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
23 struct kref {
24 atomic_t refcount;
25 };
26
27 #define KREF_INIT(n) { .refcount = ATOMIC_INIT(n), }
28
29 /**
30 * kref_init - initialize object.
31 * @kref: object in question.
32 */
kref_init(struct kref * kref)33 static inline void kref_init(struct kref *kref)
34 {
35 atomic_set(&kref->refcount, 1);
36 }
37
kref_read(const struct kref * kref)38 static inline int kref_read(const struct kref *kref)
39 {
40 return atomic_read(&kref->refcount);
41 }
42
43 /**
44 * kref_get - increment refcount for object.
45 * @kref: object.
46 */
kref_get(struct kref * kref)47 static inline void kref_get(struct kref *kref)
48 {
49 /* If refcount was 0 before incrementing then we have a race
50 * condition when this kref is freeing by some other thread right now.
51 * In this case one should use kref_get_unless_zero()
52 */
53 WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
54 }
55
56 /**
57 * kref_sub - subtract a number of refcounts for object.
58 * @kref: object.
59 * @count: Number of recounts to subtract.
60 * @release: pointer to the function that will clean up the object when the
61 * last reference to the object is released.
62 * This pointer is required, and it is not acceptable to pass kfree
63 * in as this function. If the caller does pass kfree to this
64 * function, you will be publicly mocked mercilessly by the kref
65 * maintainer, and anyone else who happens to notice it. You have
66 * been warned.
67 *
68 * Subtract @count from the refcount, and if 0, call release().
69 * Return 1 if the object was removed, otherwise return 0. Beware, if this
70 * function returns 0, you still can not count on the kref from remaining in
71 * memory. Only use the return value if you want to see if the kref is now
72 * gone, not present.
73 */
kref_sub(struct kref * kref,unsigned int count,void (* release)(struct kref * kref))74 static inline int kref_sub(struct kref *kref, unsigned int count,
75 void (*release)(struct kref *kref))
76 {
77 WARN_ON(release == NULL);
78
79 if (atomic_sub_and_test((int) count, &kref->refcount)) {
80 release(kref);
81 return 1;
82 }
83 return 0;
84 }
85
86 /**
87 * kref_put - decrement refcount for object.
88 * @kref: object.
89 * @release: pointer to the function that will clean up the object when the
90 * last reference to the object is released.
91 * This pointer is required, and it is not acceptable to pass kfree
92 * in as this function. If the caller does pass kfree to this
93 * function, you will be publicly mocked mercilessly by the kref
94 * maintainer, and anyone else who happens to notice it. You have
95 * been warned.
96 *
97 * Decrement the refcount, and if 0, call release().
98 * Return 1 if the object was removed, otherwise return 0. Beware, if this
99 * function returns 0, you still can not count on the kref from remaining in
100 * memory. Only use the return value if you want to see if the kref is now
101 * gone, not present.
102 */
kref_put(struct kref * kref,void (* release)(struct kref * kref))103 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
104 {
105 return kref_sub(kref, 1, release);
106 }
107
kref_put_mutex(struct kref * kref,void (* release)(struct kref * kref),struct mutex * lock)108 static inline int kref_put_mutex(struct kref *kref,
109 void (*release)(struct kref *kref),
110 struct mutex *lock)
111 {
112 WARN_ON(release == NULL);
113 if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
114 mutex_lock(lock);
115 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
116 mutex_unlock(lock);
117 return 0;
118 }
119 release(kref);
120 return 1;
121 }
122 return 0;
123 }
124
125 /**
126 * kref_get_unless_zero - Increment refcount for object unless it is zero.
127 * @kref: object.
128 *
129 * Return non-zero if the increment succeeded. Otherwise return 0.
130 *
131 * This function is intended to simplify locking around refcounting for
132 * objects that can be looked up from a lookup structure, and which are
133 * removed from that lookup structure in the object destructor.
134 * Operations on such objects require at least a read lock around
135 * lookup + kref_get, and a write lock around kref_put + remove from lookup
136 * structure. Furthermore, RCU implementations become extremely tricky.
137 * With a lookup followed by a kref_get_unless_zero *with return value check*
138 * locking in the kref_put path can be deferred to the actual removal from
139 * the lookup structure and RCU lookups become trivial.
140 */
kref_get_unless_zero(struct kref * kref)141 static inline int __must_check kref_get_unless_zero(struct kref *kref)
142 {
143 return atomic_add_unless(&kref->refcount, 1, 0);
144 }
145 #endif /* _KREF_H_ */
146