LXR linux/include/linux/rwsem.h

   1/* rwsem.h: R/W semaphores, public interface
   2 *
   3 * Written by David Howells (dhowells@redhat.com).
   4 * Derived from asm-i386/semaphore.h
   5 */
   6
   7#ifndef _LINUX_RWSEM_H
   8#define _LINUX_RWSEM_H
   9
  10#include <linux/linkage.h>
  11
  12#include <linux/types.h>
  13#include <linux/kernel.h>
  14#include <linux/list.h>
  15#include <linux/spinlock.h>
  16#include <linux/atomic.h>
  17#include <linux/err.h>
  18#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  19#include <linux/osq_lock.h>
  20#endif
  21
  22struct rw_semaphore;
  23
  24#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
  25#include <linux/rwsem-spinlock.h> /* use a generic implementation */
  26#define __RWSEM_INIT_COUNT(name)        .count = RWSEM_UNLOCKED_VALUE
  27#else
  28
  29#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  30struct slist_head {
  31        struct list_head *next;
  32};
  33
  34/*
  35 * Is slist_head empty?
  36 */
  37static inline bool slist_empty(struct slist_head *head)
  38{
  39        return READ_ONCE(head->next) == (void *)head;
  40}
  41
  42#define SLIST_HEAD_INIT(name)   { (void *)&(name) }
  43#else
  44#define SLIST_HEAD_INIT(name)   LIST_HEAD_INIT(name)
  45#define slist_empty(name)       list_empty(name)
  46#endif
  47
  48/* All arch specific implementations share the same struct */
  49struct rw_semaphore {
  50        RH_KABI_REPLACE(long            count,
  51                        atomic_long_t   count)
  52        raw_spinlock_t  wait_lock;
  53#if defined(CONFIG_RWSEM_SPIN_ON_OWNER) && !defined(__GENKSYMS__)
  54        struct optimistic_spin_queue osq; /* spinner MCS lock */
  55        struct slist_head       wait_list;
  56        /*
  57         * Write owner. Used as a speculative check to see
  58         * if the owner is running on the cpu.
  59         */
  60        struct task_struct      *owner;
  61#else
  62        struct list_head        wait_list;
  63#endif
  64#ifdef CONFIG_DEBUG_LOCK_ALLOC
  65        struct lockdep_map      dep_map;
  66#endif
  67};
  68
  69extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
  70extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
  71extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
  72extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
  73
  74/* Include the arch specific part */
  75#include <asm/rwsem.h>
  76
  77/* In all implementations count != 0 means locked */
  78static inline int rwsem_is_locked(struct rw_semaphore *sem)
  79{
  80        return atomic_long_read(&sem->count) != 0;
  81}
  82
  83#define __RWSEM_INIT_COUNT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
  84#endif
  85
  86/* Common initializer macros and functions */
  87
  88#ifdef CONFIG_DEBUG_LOCK_ALLOC
  89# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  90#else
  91# define __RWSEM_DEP_MAP_INIT(lockname)
  92#endif
  93
  94#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  95#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED,  \
  96                                     .owner = (void *)&(lockname).wait_list
  97#else
  98#define __RWSEM_OPT_INIT(lockname)
  99#endif
 100
 101#define __RWSEM_INITIALIZER(name)                               \
 102        { __RWSEM_INIT_COUNT(name),                             \
 103          .wait_list = SLIST_HEAD_INIT((name).wait_list),       \
 104          .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
 105          __RWSEM_OPT_INIT(name)                                \
 106          __RWSEM_DEP_MAP_INIT(name) }
 107
 108#define DECLARE_RWSEM(name) \
 109        struct rw_semaphore name = __RWSEM_INITIALIZER(name)
 110
 111extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 112                         struct lock_class_key *key);
 113
 114#define init_rwsem(sem)                                         \
 115do {                                                            \
 116        static struct lock_class_key __key;                     \
 117                                                                \
 118        __init_rwsem((sem), #sem, &__key);                      \
 119} while (0)
 120
 121/*
 122 * This is the same regardless of which rwsem implementation that is being used.
 123 * It is just a heuristic meant to be called by somebody alreadying holding the
 124 * rwsem to see if somebody from an incompatible type is wanting access to the
 125 * lock.
 126 */
 127static inline int rwsem_is_contended(struct rw_semaphore *sem)
 128{
 129        return !slist_empty(&sem->wait_list);
 130}
 131
 132/*
 133 * lock for reading
 134 */
 135extern void down_read(struct rw_semaphore *sem);
 136
 137/*
 138 * trylock for reading -- returns 1 if successful, 0 if contention
 139 */
 140extern int down_read_trylock(struct rw_semaphore *sem);
 141
 142/*
 143 * lock for writing
 144 */
 145extern void down_write(struct rw_semaphore *sem);
 146
 147/*
 148 * trylock for writing -- returns 1 if successful, 0 if contention
 149 */
 150extern int down_write_trylock(struct rw_semaphore *sem);
 151
 152/*
 153 * release a read lock
 154 */
 155extern void up_read(struct rw_semaphore *sem);
 156
 157/*
 158 * release a write lock
 159 */
 160extern void up_write(struct rw_semaphore *sem);
 161
 162/*
 163 * downgrade write lock to read lock
 164 */
 165extern void downgrade_write(struct rw_semaphore *sem);
 166
 167#ifdef CONFIG_DEBUG_LOCK_ALLOC
 168/*
 169 * nested locking. NOTE: rwsems are not allowed to recurse
 170 * (which occurs if the same task tries to acquire the same
 171 * lock instance multiple times), but multiple locks of the
 172 * same lock class might be taken, if the order of the locks
 173 * is always the same. This ordering rule can be expressed
 174 * to lockdep via the _nested() APIs, but enumerating the
 175 * subclasses that are used. (If the nesting relationship is
 176 * static then another method for expressing nested locking is
 177 * the explicit definition of lock class keys and the use of
 178 * lockdep_set_class() at lock initialization time.
 179 * See Documentation/lockdep-design.txt for more details.)
 180 */
 181extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 182extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 183extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 184
 185# define down_write_nest_lock(sem, nest_lock)                   \
 186do {                                                            \
 187        typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
 188        _down_write_nest_lock(sem, &(nest_lock)->dep_map);      \
 189} while (0);
 190
 191/*
 192 * Take/release a lock when not the owner will release it.
 193 *
 194 * [ This API should be avoided as much as possible - the
 195 *   proper abstraction for this case is completions. ]
 196 */
 197extern void down_read_non_owner(struct rw_semaphore *sem);
 198extern void up_read_non_owner(struct rw_semaphore *sem);
 199#else
 200# define down_read_nested(sem, subclass)                down_read(sem)
 201# define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 202# define down_write_nested(sem, subclass)       down_write(sem)
 203# define down_read_non_owner(sem)               down_read(sem)
 204# define up_read_non_owner(sem)                 up_read(sem)
 205#endif
 206
 207#endif /* _LINUX_RWSEM_H */
 208