1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_PERCPU_RWSEM_H
3 #define _LINUX_PERCPU_RWSEM_H
4 
5 #include <linux/atomic.h>
6 #include <linux/percpu.h>
7 #include <linux/rcuwait.h>
8 #include <linux/wait.h>
9 #include <linux/rcu_sync.h>
10 #include <linux/lockdep.h>
11 
12 struct percpu_rw_semaphore {
13 	struct rcu_sync		rss;
14 	unsigned int __percpu	*read_count;
15 	struct rcuwait		writer;
16 	wait_queue_head_t	waiters;
17 	atomic_t		block;
18 #ifdef CONFIG_DEBUG_LOCK_ALLOC
19 	struct lockdep_map	dep_map;
20 #endif
21 };
22 
23 #ifdef CONFIG_DEBUG_LOCK_ALLOC
24 #define __PERCPU_RWSEM_DEP_MAP_INIT(lockname)	.dep_map = { .name = #lockname },
25 #else
26 #define __PERCPU_RWSEM_DEP_MAP_INIT(lockname)
27 #endif
28 
29 #define __DEFINE_PERCPU_RWSEM(name, is_static)				\
30 static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name);		\
31 is_static struct percpu_rw_semaphore name = {				\
32 	.rss = __RCU_SYNC_INITIALIZER(name.rss),			\
33 	.read_count = &__percpu_rwsem_rc_##name,			\
34 	.writer = __RCUWAIT_INITIALIZER(name.writer),			\
35 	.waiters = __WAIT_QUEUE_HEAD_INITIALIZER(name.waiters),		\
36 	.block = ATOMIC_INIT(0),					\
37 	__PERCPU_RWSEM_DEP_MAP_INIT(name)				\
38 }
39 
40 #define DEFINE_PERCPU_RWSEM(name)		\
41 	__DEFINE_PERCPU_RWSEM(name, /* not static */)
42 #define DEFINE_STATIC_PERCPU_RWSEM(name)	\
43 	__DEFINE_PERCPU_RWSEM(name, static)
44 
45 extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
46 
percpu_down_read(struct percpu_rw_semaphore * sem)47 static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
48 {
49 	might_sleep();
50 
51 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
52 
53 	preempt_disable();
54 	/*
55 	 * We are in an RCU-sched read-side critical section, so the writer
56 	 * cannot both change sem->state from readers_fast and start checking
57 	 * counters while we are here. So if we see !sem->state, we know that
58 	 * the writer won't be checking until we're past the preempt_enable()
59 	 * and that once the synchronize_rcu() is done, the writer will see
60 	 * anything we did within this RCU-sched read-size critical section.
61 	 */
62 	if (likely(rcu_sync_is_idle(&sem->rss)))
63 		this_cpu_inc(*sem->read_count);
64 	else
65 		__percpu_down_read(sem, false); /* Unconditional memory barrier */
66 	/*
67 	 * The preempt_enable() prevents the compiler from
68 	 * bleeding the critical section out.
69 	 */
70 	preempt_enable();
71 }
72 
percpu_down_read_trylock(struct percpu_rw_semaphore * sem)73 static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
74 {
75 	bool ret = true;
76 
77 	preempt_disable();
78 	/*
79 	 * Same as in percpu_down_read().
80 	 */
81 	if (likely(rcu_sync_is_idle(&sem->rss)))
82 		this_cpu_inc(*sem->read_count);
83 	else
84 		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
85 	preempt_enable();
86 	/*
87 	 * The barrier() from preempt_enable() prevents the compiler from
88 	 * bleeding the critical section out.
89 	 */
90 
91 	if (ret)
92 		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
93 
94 	return ret;
95 }
96 
percpu_up_read(struct percpu_rw_semaphore * sem)97 static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
98 {
99 	rwsem_release(&sem->dep_map, _RET_IP_);
100 
101 	preempt_disable();
102 	/*
103 	 * Same as in percpu_down_read().
104 	 */
105 	if (likely(rcu_sync_is_idle(&sem->rss))) {
106 		this_cpu_dec(*sem->read_count);
107 	} else {
108 		/*
109 		 * slowpath; reader will only ever wake a single blocked
110 		 * writer.
111 		 */
112 		smp_mb(); /* B matches C */
113 		/*
114 		 * In other words, if they see our decrement (presumably to
115 		 * aggregate zero, as that is the only time it matters) they
116 		 * will also see our critical section.
117 		 */
118 		this_cpu_dec(*sem->read_count);
119 		rcuwait_wake_up(&sem->writer);
120 	}
121 	preempt_enable();
122 }
123 
124 extern void percpu_down_write(struct percpu_rw_semaphore *);
125 extern void percpu_up_write(struct percpu_rw_semaphore *);
126 
127 extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
128 				const char *, struct lock_class_key *);
129 
130 extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
131 
132 #define percpu_init_rwsem(sem)					\
133 ({								\
134 	static struct lock_class_key rwsem_key;			\
135 	__percpu_init_rwsem(sem, #sem, &rwsem_key);		\
136 })
137 
138 #define percpu_rwsem_is_held(sem)	lockdep_is_held(sem)
139 #define percpu_rwsem_assert_held(sem)	lockdep_assert_held(sem)
140 
percpu_rwsem_release(struct percpu_rw_semaphore * sem,bool read,unsigned long ip)141 static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
142 					bool read, unsigned long ip)
143 {
144 	lock_release(&sem->dep_map, ip);
145 }
146 
percpu_rwsem_acquire(struct percpu_rw_semaphore * sem,bool read,unsigned long ip)147 static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
148 					bool read, unsigned long ip)
149 {
150 	lock_acquire(&sem->dep_map, 0, 1, read, 1, NULL, ip);
151 }
152 
153 #endif
154