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/* In all implementations count != 0 means locked */
  57static 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
 101extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 102                         struct lock_class_key *key);
 103
 104#define init_rwsem(sem)                                         \
 105do {                                                            \
 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 */
 117static 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 */
 125extern void down_read(struct rw_semaphore *sem);
 126extern int __must_check down_read_killable(struct rw_semaphore *sem);
 127
 128/*
 129 * trylock for reading -- returns 1 if successful, 0 if contention
 130 */
 131extern int down_read_trylock(struct rw_semaphore *sem);
 132
 133/*
 134 * lock for writing
 135 */
 136extern void down_write(struct rw_semaphore *sem);
 137extern int __must_check down_write_killable(struct rw_semaphore *sem);
 138
 139/*
 140 * trylock for writing -- returns 1 if successful, 0 if contention
 141 */
 142extern int down_write_trylock(struct rw_semaphore *sem);
 143
 144/*
 145 * release a read lock
 146 */
 147extern void up_read(struct rw_semaphore *sem);
 148
 149/*
 150 * release a write lock
 151 */
 152extern void up_write(struct rw_semaphore *sem);
 153
 154/*
 155 * downgrade write lock to read lock
 156 */
 157extern void downgrade_write(struct rw_semaphore *sem);
 158
 159#ifdef CONFIG_DEBUG_LOCK_ALLOC
 160/*
 161 * nested locking. NOTE: rwsems are not allowed to recurse
 162 * (which occurs if the same task tries to acquire the same
 163 * lock instance multiple times), but multiple locks of the
 164 * same lock class might be taken, if the order of the locks
 165 * is always the same. This ordering rule can be expressed
 166 * to lockdep via the _nested() APIs, but enumerating the
 167 * subclasses that are used. (If the nesting relationship is
 168 * static then another method for expressing nested locking is
 169 * the explicit definition of lock class keys and the use of
 170 * lockdep_set_class() at lock initialization time.
 171 * See Documentation/locking/lockdep-design.rst for more details.)
 172 */
 173extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 174extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 175extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
 176extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 177
 178# define down_write_nest_lock(sem, nest_lock)                   \
 179do {                                                            \
 180        typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
 181        _down_write_nest_lock(sem, &(nest_lock)->dep_map);      \
 182} while (0);
 183
 184/*
 185 * Take/release a lock when not the owner will release it.
 186 *
 187 * [ This API should be avoided as much as possible - the
 188 *   proper abstraction for this case is completions. ]
 189 */
 190extern void down_read_non_owner(struct rw_semaphore *sem);
 191extern void up_read_non_owner(struct rw_semaphore *sem);
 192#else
 193# define down_read_nested(sem, subclass)                down_read(sem)
 194# define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 195# define down_write_nested(sem, subclass)       down_write(sem)
 196# define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 197# define down_read_non_owner(sem)               down_read(sem)
 198# define up_read_non_owner(sem)                 up_read(sem)
 199#endif
 200
 201#endif /* _LINUX_RWSEM_H */
 202