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 */ 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_INIT_COUNT(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 __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname 79 #else 80 # define __DEBUG_RWSEM_INITIALIZER(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_INIT_COUNT(name), \ 91 .owner = ATOMIC_LONG_INIT(0), \ 92 .wait_list = LIST_HEAD_INIT((name).wait_list), \ 93 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \ 94 __RWSEM_OPT_INIT(name) \ 95 __DEBUG_RWSEM_INITIALIZER(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 */ 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_killable(struct rw_semaphore *sem); 127 128 /* 129 * trylock for reading -- returns 1 if successful, 0 if contention 130 */ 131 extern int down_read_trylock(struct rw_semaphore *sem); 132 133 /* 134 * lock for writing 135 */ 136 extern void down_write(struct rw_semaphore *sem); 137 extern int __must_check down_write_killable(struct rw_semaphore *sem); 138 139 /* 140 * trylock for writing -- returns 1 if successful, 0 if contention 141 */ 142 extern int down_write_trylock(struct rw_semaphore *sem); 143 144 /* 145 * release a read lock 146 */ 147 extern void up_read(struct rw_semaphore *sem); 148 149 /* 150 * release a write lock 151 */ 152 extern void up_write(struct rw_semaphore *sem); 153 154 /* 155 * downgrade write lock to read lock 156 */ 157 extern void downgrade_write(struct rw_semaphore *sem); 158 159 #ifdef CONFIG_DEBUG_LOCK_ALLOC 160 /* 161 * nested locking. NOTE: rwsems are not allowed to recurse 162 * (which occurs if the same task tries to acquire the same 163 * lock instance multiple times), but multiple locks of the 164 * same lock class might be taken, if the order of the locks 165 * is always the same. This ordering rule can be expressed 166 * to lockdep via the _nested() APIs, but enumerating the 167 * subclasses that are used. (If the nesting relationship is 168 * static then another method for expressing nested locking is 169 * the explicit definition of lock class keys and the use of 170 * lockdep_set_class() at lock initialization time. 171 * See Documentation/locking/lockdep-design.rst for more details.) 172 */ 173 extern void down_read_nested(struct rw_semaphore *sem, int subclass); 174 extern void down_write_nested(struct rw_semaphore *sem, int subclass); 175 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass); 176 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock); 177 178 # define down_write_nest_lock(sem, nest_lock) \ 179 do { \ 180 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ 181 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \ 182 } while (0); 183 184 /* 185 * Take/release a lock when not the owner will release it. 186 * 187 * [ This API should be avoided as much as possible - the 188 * proper abstraction for this case is completions. ] 189 */ 190 extern void down_read_non_owner(struct rw_semaphore *sem); 191 extern void up_read_non_owner(struct rw_semaphore *sem); 192 #else 193 # define down_read_nested(sem, subclass) down_read(sem) 194 # define down_write_nest_lock(sem, nest_lock) down_write(sem) 195 # define down_write_nested(sem, subclass) down_write(sem) 196 # define down_write_killable_nested(sem, subclass) down_write_killable(sem) 197 # define down_read_non_owner(sem) down_read(sem) 198 # define up_read_non_owner(sem) up_read(sem) 199 #endif 200 201 #endif /* _LINUX_RWSEM_H */ 202