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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/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 */
35struct 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/* In all implementations count != 0 means locked */
57static inline int rwsem_is_locked(struct rw_semaphore *sem)
58{
59 return atomic_long_read(&sem->count) != 0;
60}
61
62#define RWSEM_UNLOCKED_VALUE 0L
63#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
64
65/* Common initializer macros and functions */
66
67#ifdef CONFIG_DEBUG_LOCK_ALLOC
68# define __RWSEM_DEP_MAP_INIT(lockname) \
69 , .dep_map = { \
70 .name = #lockname, \
71 .wait_type_inner = LD_WAIT_SLEEP, \
72 }
73#else
74# define __RWSEM_DEP_MAP_INIT(lockname)
75#endif
76
77#ifdef CONFIG_DEBUG_RWSEMS
78# define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
79#else
80# define __DEBUG_RWSEM_INITIALIZER(lockname)
81#endif
82
83#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
84#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
85#else
86#define __RWSEM_OPT_INIT(lockname)
87#endif
88
89#define __RWSEM_INITIALIZER(name) \
90 { __RWSEM_INIT_COUNT(name), \
91 .owner = ATOMIC_LONG_INIT(0), \
92 .wait_list = LIST_HEAD_INIT((name).wait_list), \
93 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
94 __RWSEM_OPT_INIT(name) \
95 __DEBUG_RWSEM_INITIALIZER(name) \
96 __RWSEM_DEP_MAP_INIT(name) }
97
98#define DECLARE_RWSEM(name) \
99 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
100
101extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
102 struct lock_class_key *key);
103
104#define init_rwsem(sem) \
105do { \
106 static struct lock_class_key __key; \
107 \
108 __init_rwsem((sem), #sem, &__key); \
109} while (0)
110
111/*
112 * This is the same regardless of which rwsem implementation that is being used.
113 * It is just a heuristic meant to be called by somebody alreadying holding the
114 * rwsem to see if somebody from an incompatible type is wanting access to the
115 * lock.
116 */
117static inline int rwsem_is_contended(struct rw_semaphore *sem)
118{
119 return !list_empty(&sem->wait_list);
120}
121
122/*
123 * lock for reading
124 */
125extern void down_read(struct rw_semaphore *sem);
126extern int __must_check down_read_killable(struct rw_semaphore *sem);
127
128/*
129 * trylock for reading -- returns 1 if successful, 0 if contention
130 */
131extern int down_read_trylock(struct rw_semaphore *sem);
132
133/*
134 * lock for writing
135 */
136extern void down_write(struct rw_semaphore *sem);
137extern int __must_check down_write_killable(struct rw_semaphore *sem);
138
139/*
140 * trylock for writing -- returns 1 if successful, 0 if contention
141 */
142extern int down_write_trylock(struct rw_semaphore *sem);
143
144/*
145 * release a read lock
146 */
147extern void up_read(struct rw_semaphore *sem);
148
149/*
150 * release a write lock
151 */
152extern void up_write(struct rw_semaphore *sem);
153
154/*
155 * downgrade write lock to read lock
156 */
157extern void downgrade_write(struct rw_semaphore *sem);
158
159#ifdef CONFIG_DEBUG_LOCK_ALLOC
160/*
161 * nested locking. NOTE: rwsems are not allowed to recurse
162 * (which occurs if the same task tries to acquire the same
163 * lock instance multiple times), but multiple locks of the
164 * same lock class might be taken, if the order of the locks
165 * is always the same. This ordering rule can be expressed
166 * to lockdep via the _nested() APIs, but enumerating the
167 * subclasses that are used. (If the nesting relationship is
168 * static then another method for expressing nested locking is
169 * the explicit definition of lock class keys and the use of
170 * lockdep_set_class() at lock initialization time.
171 * See Documentation/locking/lockdep-design.rst for more details.)
172 */
173extern void down_read_nested(struct rw_semaphore *sem, int subclass);
174extern void down_write_nested(struct rw_semaphore *sem, int subclass);
175extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
176extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
177
178# define down_write_nest_lock(sem, nest_lock) \
179do { \
180 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
181 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
182} while (0);
183
184/*
185 * Take/release a lock when not the owner will release it.
186 *
187 * [ This API should be avoided as much as possible - the
188 * proper abstraction for this case is completions. ]
189 */
190extern void down_read_non_owner(struct rw_semaphore *sem);
191extern void up_read_non_owner(struct rw_semaphore *sem);
192#else
193# define down_read_nested(sem, subclass) down_read(sem)
194# define down_write_nest_lock(sem, nest_lock) down_write(sem)
195# define down_write_nested(sem, subclass) down_write(sem)
196# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
197# define down_read_non_owner(sem) down_read(sem)
198# define up_read_non_owner(sem) up_read(sem)
199#endif
200
201#endif /* _LINUX_RWSEM_H */