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