2 * kref.h - library routines for handling generic reference counted objects
4 * Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
5 * Copyright (C) 2004 IBM Corp.
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
11 * This file is released under the GPLv2.
18 #include <linux/bug.h>
19 #include <linux/atomic.h>
20 #include <linux/kernel.h>
21 #include <linux/mutex.h>
27 #define KREF_INIT(n) { .refcount = ATOMIC_INIT(n), }
30 * kref_init - initialize object.
31 * @kref: object in question.
33 static inline void kref_init(struct kref *kref)
35 atomic_set(&kref->refcount, 1);
39 * kref_get - increment refcount for object.
42 static inline void kref_get(struct kref *kref)
44 /* If refcount was 0 before incrementing then we have a race
45 * condition when this kref is freeing by some other thread right now.
46 * In this case one should use kref_get_unless_zero()
48 WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
52 * kref_sub - subtract a number of refcounts for object.
54 * @count: Number of recounts to subtract.
55 * @release: pointer to the function that will clean up the object when the
56 * last reference to the object is released.
57 * This pointer is required, and it is not acceptable to pass kfree
58 * in as this function. If the caller does pass kfree to this
59 * function, you will be publicly mocked mercilessly by the kref
60 * maintainer, and anyone else who happens to notice it. You have
63 * Subtract @count from the refcount, and if 0, call release().
64 * Return 1 if the object was removed, otherwise return 0. Beware, if this
65 * function returns 0, you still can not count on the kref from remaining in
66 * memory. Only use the return value if you want to see if the kref is now
69 static inline int kref_sub(struct kref *kref, unsigned int count,
70 void (*release)(struct kref *kref))
72 WARN_ON(release == NULL);
74 if (atomic_sub_and_test((int) count, &kref->refcount)) {
82 * kref_put - decrement refcount for object.
84 * @release: pointer to the function that will clean up the object when the
85 * last reference to the object is released.
86 * This pointer is required, and it is not acceptable to pass kfree
87 * in as this function. If the caller does pass kfree to this
88 * function, you will be publicly mocked mercilessly by the kref
89 * maintainer, and anyone else who happens to notice it. You have
92 * Decrement the refcount, and if 0, call release().
93 * Return 1 if the object was removed, otherwise return 0. Beware, if this
94 * function returns 0, you still can not count on the kref from remaining in
95 * memory. Only use the return value if you want to see if the kref is now
98 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
100 return kref_sub(kref, 1, release);
103 static inline int kref_put_mutex(struct kref *kref,
104 void (*release)(struct kref *kref),
107 WARN_ON(release == NULL);
108 if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
110 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
121 * kref_get_unless_zero - Increment refcount for object unless it is zero.
124 * Return non-zero if the increment succeeded. Otherwise return 0.
126 * This function is intended to simplify locking around refcounting for
127 * objects that can be looked up from a lookup structure, and which are
128 * removed from that lookup structure in the object destructor.
129 * Operations on such objects require at least a read lock around
130 * lookup + kref_get, and a write lock around kref_put + remove from lookup
131 * structure. Furthermore, RCU implementations become extremely tricky.
132 * With a lookup followed by a kref_get_unless_zero *with return value check*
133 * locking in the kref_put path can be deferred to the actual removal from
134 * the lookup structure and RCU lookups become trivial.
136 static inline int __must_check kref_get_unless_zero(struct kref *kref)
138 return atomic_add_unless(&kref->refcount, 1, 0);
140 #endif /* _KREF_H_ */