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>
28 * kref_init - initialize object.
29 * @kref: object in question.
31 static inline void kref_init(struct kref *kref)
33 atomic_set(&kref->refcount, 1);
36 static inline int kref_read(const struct kref *kref)
38 return atomic_read(&kref->refcount);
42 * kref_get - increment refcount for object.
45 static inline void kref_get(struct kref *kref)
47 /* If refcount was 0 before incrementing then we have a race
48 * condition when this kref is freeing by some other thread right now.
49 * In this case one should use kref_get_unless_zero()
51 WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
55 * kref_sub - subtract a number of refcounts for object.
57 * @count: Number of recounts to subtract.
58 * @release: pointer to the function that will clean up the object when the
59 * last reference to the object is released.
60 * This pointer is required, and it is not acceptable to pass kfree
61 * in as this function. If the caller does pass kfree to this
62 * function, you will be publicly mocked mercilessly by the kref
63 * maintainer, and anyone else who happens to notice it. You have
66 * Subtract @count from the refcount, and if 0, call release().
67 * Return 1 if the object was removed, otherwise return 0. Beware, if this
68 * function returns 0, you still can not count on the kref from remaining in
69 * memory. Only use the return value if you want to see if the kref is now
72 static inline int kref_sub(struct kref *kref, unsigned int count,
73 void (*release)(struct kref *kref))
75 WARN_ON(release == NULL);
77 if (atomic_sub_and_test((int) count, &kref->refcount)) {
85 * kref_put - decrement refcount for object.
87 * @release: pointer to the function that will clean up the object when the
88 * last reference to the object is released.
89 * This pointer is required, and it is not acceptable to pass kfree
90 * in as this function. If the caller does pass kfree to this
91 * function, you will be publicly mocked mercilessly by the kref
92 * maintainer, and anyone else who happens to notice it. You have
95 * Decrement the refcount, and if 0, call release().
96 * Return 1 if the object was removed, otherwise return 0. Beware, if this
97 * function returns 0, you still can not count on the kref from remaining in
98 * memory. Only use the return value if you want to see if the kref is now
101 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
103 return kref_sub(kref, 1, release);
106 static inline int kref_put_mutex(struct kref *kref,
107 void (*release)(struct kref *kref),
110 WARN_ON(release == NULL);
111 if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
113 if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
124 * kref_get_unless_zero - Increment refcount for object unless it is zero.
127 * Return non-zero if the increment succeeded. Otherwise return 0.
129 * This function is intended to simplify locking around refcounting for
130 * objects that can be looked up from a lookup structure, and which are
131 * removed from that lookup structure in the object destructor.
132 * Operations on such objects require at least a read lock around
133 * lookup + kref_get, and a write lock around kref_put + remove from lookup
134 * structure. Furthermore, RCU implementations become extremely tricky.
135 * With a lookup followed by a kref_get_unless_zero *with return value check*
136 * locking in the kref_put path can be deferred to the actual removal from
137 * the lookup structure and RCU lookups become trivial.
139 static inline int __must_check kref_get_unless_zero(struct kref *kref)
141 return atomic_add_unless(&kref->refcount, 1, 0);
143 #endif /* _KREF_H_ */