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_put - decrement refcount for object. 58 * @kref: object. 59 * @release: pointer to the function that will clean up the object when the 60 * last reference to the object is released. 61 * This pointer is required, and it is not acceptable to pass kfree 62 * in as this function. If the caller does pass kfree to this 63 * function, you will be publicly mocked mercilessly by the kref 64 * maintainer, and anyone else who happens to notice it. You have 65 * been warned. 66 * 67 * Decrement the refcount, and if 0, call release(). 68 * Return 1 if the object was removed, otherwise return 0. Beware, if this 69 * function returns 0, you still can not count on the kref from remaining in 70 * memory. Only use the return value if you want to see if the kref is now 71 * gone, not present. 72 */ 73 static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref)) 74 { 75 WARN_ON(release == NULL); 76 77 if (atomic_dec_and_test(&kref->refcount)) { 78 release(kref); 79 return 1; 80 } 81 return 0; 82 } 83 84 static inline int kref_put_mutex(struct kref *kref, 85 void (*release)(struct kref *kref), 86 struct mutex *lock) 87 { 88 WARN_ON(release == NULL); 89 if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) { 90 mutex_lock(lock); 91 if (unlikely(!atomic_dec_and_test(&kref->refcount))) { 92 mutex_unlock(lock); 93 return 0; 94 } 95 release(kref); 96 return 1; 97 } 98 return 0; 99 } 100 101 /** 102 * kref_get_unless_zero - Increment refcount for object unless it is zero. 103 * @kref: object. 104 * 105 * Return non-zero if the increment succeeded. Otherwise return 0. 106 * 107 * This function is intended to simplify locking around refcounting for 108 * objects that can be looked up from a lookup structure, and which are 109 * removed from that lookup structure in the object destructor. 110 * Operations on such objects require at least a read lock around 111 * lookup + kref_get, and a write lock around kref_put + remove from lookup 112 * structure. Furthermore, RCU implementations become extremely tricky. 113 * With a lookup followed by a kref_get_unless_zero *with return value check* 114 * locking in the kref_put path can be deferred to the actual removal from 115 * the lookup structure and RCU lookups become trivial. 116 */ 117 static inline int __must_check kref_get_unless_zero(struct kref *kref) 118 { 119 return atomic_add_unless(&kref->refcount, 1, 0); 120 } 121 #endif /* _KREF_H_ */ 122