264 * lockdep_set_class() at lock initialization time.
265 * See Documentation/locking/lockdep-design.rst for more details.)
266 */
267 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
268 extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
269 extern void down_write_nested(struct rw_semaphore *sem, int subclass);
270 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
271 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
272
273 # define down_write_nest_lock(sem, nest_lock) \
274 do { \
275 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
276 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
277 } while (0)
278
279 /*
280 * Take/release a lock when not the owner will release it.
281 *
282 * [ This API should be avoided as much as possible - the
283 * proper abstraction for this case is completions. ]
284 */
285 extern void down_read_non_owner(struct rw_semaphore *sem);
286 extern void up_read_non_owner(struct rw_semaphore *sem);
287 #else
288 # define down_read_nested(sem, subclass) down_read(sem)
289 # define down_read_killable_nested(sem, subclass) down_read_killable(sem)
290 # define down_write_nest_lock(sem, nest_lock) down_write(sem)
291 # define down_write_nested(sem, subclass) down_write(sem)
292 # define down_write_killable_nested(sem, subclass) down_write_killable(sem)
293 # define down_read_non_owner(sem) down_read(sem)
294 # define up_read_non_owner(sem) up_read(sem)
295 #endif
296
297 #endif /* _LINUX_RWSEM_H */
298
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