include/linux/rwsem.h

   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 #include <linux/cleanup.h>
  19
  20 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  21 # define __RWSEM_DEP_MAP_INIT(lockname)                 \
  22         .dep_map = {                                    \
  23                 .name = #lockname,                      \
  24                 .wait_type_inner = LD_WAIT_SLEEP,       \
  25         },
  26 #else
  27 # define __RWSEM_DEP_MAP_INIT(lockname)
  28 #endif
  29
  30 #ifndef CONFIG_PREEMPT_RT
  31
  32 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  33 #include <linux/osq_lock.h>
  34 #endif
  35
  36 /*
  37  * For an uncontended rwsem, count and owner are the only fields a task
  38  * needs to touch when acquiring the rwsem. So they are put next to each
  39  * other to increase the chance that they will share the same cacheline.
  40  *
  41  * In a contended rwsem, the owner is likely the most frequently accessed
  42  * field in the structure as the optimistic waiter that holds the osq lock
  43  * will spin on owner. For an embedded rwsem, other hot fields in the
  44  * containing structure should be moved further away from the rwsem to
  45  * reduce the chance that they will share the same cacheline causing
  46  * cacheline bouncing problem.
  47  */
  48 struct rw_semaphore {
  49         atomic_long_t count;
  50         /*
  51          * Write owner or one of the read owners as well flags regarding
  52          * the current state of the rwsem. Can be used as a speculative
  53          * check to see if the write owner is running on the cpu.
  54          */
  55         atomic_long_t owner;
  56 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  57         struct optimistic_spin_queue osq; /* spinner MCS lock */
  58 #endif
  59         raw_spinlock_t wait_lock;
  60         struct list_head wait_list;
  61 #ifdef CONFIG_DEBUG_RWSEMS
  62         void *magic;
  63 #endif
  64 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  65         struct lockdep_map      dep_map;
  66 #endif
  67 };
  68
  69 #define RWSEM_UNLOCKED_VALUE            0UL
  70 #define RWSEM_WRITER_LOCKED             (1UL << 0)
  71 #define __RWSEM_COUNT_INIT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
  72
  73 static inline int rwsem_is_locked(struct rw_semaphore *sem)
  74 {
  75         return atomic_long_read(&sem->count) != RWSEM_UNLOCKED_VALUE;
  76 }
  77
  78 static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
  79 {
  80         WARN_ON(atomic_long_read(&sem->count) == RWSEM_UNLOCKED_VALUE);
  81 }
  82
  83 static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
  84 {
  85         WARN_ON(!(atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED));
  86 }
  87
  88 /* Common initializer macros and functions */
  89
  90 #ifdef CONFIG_DEBUG_RWSEMS
  91 # define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
  92 #else
  93 # define __RWSEM_DEBUG_INIT(lockname)
  94 #endif
  95
  96 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  97 #define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
  98 #else
  99 #define __RWSEM_OPT_INIT(lockname)
 100 #endif
 101
 102 #define __RWSEM_INITIALIZER(name)                               \
 103         { __RWSEM_COUNT_INIT(name),                             \
 104           .owner = ATOMIC_LONG_INIT(0),                         \
 105           __RWSEM_OPT_INIT(name)                                \
 106           .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
 107           .wait_list = LIST_HEAD_INIT((name).wait_list),        \
 108           __RWSEM_DEBUG_INIT(name)                              \
 109           __RWSEM_DEP_MAP_INIT(name) }
 110
 111 #define DECLARE_RWSEM(name) \
 112         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
 113
 114 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 115                          struct lock_class_key *key);
 116
 117 #define init_rwsem(sem)                                         \
 118 do {                                                            \
 119         static struct lock_class_key __key;                     \
 120                                                                 \
 121         __init_rwsem((sem), #sem, &__key);                      \
 122 } while (0)
 123
 124 /*
 125  * This is the same regardless of which rwsem implementation that is being used.
 126  * It is just a heuristic meant to be called by somebody already holding the
 127  * rwsem to see if somebody from an incompatible type is wanting access to the
 128  * lock.
 129  */
 130 static inline int rwsem_is_contended(struct rw_semaphore *sem)
 131 {
 132         return !list_empty(&sem->wait_list);
 133 }
 134
 135 #else /* !CONFIG_PREEMPT_RT */
 136
 137 #include <linux/rwbase_rt.h>
 138
 139 struct rw_semaphore {
 140         struct rwbase_rt        rwbase;
 141 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 142         struct lockdep_map      dep_map;
 143 #endif
 144 };
 145
 146 #define __RWSEM_INITIALIZER(name)                               \
 147         {                                                       \
 148                 .rwbase = __RWBASE_INITIALIZER(name),           \
 149                 __RWSEM_DEP_MAP_INIT(name)                      \
 150         }
 151
 152 #define DECLARE_RWSEM(lockname) \
 153         struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
 154
 155 extern void  __init_rwsem(struct rw_semaphore *rwsem, const char *name,
 156                           struct lock_class_key *key);
 157
 158 #define init_rwsem(sem)                                         \
 159 do {                                                            \
 160         static struct lock_class_key __key;                     \
 161                                                                 \
 162         __init_rwsem((sem), #sem, &__key);                      \
 163 } while (0)
 164
 165 static __always_inline int rwsem_is_locked(const struct rw_semaphore *sem)
 166 {
 167         return rw_base_is_locked(&sem->rwbase);
 168 }
 169
 170 static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
 171 {
 172         WARN_ON(!rwsem_is_locked(sem));
 173 }
 174
 175 static __always_inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
 176 {
 177         WARN_ON(!rw_base_is_write_locked(&sem->rwbase));
 178 }
 179
 180 static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
 181 {
 182         return rw_base_is_contended(&sem->rwbase);
 183 }
 184
 185 #endif /* CONFIG_PREEMPT_RT */
 186
 187 /*
 188  * The functions below are the same for all rwsem implementations including
 189  * the RT specific variant.
