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