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_LOCK_ALLOC 49 struct lockdep_map dep_map; 50 #endif 51 }; 52 53 /* 54 * Setting all bits of the owner field except bit 0 will indicate 55 * that the rwsem is writer-owned with an unknown owner. 56 */ 57 #define RWSEM_OWNER_UNKNOWN (-2L) 58 59 /* In all implementations count != 0 means locked */ 60 static inline int rwsem_is_locked(struct rw_semaphore *sem) 61 { 62 return atomic_long_read(&sem->count) != 0; 63 } 64 65 #define RWSEM_UNLOCKED_VALUE 0L 66 #define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE) 67 68 /* Common initializer macros and functions */ 69 70 #ifdef CONFIG_DEBUG_LOCK_ALLOC 71 # define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname } 72 #else 73 # define __RWSEM_DEP_MAP_INIT(lockname) 74 #endif 75 76 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER 77 #define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED 78 #else 79 #define __RWSEM_OPT_INIT(lockname) 80 #endif 81 82 #define __RWSEM_INITIALIZER(name) \ 83 { __RWSEM_INIT_COUNT(name), \ 84 .owner = ATOMIC_LONG_INIT(0), \ 85 .wait_list = LIST_HEAD_INIT((name).wait_list), \ 86 .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \ 87 __RWSEM_OPT_INIT(name) \ 88 __RWSEM_DEP_MAP_INIT(name) } 89 90 #define DECLARE_RWSEM(name) \ 91 struct rw_semaphore name = __RWSEM_INITIALIZER(name) 92 93 extern void __init_rwsem(struct rw_semaphore *sem, const char *name, 94 struct lock_class_key *key); 95 96 #define init_rwsem(sem) \ 97 do { \ 98 static struct lock_class_key __key; \ 99 \ 100 __init_rwsem((sem), #sem, &__key); \ 101 } while (0) 102 103 /* 104 * This is the same regardless of which rwsem implementation that is being used. 105 * It is just a heuristic meant to be called by somebody alreadying holding the 106 * rwsem to see if somebody from an incompatible type is wanting access to the 107 * lock. 108 */ 109 static inline int rwsem_is_contended(struct rw_semaphore *sem) 110 { 111 return !list_empty(&sem->wait_list); 112 } 113 114 /* 115 * lock for reading 116 */ 117 extern void down_read(struct rw_semaphore *sem); 118 extern int __must_check down_read_killable(struct rw_semaphore *sem); 119 120 /* 121 * trylock for reading -- returns 1 if successful, 0 if contention 122 */ 123 extern int down_read_trylock(struct rw_semaphore *sem); 124 125 /* 126 * lock for writing 127 */ 128 extern void down_write(struct rw_semaphore *sem); 129 extern int __must_check down_write_killable(struct rw_semaphore *sem); 130 131 /* 132 * trylock for writing -- returns 1 if successful, 0 if contention 133 */ 134 extern int down_write_trylock(struct rw_semaphore *sem); 135 136 /* 137 * release a read lock 138 */ 139 extern void up_read(struct rw_semaphore *sem); 140 141 /* 142 * release a write lock 143 */ 144 extern void up_write(struct rw_semaphore *sem); 145 146 /* 147 * downgrade write lock to read lock 148 */ 149 extern void downgrade_write(struct rw_semaphore *sem); 150 151 #ifdef CONFIG_DEBUG_LOCK_ALLOC 152 /* 153 * nested locking. NOTE: rwsems are not allowed to recurse 154 * (which occurs if the same task tries to acquire the same 155 * lock instance multiple times), but multiple locks of the 156 * same lock class might be taken, if the order of the locks 157 * is always the same. This ordering rule can be expressed 158 * to lockdep via the _nested() APIs, but enumerating the 159 * subclasses that are used. (If the nesting relationship is 160 * static then another method for expressing nested locking is 161 * the explicit definition of lock class keys and the use of 162 * lockdep_set_class() at lock initialization time. 163 * See Documentation/locking/lockdep-design.txt for more details.) 164 */ 165 extern void down_read_nested(struct rw_semaphore *sem, int subclass); 166 extern void down_write_nested(struct rw_semaphore *sem, int subclass); 167 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass); 168 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock); 169 170 # define down_write_nest_lock(sem, nest_lock) \ 171 do { \ 172 typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ 173 _down_write_nest_lock(sem, &(nest_lock)->dep_map); \ 174 } while (0); 175 176 /* 177 * Take/release a lock when not the owner will release it. 178 * 179 * [ This API should be avoided as much as possible - the 180 * proper abstraction for this case is completions. ] 181 */ 182 extern void down_read_non_owner(struct rw_semaphore *sem); 183 extern void up_read_non_owner(struct rw_semaphore *sem); 184 #else 185 # define down_read_nested(sem, subclass) down_read(sem) 186 # define down_write_nest_lock(sem, nest_lock) down_write(sem) 187 # define down_write_nested(sem, subclass) down_write(sem) 188 # define down_write_killable_nested(sem, subclass) down_write_killable(sem) 189 # define down_read_non_owner(sem) down_read(sem) 190 # define up_read_non_owner(sem) up_read(sem) 191 #endif 192 193 #endif /* _LINUX_RWSEM_H */ 194