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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_LOCK_ALLOC
49 struct lockdep_map dep_map;
50#endif
51};
52
53/*
54 * Setting all bits of the owner field except bit 0 will indicate
55 * that the rwsem is writer-owned with an unknown owner.
56 */
57#define RWSEM_OWNER_UNKNOWN (-2L)
58
59/* In all implementations count != 0 means locked */
60static inline int rwsem_is_locked(struct rw_semaphore *sem)
61{
62 return atomic_long_read(&sem->count) != 0;
63}
64
65#define RWSEM_UNLOCKED_VALUE 0L
66#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
67
68/* Common initializer macros and functions */
69
70#ifdef CONFIG_DEBUG_LOCK_ALLOC
71# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
72#else
73# define __RWSEM_DEP_MAP_INIT(lockname)
74#endif
75
76#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
77#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
78#else
79#define __RWSEM_OPT_INIT(lockname)
80#endif
81
82#define __RWSEM_INITIALIZER(name) \
83 { __RWSEM_INIT_COUNT(name), \
84 .owner = ATOMIC_LONG_INIT(0), \
85 .wait_list = LIST_HEAD_INIT((name).wait_list), \
86 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
87 __RWSEM_OPT_INIT(name) \
88 __RWSEM_DEP_MAP_INIT(name) }
89
90#define DECLARE_RWSEM(name) \
91 struct rw_semaphore name = __RWSEM_INITIALIZER(name)
92
93extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
94 struct lock_class_key *key);
95
96#define init_rwsem(sem) \
97do { \
98 static struct lock_class_key __key; \
99 \
100 __init_rwsem((sem), #sem, &__key); \
101} while (0)
102
103/*
104 * This is the same regardless of which rwsem implementation that is being used.
105 * It is just a heuristic meant to be called by somebody alreadying holding the
106 * rwsem to see if somebody from an incompatible type is wanting access to the
107 * lock.
108 */
109static inline int rwsem_is_contended(struct rw_semaphore *sem)
110{
111 return !list_empty(&sem->wait_list);
112}
113
114/*
115 * lock for reading
116 */
117extern void down_read(struct rw_semaphore *sem);
118extern int __must_check down_read_killable(struct rw_semaphore *sem);
119
120/*
121 * trylock for reading -- returns 1 if successful, 0 if contention
122 */
123extern int down_read_trylock(struct rw_semaphore *sem);
124
125/*
126 * lock for writing
127 */
128extern void down_write(struct rw_semaphore *sem);
129extern int __must_check down_write_killable(struct rw_semaphore *sem);
130
131/*
132 * trylock for writing -- returns 1 if successful, 0 if contention
133 */
134extern int down_write_trylock(struct rw_semaphore *sem);
135
136/*
137 * release a read lock
138 */
139extern void up_read(struct rw_semaphore *sem);
140
141/*
142 * release a write lock
143 */
144extern void up_write(struct rw_semaphore *sem);
145
146/*
147 * downgrade write lock to read lock
148 */
149extern void downgrade_write(struct rw_semaphore *sem);
150
151#ifdef CONFIG_DEBUG_LOCK_ALLOC
152/*
153 * nested locking. NOTE: rwsems are not allowed to recurse
154 * (which occurs if the same task tries to acquire the same
155 * lock instance multiple times), but multiple locks of the
156 * same lock class might be taken, if the order of the locks
157 * is always the same. This ordering rule can be expressed
158 * to lockdep via the _nested() APIs, but enumerating the
159 * subclasses that are used. (If the nesting relationship is
160 * static then another method for expressing nested locking is
161 * the explicit definition of lock class keys and the use of
162 * lockdep_set_class() at lock initialization time.
163 * See Documentation/locking/lockdep-design.rst for more details.)
164 */
165extern void down_read_nested(struct rw_semaphore *sem, int subclass);
166extern void down_write_nested(struct rw_semaphore *sem, int subclass);
167extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
168extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
169
170# define down_write_nest_lock(sem, nest_lock) \
171do { \
172 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
173 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
174} while (0);
175
176/*
177 * Take/release a lock when not the owner will release it.
178 *
179 * [ This API should be avoided as much as possible - the
180 * proper abstraction for this case is completions. ]
181 */
182extern void down_read_non_owner(struct rw_semaphore *sem);
183extern void up_read_non_owner(struct rw_semaphore *sem);
184#else
185# define down_read_nested(sem, subclass) down_read(sem)
186# define down_write_nest_lock(sem, nest_lock) down_write(sem)
187# define down_write_nested(sem, subclass) down_write(sem)
188# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
189# define down_read_non_owner(sem) down_read(sem)
190# define up_read_non_owner(sem) up_read(sem)
191#endif
192
193#endif /* _LINUX_RWSEM_H */