linux/include/linux/kref.h
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   1/*
   2 * kref.h - library routines for handling generic reference counted objects
   3 *
   4 * Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
   5 * Copyright (C) 2004 IBM Corp.
   6 *
   7 * based on kobject.h which was:
   8 * Copyright (C) 2002-2003 Patrick Mochel <mochel@osdl.org>
   9 * Copyright (C) 2002-2003 Open Source Development Labs
  10 *
  11 * This file is released under the GPLv2.
  12 *
  13 */
  14
  15#ifndef _KREF_H_
  16#define _KREF_H_
  17
  18#include <linux/bug.h>
  19#include <linux/atomic.h>
  20#include <linux/kernel.h>
  21#include <linux/mutex.h>
  22
  23struct kref {
  24        atomic_t refcount;
  25};
  26
  27/**
  28 * kref_init - initialize object.
  29 * @kref: object in question.
  30 */
  31static inline void kref_init(struct kref *kref)
  32{
  33        atomic_set(&kref->refcount, 1);
  34}
  35
  36/**
  37 * kref_get - increment refcount for object.
  38 * @kref: object.
  39 */
  40static inline void kref_get(struct kref *kref)
  41{
  42        WARN_ON(!atomic_read(&kref->refcount));
  43        atomic_inc(&kref->refcount);
  44}
  45
  46/**
  47 * kref_sub - subtract a number of refcounts for object.
  48 * @kref: object.
  49 * @count: Number of recounts to subtract.
  50 * @release: pointer to the function that will clean up the object when the
  51 *           last reference to the object is released.
  52 *           This pointer is required, and it is not acceptable to pass kfree
  53 *           in as this function.  If the caller does pass kfree to this
  54 *           function, you will be publicly mocked mercilessly by the kref
  55 *           maintainer, and anyone else who happens to notice it.  You have
  56 *           been warned.
  57 *
  58 * Subtract @count from the refcount, and if 0, call release().
  59 * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  60 * function returns 0, you still can not count on the kref from remaining in
  61 * memory.  Only use the return value if you want to see if the kref is now
  62 * gone, not present.
  63 */
  64static inline int kref_sub(struct kref *kref, unsigned int count,
  65             void (*release)(struct kref *kref))
  66{
  67        WARN_ON(release == NULL);
  68
  69        if (atomic_sub_and_test((int) count, &kref->refcount)) {
  70                release(kref);
  71                return 1;
  72        }
  73        return 0;
  74}
  75
  76/**
  77 * kref_put - decrement refcount for object.
  78 * @kref: object.
  79 * @release: pointer to the function that will clean up the object when the
  80 *           last reference to the object is released.
  81 *           This pointer is required, and it is not acceptable to pass kfree
  82 *           in as this function.  If the caller does pass kfree to this
  83 *           function, you will be publicly mocked mercilessly by the kref
  84 *           maintainer, and anyone else who happens to notice it.  You have
  85 *           been warned.
  86 *
  87 * Decrement the refcount, and if 0, call release().
  88 * Return 1 if the object was removed, otherwise return 0.  Beware, if this
  89 * function returns 0, you still can not count on the kref from remaining in
  90 * memory.  Only use the return value if you want to see if the kref is now
  91 * gone, not present.
  92 */
  93static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
  94{
  95        return kref_sub(kref, 1, release);
  96}
  97
  98static inline int kref_put_mutex(struct kref *kref,
  99                                 void (*release)(struct kref *kref),
 100                                 struct mutex *lock)
 101{
 102        WARN_ON(release == NULL);
 103        if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
 104                mutex_lock(lock);
 105                if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
 106                        mutex_unlock(lock);
 107                        return 0;
 108                }
 109                release(kref);
 110                return 1;
 111        }
 112        return 0;
 113}
 114
 115/**
 116 * kref_get_unless_zero - Increment refcount for object unless it is zero.
 117 * @kref: object.
 118 *
 119 * Return non-zero if the increment succeeded. Otherwise return 0.
 120 *
 121 * This function is intended to simplify locking around refcounting for
 122 * objects that can be looked up from a lookup structure, and which are
 123 * removed from that lookup structure in the object destructor.
 124 * Operations on such objects require at least a read lock around
 125 * lookup + kref_get, and a write lock around kref_put + remove from lookup
 126 * structure. Furthermore, RCU implementations become extremely tricky.
 127 * With a lookup followed by a kref_get_unless_zero *with return value check*
 128 * locking in the kref_put path can be deferred to the actual removal from
 129 * the lookup structure and RCU lookups become trivial.
 130 */
 131static inline int __must_check kref_get_unless_zero(struct kref *kref)
 132{
 133        return atomic_add_unless(&kref->refcount, 1, 0);
 134}
 135#endif /* _KREF_H_ */
 136
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