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 #include <linux/android_vendor.h>
23
24 /*
25 * For an uncontended rwsem, count and owner are the only fields a task
26 * needs to touch when acquiring the rwsem. So they are put next to each
27 * other to increase the chance that they will share the same cacheline.
28 *
29 * In a contended rwsem, the owner is likely the most frequently accessed
30 * field in the structure as the optimistic waiter that holds the osq lock
31 * will spin on owner. For an embedded rwsem, other hot fields in the
32 * containing structure should be moved further away from the rwsem to
33 * reduce the chance that they will share the same cacheline causing
34 * cacheline bouncing problem.
35 */
36 struct rw_semaphore {
37 atomic_long_t count;
38 /*
39 * Write owner or one of the read owners as well flags regarding
40 * the current state of the rwsem. Can be used as a speculative
41 * check to see if the write owner is running on the cpu.
42 */
43 atomic_long_t owner;
44 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
45 struct optimistic_spin_queue osq; /* spinner MCS lock */
46 #endif
47 raw_spinlock_t wait_lock;
48 struct list_head wait_list;
49 #ifdef CONFIG_DEBUG_RWSEMS
50 void *magic;
51 #endif
52 #ifdef CONFIG_DEBUG_LOCK_ALLOC
53 struct lockdep_map dep_map;
54 #endif
55 ANDROID_VENDOR_DATA(1);
56 ANDROID_OEM_DATA_ARRAY(1, 2);
57 };
58
59 enum rwsem_waiter_type {
60 RWSEM_WAITING_FOR_WRITE,
61 RWSEM_WAITING_FOR_READ
62 };
63
64 struct rwsem_waiter {
65 struct list_head list;
66 struct task_struct *task;
67 enum rwsem_waiter_type type;
68 unsigned long timeout;
69 unsigned long last_rowner;
70 };
71
72 /* In all implementations count != 0 means locked */
rwsem_is_locked(struct rw_semaphore * sem)73 static inline int rwsem_is_locked(struct rw_semaphore *sem)
74 {
75 return atomic_long_read(&sem->count) != 0;
76 }
77
78 #define RWSEM_UNLOCKED_VALUE 0L
79 #define __RWSEM_COUNT_INIT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
80
81 /* Common initializer macros and functions */
82
83 #ifdef CONFIG_DEBUG_LOCK_ALLOC
84 # define __RWSEM_DEP_MAP_INIT(lockname) \
85 .dep_map = { \
86 .name = #lockname, \
87 .wait_type_inner = LD_WAIT_SLEEP, \
88 },
89 #else
90 # define __RWSEM_DEP_MAP_INIT(lockname)
91 #endif
92
93 #ifdef CONFIG_DEBUG_RWSEMS
94 # define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
95 #else
96 # define __RWSEM_DEBUG_INIT(lockname)
97 #endif
98
99 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
100 #define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
101 #else
102 #define __RWSEM_OPT_INIT(lockname)
103 #endif
104
105 #define __RWSEM_INITIALIZER(name) \
106 { __RWSEM_COUNT_INIT(name), \
107 .owner = ATOMIC_LONG_INIT(0), \
108 __RWSEM_OPT_INIT(name) \
109 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
110 .wait_list = LIST_HEAD_INIT((name).wait_list), \
111 __RWSEM_DEBUG_INIT(name) \
112 __RWSEM_DEP_MAP_INIT(name) }
113
114 #define DECLARE_RWSEM(name) \
115 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
116
117 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
118 struct lock_class_key *key);
119
120 #define init_rwsem(sem) \
121 do { \
122 static struct lock_class_key __key; \
123 \
124 __init_rwsem((sem), #sem, &__key); \
125 } while (0)
126
127 /*
128 * This is the same regardless of which rwsem implementation that is being used.
129 * It is just a heuristic meant to be called by somebody alreadying holding the
130 * rwsem to see if somebody from an incompatible type is wanting access to the
131 * lock.
132 */
rwsem_is_contended(struct rw_semaphore * sem)133 static inline int rwsem_is_contended(struct rw_semaphore *sem)
134 {
135 return !list_empty(&sem->wait_list);
136 }
137
138 /*
139 * lock for reading
140 */
141 extern void down_read(struct rw_semaphore *sem);
142 extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
143 extern int __must_check down_read_killable(struct rw_semaphore *sem);
144
145 /*
146 * trylock for reading -- returns 1 if successful, 0 if contention
147 */
148 extern int down_read_trylock(struct rw_semaphore *sem);
149
150 /*
151 * lock for writing
152 */
153 extern void down_write(struct rw_semaphore *sem);
154 extern int __must_check down_write_killable(struct rw_semaphore *sem);
155
156 /*
157 * trylock for writing -- returns 1 if successful, 0 if contention
158 */
159 extern int down_write_trylock(struct rw_semaphore *sem);
160
161 /*
162 * release a read lock
163 */
164 extern void up_read(struct rw_semaphore *sem);
165
166 /*
167 * release a write lock
168 */
169 extern void up_write(struct rw_semaphore *sem);
170
171 /*
172 * downgrade write lock to read lock
173 */
174 extern void downgrade_write(struct rw_semaphore *sem);
175
176 #ifdef CONFIG_DEBUG_LOCK_ALLOC
177 /*
178 * nested locking. NOTE: rwsems are not allowed to recurse
179 * (which occurs if the same task tries to acquire the same
180 * lock instance multiple times), but multiple locks of the
181 * same lock class might be taken, if the order of the locks
182 * is always the same. This ordering rule can be expressed
183 * to lockdep via the _nested() APIs, but enumerating the
184 * subclasses that are used. (If the nesting relationship is
185 * static then another method for expressing nested locking is
186 * the explicit definition of lock class keys and the use of
187 * lockdep_set_class() at lock initialization time.
188 * See Documentation/locking/lockdep-design.rst for more details.)
189 */
190 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
191 extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
192 extern void down_write_nested(struct rw_semaphore *sem, int subclass);
193 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
194 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
195
196 # define down_write_nest_lock(sem, nest_lock) \
197 do { \
198 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
199 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
200 } while (0);
201
202 /*
203 * Take/release a lock when not the owner will release it.
204 *
205 * [ This API should be avoided as much as possible - the
206 * proper abstraction for this case is completions. ]
207 */
208 extern void down_read_non_owner(struct rw_semaphore *sem);
209 extern void up_read_non_owner(struct rw_semaphore *sem);
210 #else
211 # define down_read_nested(sem, subclass) down_read(sem)
212 # define down_read_killable_nested(sem, subclass) down_read_killable(sem)
213 # define down_write_nest_lock(sem, nest_lock) down_write(sem)
214 # define down_write_nested(sem, subclass) down_write(sem)
215 # define down_write_killable_nested(sem, subclass) down_write_killable(sem)
216 # define down_read_non_owner(sem) down_read(sem)
217 # define up_read_non_owner(sem) up_read(sem)
218 #endif
219
220 #endif /* _LINUX_RWSEM_H */
221