 190  */
 191
 192 static inline void rwsem_assert_held(const struct rw_semaphore *sem)
 193 {
 194         if (IS_ENABLED(CONFIG_LOCKDEP))
 195                 lockdep_assert_held(sem);
 196         else
 197                 rwsem_assert_held_nolockdep(sem);
 198 }
 199
 200 static inline void rwsem_assert_held_write(const struct rw_semaphore *sem)
 201 {
 202         if (IS_ENABLED(CONFIG_LOCKDEP))
 203                 lockdep_assert_held_write(sem);
 204         else
 205                 rwsem_assert_held_write_nolockdep(sem);
 206 }
 207
 208 /*
 209  * lock for reading
 210  */
 211 extern void down_read(struct rw_semaphore *sem);
 212 extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
 213 extern int __must_check down_read_killable(struct rw_semaphore *sem);
 214
 215 /*
 216  * trylock for reading -- returns 1 if successful, 0 if contention
 217  */
 218 extern int down_read_trylock(struct rw_semaphore *sem);
 219
 220 /*
 221  * lock for writing
 222  */
 223 extern void down_write(struct rw_semaphore *sem);
 224 extern int __must_check down_write_killable(struct rw_semaphore *sem);
 225
 226 /*
 227  * trylock for writing -- returns 1 if successful, 0 if contention
 228  */
 229 extern int down_write_trylock(struct rw_semaphore *sem);
 230
 231 /*
 232  * release a read lock
 233  */
 234 extern void up_read(struct rw_semaphore *sem);
 235
 236 /*
 237  * release a write lock
 238  */
 239 extern void up_write(struct rw_semaphore *sem);
 240
 241 DEFINE_GUARD(rwsem_read, struct rw_semaphore *, down_read(_T), up_read(_T))
 242 DEFINE_GUARD_COND(rwsem_read, _try, down_read_trylock(_T))
 243 DEFINE_GUARD_COND(rwsem_read, _intr, down_read_interruptible(_T) == 0)
 244
 245 DEFINE_GUARD(rwsem_write, struct rw_semaphore *, down_write(_T), up_write(_T))
 246 DEFINE_GUARD_COND(rwsem_write, _try, down_write_trylock(_T))
 247
 248 /*
 249  * downgrade write lock to read lock
 250  */
 251 extern void downgrade_write(struct rw_semaphore *sem);
 252
 253 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 254 /*
 255  * nested locking. NOTE: rwsems are not allowed to recurse
 256  * (which occurs if the same task tries to acquire the same
 257  * lock instance multiple times), but multiple locks of the
 258  * same lock class might be taken, if the order of the locks
 259  * is always the same. This ordering rule can be expressed
 260  * to lockdep via the _nested() APIs, but enumerating the
 261  * subclasses that are used. (If the nesting relationship is
 262  * static then another method for expressing nested locking is
 263  * the explicit definition of lock class keys and the use of
 264  * lockdep_set_class() at lock initialization time.
 265  * See Documentation/locking/lockdep-design.rst for more details.)
 266  */
 267 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 268 extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
 269 extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 270 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
 271 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 272
 273 # define down_write_nest_lock(sem, nest_lock)                   \
 274 do {                                                            \
 275         typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
 276         _down_write_nest_lock(sem, &(nest_lock)->dep_map);      \
 277 } while (0)
 278
 279 /*
 280  * Take/release a lock when not the owner will release it.
 281  *
 282  * [ This API should be avoided as much as possible - the
 283  *   proper abstraction for this case is completions. ]
 284  */
 285 extern void down_read_non_owner(struct rw_semaphore *sem);
 286 extern void up_read_non_owner(struct rw_semaphore *sem);
 287 #else
 288 # define down_read_nested(sem, subclass)                down_read(sem)
 289 # define down_read_killable_nested(sem, subclass)       down_read_killable(sem)
 290 # define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 291 # define down_write_nested(sem, subclass)       down_write(sem)
 292 # define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 293 # define down_read_non_owner(sem)               down_read(sem)
 294 # define up_read_non_owner(sem)                 up_read(sem)
 295 #endif
 296
 297 #endif /* _LINUX_RWSEM_H */