1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* rwsem.h: R/W semaphores, public interface 3 * 4 * Written by David Howells ([email protected]). 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 /* 57 * Setting all bits of the owner field except bit 0 will indicate 58 * that the rwsem is writer-owned with an unknown owner. 59 */ 60 #define RWSEM_OWNER_UNKNOWN (-2L) 61 62 /* In all implementations count != 0 means locked */ 63 static inline int rwsem_is_locked(struct rw_semaphore *sem) 64 { 65 return atomic_long_read(&sem->count) != 0; 66 } 67 68 #define RWSEM_UNLOCKED_VALUE 0L 69 #define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE) 70 71 /* Common initializer macros and functions */ 72 73 #ifdef CONFIG_DEBUG_LOCK_ALLOC 74 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname } 75 #else 76 # define __RWSEM_DEP_MAP_INIT(lockname) 77 #endif 78 79 #ifdef CONFIG_DEBUG_RWSEMS 80 # define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname 81 #else 82 # define __DEBUG_RWSEM_INITIALIZER(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_INIT_COUNT(name), \ 93 .owner = ATOMIC_LONG_INIT(0), \ 94 .wait_list = LIST_HEAD_INIT((name).wait_list), \ 95 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \ 96 __RWSEM_OPT_INIT(name) \ 97 __DEBUG_RWSEM_INITIALIZER(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 alreadying 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 /* 125 * lock for reading 126 */ 127 extern void down_read(struct rw_semaphore *sem); 128 extern int __must_check down_read_killable(struct rw_semaphore *sem); 129 130 /* 131 * trylock for reading -- returns 1 if successful, 0 if contention 132 */ 133 extern int down_read_trylock(struct rw_semaphore *sem); 134 135 /* 136 * lock for writing 137 */ 138 extern void down_write(struct rw_semaphore *sem); 139 extern int __must_check down_write_killable(struct rw_semaphore *sem); 140 141 /* 142 * trylock for writing -- returns 1 if successful, 0 if contention 143 */ 144 extern int down_write_trylock(struct rw_semaphore *sem); 145 146 /* 147 * release a read lock 148 */ 149 extern void up_read(struct rw_semaphore *sem); 150 151 /* 152 * release a write lock 153 */ 154 extern void up_write(struct rw_semaphore *sem); 155 156 /* 157 * downgrade write lock to read lock 158 */ 159 extern void downgrade_write(struct rw_semaphore *sem); 160 161 #ifdef CONFIG_DEBUG_LOCK_ALLOC 162 /* 163 * nested locking. NOTE: rwsems are not allowed to recurse 164 * (which occurs if the same task tries to acquire the same 165 * lock instance multiple times), but multiple locks of the 166 * same lock class might be taken, if the order of the locks 167 * is always the same. This ordering rule can be expressed 168 * to lockdep via the _nested() APIs, but enumerating the 169 * subclasses that are used. (If the nesting relationship is 170 * static then another method for expressing nested locking is 171 * the explicit definition of lock class keys and the use of 172 * lockdep_set_class() at lock initialization time. 173 * See Documentation/locking/lockdep-design.rst for more details.) 174 */ 175 extern void down_read_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_write_nest_lock(sem, nest_lock) down_write(sem) 197 # define down_write_nested(sem, subclass) down_write(sem) 198 # define down_write_killable_nested(sem, subclass) down_write_killable(sem) 199 # define down_read_non_owner(sem) down_read(sem) 200 # define up_read_non_owner(sem) up_read(sem) 201 #endif 202 203 #endif /* _LINUX_RWSEM_H */ 204