root/include/linux/rwsem.h

INCLUDED FROM

DEFINITIONS

1. rwsem_is_locked
2. rwsem_is_contended

   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 /*
  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 */
  63 static 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 
 103 extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
 104                          struct lock_class_key *key);
 105 
 106 #define init_rwsem(sem)                                         \
 107 do {                                                            \
 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  */
 119 static 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  */
 127 extern void down_read(struct rw_semaphore *sem);
 128 extern int __must_check down_read_killable(struct rw_semaphore *sem);
 129 
 130 /*
 131  * trylock for reading -- returns 1 if successful, 0 if contention
 132  */
 133 extern int down_read_trylock(struct rw_semaphore *sem);
 134 
 135 /*
 136  * lock for writing
 137  */
 138 extern void down_write(struct rw_semaphore *sem);
 139 extern int __must_check down_write_killable(struct rw_semaphore *sem);
 140 
 141 /*
 142  * trylock for writing -- returns 1 if successful, 0 if contention
 143  */
 144 extern int down_write_trylock(struct rw_semaphore *sem);
 145 
 146 /*
 147  * release a read lock
 148  */
 149 extern void up_read(struct rw_semaphore *sem);
 150 
 151 /*
 152  * release a write lock
 153  */
 154 extern void up_write(struct rw_semaphore *sem);
 155 
 156 /*
 157  * downgrade write lock to read lock
 158  */
 159 extern 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  */
 175 extern void down_read_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_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 */