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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 : */
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 : /* In all implementations count != 0 means locked */
70 : static inline int rwsem_is_locked(struct rw_semaphore *sem)
71 : {
72 10 : 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 :
104 : extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
105 : struct lock_class_key *key);
106 :
107 : #define init_rwsem(sem) \
108 : do { \
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 : */
120 : static inline int rwsem_is_contended(struct rw_semaphore *sem)
121 : {
122 0 : return !list_empty(&sem->wait_list);
123 : }
124 :
125 : #else /* !CONFIG_PREEMPT_RT */
126 :
127 : #include <linux/rwbase_rt.h>
128 :
129 : struct 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 :
145 : extern void __init_rwsem(struct rw_semaphore *rwsem, const char *name,
146 : struct lock_class_key *key);
147 :
148 : #define init_rwsem(sem) \
149 : do { \
150 : static struct lock_class_key __key; \
151 : \
152 : __init_rwsem((sem), #sem, &__key); \
153 : } while (0)
154 :
155 : static __always_inline int rwsem_is_locked(struct rw_semaphore *sem)
156 : {
157 : return rw_base_is_locked(&sem->rwbase);
158 : }
159 :
160 : static __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 : */
175 : extern void down_read(struct rw_semaphore *sem);
176 : extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
177 : extern int __must_check down_read_killable(struct rw_semaphore *sem);
178 :
179 : /*
180 : * trylock for reading -- returns 1 if successful, 0 if contention
181 : */
182 : extern int down_read_trylock(struct rw_semaphore *sem);
183 :
184 : /*
185 : * lock for writing
186 : */
187 : extern void down_write(struct rw_semaphore *sem);
188 : extern int __must_check down_write_killable(struct rw_semaphore *sem);
189 :
190 : /*
191 : * trylock for writing -- returns 1 if successful, 0 if contention
192 : */
193 : extern int down_write_trylock(struct rw_semaphore *sem);
194 :
195 : /*
196 : * release a read lock
197 : */
198 : extern void up_read(struct rw_semaphore *sem);
199 :
200 : /*
201 : * release a write lock
202 : */
203 : extern void up_write(struct rw_semaphore *sem);
204 :
205 : DEFINE_GUARD(rwsem_read, struct rw_semaphore *, down_read(_T), up_read(_T))
206 : DEFINE_GUARD(rwsem_write, struct rw_semaphore *, down_write(_T), up_write(_T))
207 :
208 : DEFINE_FREE(up_read, struct rw_semaphore *, if (_T) up_read(_T))
209 : DEFINE_FREE(up_write, struct rw_semaphore *, if (_T) up_write(_T))
210 :
211 :
212 : /*
213 : * downgrade write lock to read lock
214 : */
215 : extern 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 : */
231 : extern void down_read_nested(struct rw_semaphore *sem, int subclass);
232 : extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
233 : extern void down_write_nested(struct rw_semaphore *sem, int subclass);
234 : extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
235 : extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
236 :
237 : # define down_write_nest_lock(sem, nest_lock) \
238 : do { \
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 : */
249 : extern void down_read_non_owner(struct rw_semaphore *sem);
250 : extern 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 */
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