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/kernel.h>
15 #include <linux/list.h>
16 #include <linux/spinlock.h>
17 #include <linux/atomic.h>
18 #include <linux/err.h>
19 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
20 #include <linux/osq_lock.h>
21 #endif
22
23 /*
24 * For an uncontended rwsem, count and owner are the only fields a task
25 * needs to touch when acquiring the rwsem. So they are put next to each
26 * other to increase the chance that they will share the same cacheline.
27 *
28 * In a contended rwsem, the owner is likely the most frequently accessed
29 * field in the structure as the optimistic waiter that holds the osq lock
30 * will spin on owner. For an embedded rwsem, other hot fields in the
31 * containing structure should be moved further away from the rwsem to
32 * reduce the chance that they will share the same cacheline causing
33 * cacheline bouncing problem.
34 */
35 struct rw_semaphore {
36 atomic_long_t count;
37 /*
38 * Write owner or one of the read owners as well flags regarding
39 * the current state of the rwsem. Can be used as a speculative
40 * check to see if the write owner is running on the cpu.
41 */
42 atomic_long_t owner;
43 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
44 struct optimistic_spin_queue osq; /* spinner MCS lock */
45 #endif
46 raw_spinlock_t wait_lock;
47 struct list_head wait_list;
48 #ifdef CONFIG_DEBUG_RWSEMS
49 void *magic;
50 #endif
51 #ifdef CONFIG_DEBUG_LOCK_ALLOC
52 struct lockdep_map dep_map;
53 #endif
54 };
55
56 /*
57 * Setting all bits of the owner field except bit 0 will indicate
58 * that the rwsem is writer-owned with an unknown owner.
59 */
60 #define RWSEM_OWNER_UNKNOWN (-2L)
61
62 /* In all implementations count != 0 means locked */
rwsem_is_locked(struct rw_semaphore * sem)63 static inline int rwsem_is_locked(struct rw_semaphore *sem)
64 {
65 return atomic_long_read(&sem->count) != 0;
66 }
67
68 #define RWSEM_UNLOCKED_VALUE 0L
69 #define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
70
71 /* Common initializer macros and functions */
72
73 #ifdef CONFIG_DEBUG_LOCK_ALLOC
74 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
75 #else
76 # define __RWSEM_DEP_MAP_INIT(lockname)
77 #endif
78
79 #ifdef CONFIG_DEBUG_RWSEMS
80 # define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
81 #else
82 # define __DEBUG_RWSEM_INITIALIZER(lockname)
83 #endif
84
85 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
86 #define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
87 #else
88 #define __RWSEM_OPT_INIT(lockname)
89 #endif
90
91 #define __RWSEM_INITIALIZER(name) \
92 { __RWSEM_INIT_COUNT(name), \
93 .owner = ATOMIC_LONG_INIT(0), \
94 .wait_list = LIST_HEAD_INIT((name).wait_list), \
95 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
96 __RWSEM_OPT_INIT(name) \
97 __DEBUG_RWSEM_INITIALIZER(name) \
98 __RWSEM_DEP_MAP_INIT(name) }
99
100 #define DECLARE_RWSEM(name) \
101 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
102
103 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
104 struct lock_class_key *key);
105
106 #define init_rwsem(sem) \
107 do { \
108 static struct lock_class_key __key; \
109 \
110 __init_rwsem((sem), #sem, &__key); \
111 } while (0)
112
113 /*
114 * This is the same regardless of which rwsem implementation that is being used.
115 * It is just a heuristic meant to be called by somebody alreadying holding the
116 * rwsem to see if somebody from an incompatible type is wanting access to the
117 * lock.
118 */
rwsem_is_contended(struct rw_semaphore * sem)119 static inline int rwsem_is_contended(struct rw_semaphore *sem)
120 {
121 return !list_empty(&sem->wait_list);
122 }
123
124 /*
125 * lock for reading
126 */
127 extern void down_read(struct rw_semaphore *sem);
128 extern int __must_check down_read_killable(struct rw_semaphore *sem);
129
130 /*
131 * trylock for reading -- returns 1 if successful, 0 if contention
132 */
133 extern int down_read_trylock(struct rw_semaphore *sem);
134
135 /*
136 * lock for writing
137 */
138 extern void down_write(struct rw_semaphore *sem);
139 extern int __must_check down_write_killable(struct rw_semaphore *sem);
140
141 /*
142 * trylock for writing -- returns 1 if successful, 0 if contention
143 */
144 extern int down_write_trylock(struct rw_semaphore *sem);
145
146 /*
147 * release a read lock
148 */
149 extern void up_read(struct rw_semaphore *sem);
150
151 /*
152 * release a write lock
153 */
154 extern void up_write(struct rw_semaphore *sem);
155
156 /*
157 * downgrade write lock to read lock
158 */
159 extern void downgrade_write(struct rw_semaphore *sem);
160
161 #ifdef CONFIG_DEBUG_LOCK_ALLOC
162 /*
163 * nested locking. NOTE: rwsems are not allowed to recurse
164 * (which occurs if the same task tries to acquire the same
165 * lock instance multiple times), but multiple locks of the
166 * same lock class might be taken, if the order of the locks
167 * is always the same. This ordering rule can be expressed
168 * to lockdep via the _nested() APIs, but enumerating the
169 * subclasses that are used. (If the nesting relationship is
170 * static then another method for expressing nested locking is
171 * the explicit definition of lock class keys and the use of
172 * lockdep_set_class() at lock initialization time.
173 * See Documentation/locking/lockdep-design.rst for more details.)
174 */
175 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
176 extern void down_write_nested(struct rw_semaphore *sem, int subclass);
177 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
178 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
179
180 # define down_write_nest_lock(sem, nest_lock) \
181 do { \
182 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
183 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
184 } while (0);
185
186 /*
187 * Take/release a lock when not the owner will release it.
188 *
189 * [ This API should be avoided as much as possible - the
190 * proper abstraction for this case is completions. ]
191 */
192 extern void down_read_non_owner(struct rw_semaphore *sem);
193 extern void up_read_non_owner(struct rw_semaphore *sem);
194 #else
195 # define down_read_nested(sem, subclass) down_read(sem)
196 # define down_write_nest_lock(sem, nest_lock) down_write(sem)
197 # define down_write_nested(sem, subclass) down_write(sem)
198 # define down_write_killable_nested(sem, subclass) down_write_killable(sem)
199 # define down_read_non_owner(sem) down_read(sem)
200 # define up_read_non_owner(sem) up_read(sem)
201 #endif
202
203 #endif /* _LINUX_RWSEM_H */
204