Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 2 krefs allow you to add reference counters to your objects. If you 3 have objects that are used in multiple places and passed around, and 4 you don't have refcounts, your code is almost certainly broken. If 5 you want refcounts, krefs are the way to go. 6 7 To use a kref, add one to your data structures like: 8 9 struct my_data 10 { 11 . 12 . 13 struct kref refcount; 14 . 15 . 16 }; 17 18 The kref can occur anywhere within the data structure. 19 20 You must initialize the kref after you allocate it. To do this, call 21 kref_init as so: 22 23 struct my_data *data; 24 25 data = kmalloc(sizeof(*data), GFP_KERNEL); 26 if (!data) 27 return -ENOMEM; 28 kref_init(&data->refcount); 29 30 This sets the refcount in the kref to 1. 31 32 Once you have an initialized kref, you must follow the following 33 rules: 34 35 1) If you make a non-temporary copy of a pointer, especially if 36 it can be passed to another thread of execution, you must 37 increment the refcount with kref_get() before passing it off: 38 kref_get(&data->refcount); 39 If you already have a valid pointer to a kref-ed structure (the 40 refcount cannot go to zero) you may do this without a lock. 41 42 2) When you are done with a pointer, you must call kref_put(): 43 kref_put(&data->refcount, data_release); 44 If this is the last reference to the pointer, the release 45 routine will be called. If the code never tries to get 46 a valid pointer to a kref-ed structure without already 47 holding a valid pointer, it is safe to do this without 48 a lock. 49 50 3) If the code attempts to gain a reference to a kref-ed structure 51 without already holding a valid pointer, it must serialize access 52 where a kref_put() cannot occur during the kref_get(), and the 53 structure must remain valid during the kref_get(). 54 55 For example, if you allocate some data and then pass it to another 56 thread to process: 57 58 void data_release(struct kref *ref) 59 { 60 struct my_data *data = container_of(ref, struct my_data, refcount); 61 kfree(data); 62 } 63 64 void more_data_handling(void *cb_data) 65 { 66 struct my_data *data = cb_data; 67 . 68 . do stuff with data here 69 . 70 kref_put(&data->refcount, data_release); 71 } 72 73 int my_data_handler(void) 74 { 75 int rv = 0; 76 struct my_data *data; 77 struct task_struct *task; 78 data = kmalloc(sizeof(*data), GFP_KERNEL); 79 if (!data) 80 return -ENOMEM; 81 kref_init(&data->refcount); 82 83 kref_get(&data->refcount); 84 task = kthread_run(more_data_handling, data, "more_data_handling"); 85 if (task == ERR_PTR(-ENOMEM)) { 86 rv = -ENOMEM; 87 kref_put(&data->refcount, data_release); 88 goto out; 89 } 90 91 . 92 . do stuff with data here 93 . 94 out: 95 kref_put(&data->refcount, data_release); 96 return rv; 97 } 98 99 This way, it doesn't matter what order the two threads handle the 100 data, the kref_put() handles knowing when the data is not referenced 101 any more and releasing it. The kref_get() does not require a lock, 102 since we already have a valid pointer that we own a refcount for. The 103 put needs no lock because nothing tries to get the data without 104 already holding a pointer. 105 106 Note that the "before" in rule 1 is very important. You should never 107 do something like: 108 109 task = kthread_run(more_data_handling, data, "more_data_handling"); 110 if (task == ERR_PTR(-ENOMEM)) { 111 rv = -ENOMEM; 112 goto out; 113 } else 114 /* BAD BAD BAD - get is after the handoff */ 115 kref_get(&data->refcount); 116 117 Don't assume you know what you are doing and use the above construct. 118 First of all, you may not know what you are doing. Second, you may 119 know what you are doing (there are some situations where locking is 120 involved where the above may be legal) but someone else who doesn't 121 know what they are doing may change the code or copy the code. It's 122 bad style. Don't do it. 123 124 There are some situations where you can optimize the gets and puts. 125 For instance, if you are done with an object and enqueuing it for 126 something else or passing it off to something else, there is no reason 127 to do a get then a put: 128 129 /* Silly extra get and put */ 130 kref_get(&obj->ref); 131 enqueue(obj); 132 kref_put(&obj->ref, obj_cleanup); 133 134 Just do the enqueue. A comment about this is always welcome: 135 136 enqueue(obj); 137 /* We are done with obj, so we pass our refcount off 138 to the queue. DON'T TOUCH obj AFTER HERE! */ 139 140 The last rule (rule 3) is the nastiest one to handle. Say, for 141 instance, you have a list of items that are each kref-ed, and you wish 142 to get the first one. You can't just pull the first item off the list 143 and kref_get() it. That violates rule 3 because you are not already 144 holding a valid pointer. You must add a mutex (or some other lock). 145 For instance: 146 147 static DEFINE_MUTEX(mutex); 148 static LIST_HEAD(q); 149 struct my_data 150 { 151 struct kref refcount; 152 struct list_head link; 153 }; 154 155 static struct my_data *get_entry() 156 { 157 struct my_data *entry = NULL; 158 mutex_lock(&mutex); 159 if (!list_empty(&q)) { 160 entry = container_of(q.next, struct my_q_entry, link); 161 kref_get(&entry->refcount); 162 } 163 mutex_unlock(&mutex); 164 return entry; 165 } 166 167 static void release_entry(struct kref *ref) 168 { 169 struct my_data *entry = container_of(ref, struct my_data, refcount); 170 171 list_del(&entry->link); 172 kfree(entry); 173 } 174 175 static void put_entry(struct my_data *entry) 176 { 177 mutex_lock(&mutex); 178 kref_put(&entry->refcount, release_entry); 179 mutex_unlock(&mutex); 180 } 181 182 The kref_put() return value is useful if you do not want to hold the 183 lock during the whole release operation. Say you didn't want to call 184 kfree() with the lock held in the example above (since it is kind of 185 pointless to do so). You could use kref_put() as follows: 186 187 static void release_entry(struct kref *ref) 188 { 189 /* All work is done after the return from kref_put(). */ 190 } 191 192 static void put_entry(struct my_data *entry) 193 { 194 mutex_lock(&mutex); 195 if (kref_put(&entry->refcount, release_entry)) { 196 list_del(&entry->link); 197 mutex_unlock(&mutex); 198 kfree(entry); 199 } else 200 mutex_unlock(&mutex); 201 } 202 203 This is really more useful if you have to call other routines as part 204 of the free operations that could take a long time or might claim the 205 same lock. Note that doing everything in the release routine is still 206 preferred as it is a little neater. 207 208 209 Corey Minyard <minyard[AT]acm[DOT]org> 210 211 A lot of this was lifted from Greg Kroah-Hartman's 2004 OLS paper and 212 presentation on krefs, which can be found at: 213 http://www.kroah.com/linux/talks/ols_2004_kref_paper/Reprint-Kroah-Hartman-OLS2004.pdf 214 and: 215 http://www.kroah.com/linux/talks/ols_2004_kref_talk/
