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 */ 47 struct 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 */ 69 static 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) 102 103 extern void __init_rwsem(struct rw_semaphore *sem, const char *name, 104 struct lock_class_key *key); 105 106 #define init_rwsem(sem) \ 107 do { \ 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 */ 119 static 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> 127 128 struct 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) 143 144 extern void __init_rwsem(struct rw_semaphore *rwsem, const char *name, 145 struct lock_class_key *key); 146 147 #define init_rwsem(sem) \ 148 do { \ 149 static struct lock_class_key __key; \ 150 \ 151 __init_rwsem((sem), #sem, &__key); \ 152 } while (0) 153 154 static __always_inline int rwsem_is_locked(struct rw_semaphore *sem) 155 { 156 return rw_base_is_locked(&sem->rwbase); 157 } 158 159 static __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 */ 174 extern void down_read(struct rw_semaphore *sem); 175 extern int __must_check down_read_interruptible(struct rw_semaphore *sem); 176 extern int __must_check down_read_killable(struct rw_semaphore *sem); 177 178 /* 179 * trylock for reading -- returns 1 if successful, 0 if contention 180 */ 181 extern int down_read_trylock(struct rw_semaphore *sem); 182 183 /* 184 * lock for writing 185 */ 186 extern void down_write(struct rw_semaphore *sem); 187 extern int __must_check down_write_killable(struct rw_semaphore *sem); 188 189 /* 190 * trylock for writing -- returns 1 if successful, 0 if contention 191 */ 192 extern int down_write_trylock(struct rw_semaphore *sem); 193 194 /* 195 * release a read lock 196 */ 197 extern void up_read(struct rw_semaphore *sem); 198 199 /* 200 * release a write lock 201 */ 202 extern void up_write(struct rw_semaphore *sem); 203 204 /* 205 * downgrade write lock to read lock 206 */ 207 extern 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 */ 223 extern void down_read_nested(struct rw_semaphore *sem, int subclass); 224 extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass); 225 extern void down_write_nested(struct rw_semaphore *sem, int subclass); 226 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass); 227 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock); 228 229 # define down_write_nest_lock(sem, nest_lock) \ 230 do { \ 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 */ 241 extern void down_read_non_owner(struct rw_semaphore *sem); 242 extern 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 */ 254