LXR linux/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 */
  35struct 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/*
  57 * Setting all bits of the owner field except bit 0 will indicate
  58 * that the rwsem is writer-owned with an unknown owner.
  59 */
  60#define RWSEM_OWNER_UNKNOWN     (-2L)
  61
  62/* In all implementations count != 0 means locked */
  63static inline int rwsem_is_locked(struct rw_semaphore *sem)
  64{
  65        return atomic_long_read(&sem->count) != 0;
  66}
  67
  68#define RWSEM_UNLOCKED_VALUE            0L
  69#define __RWSEM_INIT_COUNT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
  70
  71/* Common initializer macros and functions */
  72
  73#ifdef CONFIG_DEBUG_LOCK_ALLOC
  74# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  75#else
  76# define __RWSEM_DEP_MAP_INIT(lockname)
  77#endif
  78
  79#ifdef CONFIG_DEBUG_RWSEMS
  80# define __DEBUG_RWSEM_INITIALIZER(lockname) , .magic = &lockname
  81#else
  82# define __DEBUG_RWSEM_INITIALIZER(lockname)
  83#endif
  84
  85#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  86#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
  87#else
  88#define __RWSEM_OPT_INIT(lockname)
  89#endif
  90
  91#define __RWSEM_INITIALIZER(name)                               \
  92        { __RWSEM_INIT_COUNT(name),                             \
  93          .owner = ATOMIC_LONG_INIT(0),                         \
  94          .wait_list = LIST_HEAD_INIT((name).wait_list),        \
  95          .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
  96          __RWSEM_OPT_INIT(name)                                \
  97          __DEBUG_RWSEM_INITIALIZER(name)                       \
  98          __RWSEM_DEP_MAP_INIT(name) }
  99
 100#define DECLARE_RWSEM(name) \
 101        struct rw_semaphore name = __RWSEM_INITIALIZER(name)
 102
 103extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 104                         struct lock_class_key *key);
 105
 106#define init_rwsem(sem)                                         \
 107do {                                                            \
 108        static struct lock_class_key __key;                     \
 109                                                                \
 110        __init_rwsem((sem), #sem, &__key);                      \
 111} while (0)
 112
 113/*
 114 * This is the same regardless of which rwsem implementation that is being used.
 115 * It is just a heuristic meant to be called by somebody alreadying holding the
 116 * rwsem to see if somebody from an incompatible type is wanting access to the
 117 * lock.
 118 */
 119static inline int rwsem_is_contended(struct rw_semaphore *sem)
 120{
 121        return !list_empty(&sem->wait_list);
 122}
 123
 124/*
 125 * lock for reading
 126 */
 127extern void down_read(struct rw_semaphore *sem);
 128extern int __must_check down_read_killable(struct rw_semaphore *sem);
 129
 130/*
 131 * trylock for reading -- returns 1 if successful, 0 if contention
 132 */
 133extern int down_read_trylock(struct rw_semaphore *sem);
 134
 135/*
 136 * lock for writing
 137 */
 138extern void down_write(struct rw_semaphore *sem);
 139extern int __must_check down_write_killable(struct rw_semaphore *sem);
 140
 141/*
 142 * trylock for writing -- returns 1 if successful, 0 if contention
 143 */
 144extern int down_write_trylock(struct rw_semaphore *sem);
 145
 146/*
 147 * release a read lock
 148 */
 149extern void up_read(struct rw_semaphore *sem);
 150
 151/*
 152 * release a write lock
 153 */
 154extern void up_write(struct rw_semaphore *sem);
 155
 156/*
 157 * downgrade write lock to read lock
 158 */
 159extern void downgrade_write(struct rw_semaphore *sem);
 160
 161#ifdef CONFIG_DEBUG_LOCK_ALLOC
 162/*
 163 * nested locking. NOTE: rwsems are not allowed to recurse
 164 * (which occurs if the same task tries to acquire the same
 165 * lock instance multiple times), but multiple locks of the
 166 * same lock class might be taken, if the order of the locks
 167 * is always the same. This ordering rule can be expressed
 168 * to lockdep via the _nested() APIs, but enumerating the
 169 * subclasses that are used. (If the nesting relationship is
 170 * static then another method for expressing nested locking is
 171 * the explicit definition of lock class keys and the use of
 172 * lockdep_set_class() at lock initialization time.
 173 * See Documentation/locking/lockdep-design.rst for more details.)
 174 */
 175extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 176extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 177extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
 178extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 179
 180# define down_write_nest_lock(sem, nest_lock)                   \
 181do {                                                            \
 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 */
 192extern void down_read_non_owner(struct rw_semaphore *sem);
 193extern void up_read_non_owner(struct rw_semaphore *sem);
 194#else
 195# define down_read_nested(sem, subclass)                down_read(sem)
 196# define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 197# define down_write_nested(sem, subclass)       down_write(sem)
 198# define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 199# define down_read_non_owner(sem)               down_read(sem)
 200# define up_read_non_owner(sem)                 up_read(sem)
 201#endif
 202
 203#endif /* _LINUX_RWSEM_H */
 204