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 #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_COUNT_INIT(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 __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
  79 #else
  80 # define __RWSEM_DEBUG_INIT(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_COUNT_INIT(name),                             \
  91           .owner = ATOMIC_LONG_INIT(0),                         \
  92           __RWSEM_OPT_INIT(name)                                \
  93           .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
  94           .wait_list = LIST_HEAD_INIT((name).wait_list),        \
  95           __RWSEM_DEBUG_INIT(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_interruptible(struct rw_semaphore *sem);
 127 extern int __must_check down_read_killable(struct rw_semaphore *sem);
 128
 129 /*
 130  * trylock for reading -- returns 1 if successful, 0 if contention
 131  */
 132 extern int down_read_trylock(struct rw_semaphore *sem);
 133
 134 /*
 135  * lock for writing
 136  */
 137 extern void down_write(struct rw_semaphore *sem);
 138 extern int __must_check down_write_killable(struct rw_semaphore *sem);
 139
 140 /*
 141  * trylock for writing -- returns 1 if successful, 0 if contention
 142  */
 143 extern int down_write_trylock(struct rw_semaphore *sem);
 144
 145 /*
 146  * release a read lock
 147  */
 148 extern void up_read(struct rw_semaphore *sem);
 149
 150 /*
 151  * release a write lock
 152  */
 153 extern void up_write(struct rw_semaphore *sem);
 154
 155 /*
 156  * downgrade write lock to read lock
 157  */
 158 extern void downgrade_write(struct rw_semaphore *sem);
 159
 160 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 161 /*
 162  * nested locking. NOTE: rwsems are not allowed to recurse
 163  * (which occurs if the same task tries to acquire the same
 164  * lock instance multiple times), but multiple locks of the
 165  * same lock class might be taken, if the order of the locks
 166  * is always the same. This ordering rule can be expressed
 167  * to lockdep via the _nested() APIs, but enumerating the
 168  * subclasses that are used. (If the nesting relationship is
 169  * static then another method for expressing nested locking is
 170  * the explicit definition of lock class keys and the use of
 171  * lockdep_set_class() at lock initialization time.
 172  * See Documentation/locking/lockdep-design.rst for more details.)
 173  */
 174 extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 175 extern int __must_check down_read_killable_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_read_killable_nested(sem, subclass)       down_read_killable(sem)
 197 # define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 198 # define down_write_nested(sem, subclass)       down_write(sem)
 199 # define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 200 # define down_read_non_owner(sem)               down_read(sem)
 201 # define up_read_non_owner(sem)                 up_read(sem)
 202 #endif
 203
 204 #endif /* _LINUX_RWSEM_H */