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/kernel.h>
  15 #include <linux/list.h>
  16 #include <linux/spinlock.h>
  17 #include <linux/atomic.h>
  18 #include <linux/err.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 /* In all implementations count != 0 means locked */
  70 static inline int rwsem_is_locked(struct rw_semaphore *sem)
  71 {
  72         return atomic_long_read(&sem->count) != 0;
  73 }
  74
  75 #define RWSEM_UNLOCKED_VALUE            0L
  76 #define __RWSEM_COUNT_INIT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
  77
  78 /* Common initializer macros and functions */
  79
  80 #ifdef CONFIG_DEBUG_RWSEMS
  81 # define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
  82 #else
  83 # define __RWSEM_DEBUG_INIT(lockname)
  84 #endif
  85
  86 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  87 #define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
  88 #else
  89 #define __RWSEM_OPT_INIT(lockname)
  90 #endif
  91
  92 #define __RWSEM_INITIALIZER(name)                               \
  93         { __RWSEM_COUNT_INIT(name),                             \
  94           .owner = ATOMIC_LONG_INIT(0),                         \
  95           __RWSEM_OPT_INIT(name)                                \
  96           .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
  97           .wait_list = LIST_HEAD_INIT((name).wait_list),        \
  98           __RWSEM_DEBUG_INIT(name)                              \
  99           __RWSEM_DEP_MAP_INIT(name) }
 100
 101 #define DECLARE_RWSEM(name) \
 102         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
 103
 104 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 105                          struct lock_class_key *key);
 106
 107 #define init_rwsem(sem)                                         \
 108 do {                                                            \
 109         static struct lock_class_key __key;                     \
 110                                                                 \
 111         __init_rwsem((sem), #sem, &__key);                      \
 112 } while (0)
 113
 114 /*
 115  * This is the same regardless of which rwsem implementation that is being used.
 116  * It is just a heuristic meant to be called by somebody already holding the
 117  * rwsem to see if somebody from an incompatible type is wanting access to the
 118  * lock.
 119  */
 120 static inline int rwsem_is_contended(struct rw_semaphore *sem)
 121 {
 122         return !list_empty(&sem->wait_list);
 123 }
 124
 125 #else /* !CONFIG_PREEMPT_RT */
 126
 127 #include <linux/rwbase_rt.h>
 128
 129 struct rw_semaphore {
 130         struct rwbase_rt        rwbase;
 131 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 132         struct lockdep_map      dep_map;
 133 #endif
 134 };
 135
 136 #define __RWSEM_INITIALIZER(name)                               \
 137         {                                                       \
 138                 .rwbase = __RWBASE_INITIALIZER(name),           \
 139                 __RWSEM_DEP_MAP_INIT(name)                      \
 140         }
 141
 142 #define DECLARE_RWSEM(lockname) \
 143         struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
 144
 145 extern void  __init_rwsem(struct rw_semaphore *rwsem, const char *name,
 146                           struct lock_class_key *key);
 147
 148 #define init_rwsem(sem)                                         \
 149 do {                                                            \
 150         static struct lock_class_key __key;                     \
 151                                                                 \
 152         __init_rwsem((sem), #sem, &__key);                      \
 153 } while (0)
 154
 155 static __always_inline int rwsem_is_locked(struct rw_semaphore *sem)
 156 {
 157         return rw_base_is_locked(&sem->rwbase);
 158 }
 159
 160 static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
 161 {
 162         return rw_base_is_contended(&sem->rwbase);
 163 }
 164
 165 #endif /* CONFIG_PREEMPT_RT */
 166
 167 /*
 168  * The functions below are the same for all rwsem implementations including
 169  * the RT specific variant.
 170  */
 171
 172 /*
 173  * lock for reading
 174  */
 175 extern void down_read(struct rw_semaphore *sem);
 176 extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
 177 extern int __must_check down_read_killable(struct rw_semaphore *sem);
 178
 179 /*
 180  * trylock for reading -- returns 1 if successful, 0 if contention
 181  */
 182 extern int down_read_trylock(struct rw_semaphore *sem);
 183
 184 /*
 185  * lock for writing
 186  */
 187 extern void down_write(struct rw_semaphore *sem);
 188 extern int __must_check down_write_killable(struct rw_semaphore *sem);
 189
 190 /*
 191  * trylock for writing -- returns 1 if successful, 0 if contention
 192  */
 193 extern int down_write_trylock(struct rw_semaphore *sem);
 194
 195 /*
 196  * release a read lock
 197  */
 198 extern void up_read(struct rw_semaphore *sem);
 199
 200 /*
 201  * release a write lock
 202  */
 203 extern void up_write(struct rw_semaphore *sem);
 204
 205 /*
 206  * downgrade write lock to read lock
 207  */
 208 extern void downgrade_write(struct rw_semaphore *sem);
 209
 210 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 211 /*
 212  * nested locking. NOTE: rwsems are not allowed to recurse
 213  * (which occurs if the same task tries to acquire the same
 214  * lock instance multiple times), but multiple locks of the
 215  * same lock class might be taken, if the order of the locks
 216  * is always the same. This ordering rule can be expressed
 217  * to lockdep via the _nested() APIs, but enumerating the
 218  * subclasses that are used. (If the nesting relationship is
 219  * static then another method for expressing nested locking is
 220  * the explicit definition of lock class keys and the use of
 221  * lockdep_set_class() at lock initialization time.
 222  * See Documentation/locking/lockdep-design.rst for more details.)
 223  */
 224 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 225 extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
 226 extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 227 extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
 228 extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 229
 230 # define down_write_nest_lock(sem, nest_lock)                   \
 231 do {                                                            \
 232         typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
 233         _down_write_nest_lock(sem, &(nest_lock)->dep_map);      \
 234 } while (0);
 235
 236 /*
 237  * Take/release a lock when not the owner will release it.
 238  *
 239  * [ This API should be avoided as much as possible - the
 240  *   proper abstraction for this case is completions. ]
 241  */
 242 extern void down_read_non_owner(struct rw_semaphore *sem);
 243 extern void up_read_non_owner(struct rw_semaphore *sem);
 244 #else
 245 # define down_read_nested(sem, subclass)                down_read(sem)
 246 # define down_read_killable_nested(sem, subclass)       down_read_killable(sem)
 247 # define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 248 # define down_write_nested(sem, subclass)       down_write(sem)
 249 # define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 250 # define down_read_non_owner(sem)               down_read(sem)
 251 # define up_read_non_owner(sem)                 up_read(sem)
 252 #endif
 253
 254 #endif /* _LINUX_RWSEM_H */