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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  */
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 /* In all implementations count != 0 means locked */
rwsem_is_locked(struct rw_semaphore * sem)57 static 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_COUNT_INIT(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 __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
79 #else
80 # define __RWSEM_DEBUG_INIT(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_COUNT_INIT(name),				\
91 	  .owner = ATOMIC_LONG_INIT(0),				\
92 	  __RWSEM_OPT_INIT(name)				\
93 	  .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
94 	  .wait_list = LIST_HEAD_INIT((name).wait_list),	\
95 	  __RWSEM_DEBUG_INIT(name)				\
96 	  __RWSEM_DEP_MAP_INIT(name) }
97 
98 #define DECLARE_RWSEM(name) \
99 	struct rw_semaphore name = __RWSEM_INITIALIZER(name)
100 
101 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
102 			 struct lock_class_key *key);
103 
104 #define init_rwsem(sem)						\
105 do {								\
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  */
rwsem_is_contended(struct rw_semaphore * sem)117 static 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  */
125 extern void down_read(struct rw_semaphore *sem);
126 extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
127 extern int __must_check down_read_killable(struct rw_semaphore *sem);
128 
129 /*
130  * trylock for reading -- returns 1 if successful, 0 if contention
131  */
132 extern int down_read_trylock(struct rw_semaphore *sem);
133 
134 /*
135  * lock for writing
136  */
137 extern void down_write(struct rw_semaphore *sem);
138 extern int __must_check down_write_killable(struct rw_semaphore *sem);
139 
140 /*
141  * trylock for writing -- returns 1 if successful, 0 if contention
142  */
143 extern int down_write_trylock(struct rw_semaphore *sem);
144 
145 /*
146  * release a read lock
147  */
148 extern void up_read(struct rw_semaphore *sem);
149 
150 /*
151  * release a write lock
152  */
153 extern void up_write(struct rw_semaphore *sem);
154 
155 /*
156  * downgrade write lock to read lock
157  */
158 extern void downgrade_write(struct rw_semaphore *sem);
159 
160 #ifdef CONFIG_DEBUG_LOCK_ALLOC
161 /*
162  * nested locking. NOTE: rwsems are not allowed to recurse
163  * (which occurs if the same task tries to acquire the same
164  * lock instance multiple times), but multiple locks of the
165  * same lock class might be taken, if the order of the locks
166  * is always the same. This ordering rule can be expressed
167  * to lockdep via the _nested() APIs, but enumerating the
168  * subclasses that are used. (If the nesting relationship is
169  * static then another method for expressing nested locking is
170  * the explicit definition of lock class keys and the use of
171  * lockdep_set_class() at lock initialization time.
172  * See Documentation/locking/lockdep-design.rst for more details.)
173  */
174 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
175 extern int __must_check down_read_killable_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_read_killable_nested(sem, subclass)	down_read_killable(sem)
197 # define down_write_nest_lock(sem, nest_lock)	down_write(sem)
198 # define down_write_nested(sem, subclass)	down_write(sem)
199 # define down_write_killable_nested(sem, subclass)	down_write_killable(sem)
200 # define down_read_non_owner(sem)		down_read(sem)
201 # define up_read_non_owner(sem)			up_read(sem)
202 #endif
203 
204 #endif /* _LINUX_RWSEM_H */
205