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_LOCK_ALLOC
  49        struct lockdep_map      dep_map;
  50#endif
  51};
  52
  53/*
  54 * Setting all bits of the owner field except bit 0 will indicate
  55 * that the rwsem is writer-owned with an unknown owner.
  56 */
  57#define RWSEM_OWNER_UNKNOWN     (-2L)
  58
  59/* In all implementations count != 0 means locked */
  60static inline int rwsem_is_locked(struct rw_semaphore *sem)
  61{
  62        return atomic_long_read(&sem->count) != 0;
  63}
  64
  65#define RWSEM_UNLOCKED_VALUE            0L
  66#define __RWSEM_INIT_COUNT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
  67
  68/* Common initializer macros and functions */
  69
  70#ifdef CONFIG_DEBUG_LOCK_ALLOC
  71# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
  72#else
  73# define __RWSEM_DEP_MAP_INIT(lockname)
  74#endif
  75
  76#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  77#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED
  78#else
  79#define __RWSEM_OPT_INIT(lockname)
  80#endif
  81
  82#define __RWSEM_INITIALIZER(name)                               \
  83        { __RWSEM_INIT_COUNT(name),                             \
  84          .owner = ATOMIC_LONG_INIT(0),                         \
  85          .wait_list = LIST_HEAD_INIT((name).wait_list),        \
  86          .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
  87          __RWSEM_OPT_INIT(name)                                \
  88          __RWSEM_DEP_MAP_INIT(name) }
  89
  90#define DECLARE_RWSEM(name) \
  91        struct rw_semaphore name = __RWSEM_INITIALIZER(name)
  92
  93extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
  94                         struct lock_class_key *key);
  95
  96#define init_rwsem(sem)                                         \
  97do {                                                            \
  98        static struct lock_class_key __key;                     \
  99                                                                \
 100        __init_rwsem((sem), #sem, &__key);                      \
 101} while (0)
 102
 103/*
 104 * This is the same regardless of which rwsem implementation that is being used.
 105 * It is just a heuristic meant to be called by somebody alreadying holding the
 106 * rwsem to see if somebody from an incompatible type is wanting access to the
 107 * lock.
 108 */
 109static inline int rwsem_is_contended(struct rw_semaphore *sem)
 110{
 111        return !list_empty(&sem->wait_list);
 112}
 113
 114/*
 115 * lock for reading
 116 */
 117extern void down_read(struct rw_semaphore *sem);
 118extern int __must_check down_read_killable(struct rw_semaphore *sem);
 119
 120/*
 121 * trylock for reading -- returns 1 if successful, 0 if contention
 122 */
 123extern int down_read_trylock(struct rw_semaphore *sem);
 124
 125/*
 126 * lock for writing
 127 */
 128extern void down_write(struct rw_semaphore *sem);
 129extern int __must_check down_write_killable(struct rw_semaphore *sem);
 130
 131/*
 132 * trylock for writing -- returns 1 if successful, 0 if contention
 133 */
 134extern int down_write_trylock(struct rw_semaphore *sem);
 135
 136/*
 137 * release a read lock
 138 */
 139extern void up_read(struct rw_semaphore *sem);
 140
 141/*
 142 * release a write lock
 143 */
 144extern void up_write(struct rw_semaphore *sem);
 145
 146/*
 147 * downgrade write lock to read lock
 148 */
 149extern void downgrade_write(struct rw_semaphore *sem);
 150
 151#ifdef CONFIG_DEBUG_LOCK_ALLOC
 152/*
 153 * nested locking. NOTE: rwsems are not allowed to recurse
 154 * (which occurs if the same task tries to acquire the same
 155 * lock instance multiple times), but multiple locks of the
 156 * same lock class might be taken, if the order of the locks
 157 * is always the same. This ordering rule can be expressed
 158 * to lockdep via the _nested() APIs, but enumerating the
 159 * subclasses that are used. (If the nesting relationship is
 160 * static then another method for expressing nested locking is
 161 * the explicit definition of lock class keys and the use of
 162 * lockdep_set_class() at lock initialization time.
 163 * See Documentation/locking/lockdep-design.rst for more details.)
 164 */
 165extern void down_read_nested(struct rw_semaphore *sem, int subclass);
 166extern void down_write_nested(struct rw_semaphore *sem, int subclass);
 167extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
 168extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
 169
 170# define down_write_nest_lock(sem, nest_lock)                   \
 171do {                                                            \
 172        typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
 173        _down_write_nest_lock(sem, &(nest_lock)->dep_map);      \
 174} while (0);
 175
 176/*
 177 * Take/release a lock when not the owner will release it.
 178 *
 179 * [ This API should be avoided as much as possible - the
 180 *   proper abstraction for this case is completions. ]
 181 */
 182extern void down_read_non_owner(struct rw_semaphore *sem);
 183extern void up_read_non_owner(struct rw_semaphore *sem);
 184#else
 185# define down_read_nested(sem, subclass)                down_read(sem)
 186# define down_write_nest_lock(sem, nest_lock)   down_write(sem)
 187# define down_write_nested(sem, subclass)       down_write(sem)
 188# define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
 189# define down_read_non_owner(sem)               down_read(sem)
 190# define up_read_non_owner(sem)                 up_read(sem)
 191#endif
 192
 193#endif /* _LINUX_RWSEM_H */
 194