1 /* 2 * kref.h - library routines for handling generic reference counted objects 3 * 4 * Copyright (C) 2004 Greg Kroah-Hartman <[email protected]> 5 * Copyright (C) 2004 IBM Corp. 6 * 7 * based on kobject.h which was: 8 * Copyright (C) 2002-2003 Patrick Mochel <[email protected]> 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 */ 33 static inline void kref_init(struct kref *kref) 34 { 35 atomic_set(&kref->refcount, 1); 36 } 37 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 */ 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 */ 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 */ 103 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref)) 104 { 105 return kref_sub(kref, 1, release); 106 } 107 108 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 */ 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