rwsem.h (7466B)
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/list.h> 15#include <linux/spinlock.h> 16#include <linux/atomic.h> 17#include <linux/err.h> 18 19#ifdef CONFIG_DEBUG_LOCK_ALLOC 20# define __RWSEM_DEP_MAP_INIT(lockname) \ 21 .dep_map = { \ 22 .name = #lockname, \ 23 .wait_type_inner = LD_WAIT_SLEEP, \ 24 }, 25#else 26# define __RWSEM_DEP_MAP_INIT(lockname) 27#endif 28 29#ifndef CONFIG_PREEMPT_RT 30 31#ifdef CONFIG_RWSEM_SPIN_ON_OWNER 32#include <linux/osq_lock.h> 33#endif 34 35/* 36 * For an uncontended rwsem, count and owner are the only fields a task 37 * needs to touch when acquiring the rwsem. So they are put next to each 38 * other to increase the chance that they will share the same cacheline. 39 * 40 * In a contended rwsem, the owner is likely the most frequently accessed 41 * field in the structure as the optimistic waiter that holds the osq lock 42 * will spin on owner. For an embedded rwsem, other hot fields in the 43 * containing structure should be moved further away from the rwsem to 44 * reduce the chance that they will share the same cacheline causing 45 * cacheline bouncing problem. 46 */ 47struct rw_semaphore { 48 atomic_long_t count; 49 /* 50 * Write owner or one of the read owners as well flags regarding 51 * the current state of the rwsem. Can be used as a speculative 52 * check to see if the write owner is running on the cpu. 53 */ 54 atomic_long_t owner; 55#ifdef CONFIG_RWSEM_SPIN_ON_OWNER 56 struct optimistic_spin_queue osq; /* spinner MCS lock */ 57#endif 58 raw_spinlock_t wait_lock; 59 struct list_head wait_list; 60#ifdef CONFIG_DEBUG_RWSEMS 61 void *magic; 62#endif 63#ifdef CONFIG_DEBUG_LOCK_ALLOC 64 struct lockdep_map dep_map; 65#endif 66}; 67 68/* In all implementations count != 0 means locked */ 69static inline int rwsem_is_locked(struct rw_semaphore *sem) 70{ 71 return atomic_long_read(&sem->count) != 0; 72} 73 74#define RWSEM_UNLOCKED_VALUE 0L 75#define __RWSEM_COUNT_INIT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE) 76 77/* Common initializer macros and functions */ 78 79#ifdef CONFIG_DEBUG_RWSEMS 80# define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname, 81#else 82# define __RWSEM_DEBUG_INIT(lockname) 83#endif 84 85#ifdef CONFIG_RWSEM_SPIN_ON_OWNER 86#define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED, 87#else 88#define __RWSEM_OPT_INIT(lockname) 89#endif 90 91#define __RWSEM_INITIALIZER(name) \ 92 { __RWSEM_COUNT_INIT(name), \ 93 .owner = ATOMIC_LONG_INIT(0), \ 94 __RWSEM_OPT_INIT(name) \ 95 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\ 96 .wait_list = LIST_HEAD_INIT((name).wait_list), \ 97 __RWSEM_DEBUG_INIT(name) \ 98 __RWSEM_DEP_MAP_INIT(name) } 99 100#define DECLARE_RWSEM(name) \ 101 struct rw_semaphore name = __RWSEM_INITIALIZER(name) 102 103extern void __init_rwsem(struct rw_semaphore *sem, const char *name, 104 struct lock_class_key *key); 105 106#define init_rwsem(sem) \ 107do { \ 108 static struct lock_class_key __key; \ 109 \ 110 __init_rwsem((sem), #sem, &__key); \ 111} while (0) 112 113/* 114 * This is the same regardless of which rwsem implementation that is being used. 115 * It is just a heuristic meant to be called by somebody already holding the 116 * rwsem to see if somebody from an incompatible type is wanting access to the 117 * lock. 118 */ 119static inline int rwsem_is_contended(struct rw_semaphore *sem) 120{ 121 return !list_empty(&sem->wait_list); 122} 123 124#else /* !CONFIG_PREEMPT_RT */ 125 126#include <linux/rwbase_rt.h> 127 128struct rw_semaphore { 129 struct rwbase_rt rwbase; 130#ifdef CONFIG_DEBUG_LOCK_ALLOC 131 struct lockdep_map dep_map; 132#endif 133}; 134 135#define __RWSEM_INITIALIZER(name) \ 136 { \ 137 .rwbase = __RWBASE_INITIALIZER(name), \ 138 __RWSEM_DEP_MAP_INIT(name) \ 139 } 140 141#define DECLARE_RWSEM(lockname) \ 142 struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname) 143 144extern void __init_rwsem(struct rw_semaphore *rwsem, const char *name, 145 struct lock_class_key *key); 146 147#define init_rwsem(sem) \ 148do { \ 149 static struct lock_class_key __key; \ 150 \ 151 __init_rwsem((sem), #sem, &__key); \ 152} while (0) 153 154static __always_inline int rwsem_is_locked(struct rw_semaphore *sem) 155{ 156 return rw_base_is_locked(&sem->rwbase); 157} 158 159static __always_inline int rwsem_is_contended(struct rw_semaphore *sem) 160{ 161 return rw_base_is_contended(&sem->rwbase); 162} 163 164#endif /* CONFIG_PREEMPT_RT */ 165 166/* 167 * The functions below are the same for all rwsem implementations including 168 * the RT specific variant. 169 */ 170 171/* 172 * lock for reading 173 */ 174extern void down_read(struct rw_semaphore *sem); 175extern int __must_check down_read_interruptible(struct rw_semaphore *sem); 176extern int __must_check down_read_killable(struct rw_semaphore *sem); 177 178/* 179 * trylock for reading -- returns 1 if successful, 0 if contention 180 */ 181extern int down_read_trylock(struct rw_semaphore *sem); 182 183/* 184 * lock for writing 185 */ 186extern void down_write(struct rw_semaphore *sem); 187extern int __must_check down_write_killable(struct rw_semaphore *sem); 188 189/* 190 * trylock for writing -- returns 1 if successful, 0 if contention 191 */ 192extern int down_write_trylock(struct rw_semaphore *sem); 193 194/* 195 * release a read lock 196 */ 197extern void up_read(struct rw_semaphore *sem); 198 199/* 200 * release a write lock 201 */ 202extern void up_write(struct rw_semaphore *sem); 203 204/* 205 * downgrade write lock to read lock 206 */ 207extern void downgrade_write(struct rw_semaphore *sem); 208 209#ifdef CONFIG_DEBUG_LOCK_ALLOC 210/* 211 * nested locking. NOTE: rwsems are not allowed to recurse 212 * (which occurs if the same task tries to acquire the same 213 * lock instance multiple times), but multiple locks of the 214 * same lock class might be taken, if the order of the locks 215 * is always the same. This ordering rule can be expressed 216 * to lockdep via the _nested() APIs, but enumerating the 217 * subclasses that are used. (If the nesting relationship is 218 * static then another method for expressing nested locking is 219 * the explicit definition of lock class keys and the use of 220 * lockdep_set_class() at lock initialization time. 221 * See Documentation/locking/lockdep-design.rst for more details.) 222 */ 223extern void down_read_nested(struct rw_semaphore *sem, int subclass); 224extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass); 225extern void down_write_nested(struct rw_semaphore *sem, int subclass); 226extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass); 227extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock); 228 229# define down_write_nest_lock(sem, nest_lock) \ 230do { \ 231 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ 232 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \ 233} while (0) 234 235/* 236 * Take/release a lock when not the owner will release it. 237 * 238 * [ This API should be avoided as much as possible - the 239 * proper abstraction for this case is completions. ] 240 */ 241extern void down_read_non_owner(struct rw_semaphore *sem); 242extern void up_read_non_owner(struct rw_semaphore *sem); 243#else 244# define down_read_nested(sem, subclass) down_read(sem) 245# define down_read_killable_nested(sem, subclass) down_read_killable(sem) 246# define down_write_nest_lock(sem, nest_lock) down_write(sem) 247# define down_write_nested(sem, subclass) down_write(sem) 248# define down_write_killable_nested(sem, subclass) down_write_killable(sem) 249# define down_read_non_owner(sem) down_read(sem) 250# define up_read_non_owner(sem) up_read(sem) 251#endif 252 253#endif /* _LINUX_RWSEM_H */