[PATCH] kref: add documentation
Add some documentation for krefs. Signed-off-by: Corey Minyard <minyard@acm.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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minyard@acm.org
Greg KH
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211
Documentation/kref.txt
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211
Documentation/kref.txt
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krefs allow you to add reference counters to your objects. If you
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have objects that are used in multiple places and passed around, and
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you don't have refcounts, your code is almost certainly broken. If
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you want refcounts, krefs are the way to go.
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To use a kref, add one to your data structures like:
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struct my_data
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{
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.
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.
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struct kref refcount;
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.
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.
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};
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The kref can occur anywhere within the data structure.
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You must initialize the kref after you allocate it. To do this, call
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kref_init as so:
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struct my_data *data;
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data = kmalloc(sizeof(*data), GFP_KERNEL);
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if (!data)
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return -ENOMEM;
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kref_init(&data->refcount);
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This sets the refcount in the kref to 1.
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Once you have an initialized kref, you must follow the following
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rules:
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1) If you make a non-temporary copy of a pointer, especially if
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it can be passed to another thread of execution, you must
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increment the refcount with kref_get() before passing it off:
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kref_get(&data->refcount);
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If you already have a valid pointer to a kref-ed structure (the
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refcount cannot go to zero) you may do this without a lock.
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2) When you are done with a pointer, you must call kref_put():
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kref_put(&data->refcount, data_release);
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If this is the last reference to the pointer, the release
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routine will be called. If the code never tries to get
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a valid pointer to a kref-ed structure without already
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holding a valid pointer, it is safe to do this without
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a lock.
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3) If the code attempts to gain a reference to a kref-ed structure
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without already holding a valid pointer, it must serialize access
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where a kref_put() cannot occur during the kref_get(), and the
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structure must remain valid during the kref_get().
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For example, if you allocate some data and then pass it to another
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thread to process:
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void data_release(struct kref *ref)
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{
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struct my_data *data = container_of(ref, struct my_data, refcount);
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kfree(data);
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}
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void more_data_handling(void *cb_data)
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{
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struct my_data *data = cb_data;
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.
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. do stuff with data here
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.
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kref_put(data, data_release);
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}
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int my_data_handler(void)
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{
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int rv = 0;
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struct my_data *data;
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struct task_struct *task;
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data = kmalloc(sizeof(*data), GFP_KERNEL);
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if (!data)
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return -ENOMEM;
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kref_init(&data->refcount);
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kref_get(&data->refcount);
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task = kthread_run(more_data_handling, data, "more_data_handling");
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if (task == ERR_PTR(-ENOMEM)) {
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rv = -ENOMEM;
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kref_put(&data->refcount, data_release);
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goto out;
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}
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.
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. do stuff with data here
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.
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out:
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kref_put(&data->refcount, data_release);
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return rv;
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}
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This way, it doesn't matter what order the two threads handle the
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data, the kref_put() handles knowing when the data is not referenced
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any more and releasing it. The kref_get() does not require a lock,
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since we already have a valid pointer that we own a refcount for. The
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put needs no lock because nothing tries to get the data without
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already holding a pointer.
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Note that the "before" in rule 1 is very important. You should never
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do something like:
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task = kthread_run(more_data_handling, data, "more_data_handling");
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if (task == ERR_PTR(-ENOMEM)) {
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rv = -ENOMEM;
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goto out;
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} else
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/* BAD BAD BAD - get is after the handoff */
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kref_get(&data->refcount);
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Don't assume you know what you are doing and use the above construct.
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First of all, you may not know what you are doing. Second, you may
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know what you are doing (there are some situations where locking is
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involved where the above may be legal) but someone else who doesn't
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know what they are doing may change the code or copy the code. It's
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bad style. Don't do it.
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There are some situations where you can optimize the gets and puts.
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For instance, if you are done with an object and enqueuing it for
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something else or passing it off to something else, there is no reason
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to do a get then a put:
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/* Silly extra get and put */
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kref_get(&obj->ref);
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enqueue(obj);
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kref_put(&obj->ref, obj_cleanup);
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Just do the enqueue. A comment about this is always welcome:
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enqueue(obj);
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/* We are done with obj, so we pass our refcount off
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to the queue. DON'T TOUCH obj AFTER HERE! */
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The last rule (rule 3) is the nastiest one to handle. Say, for
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instance, you have a list of items that are each kref-ed, and you wish
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to get the first one. You can't just pull the first item off the list
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and kref_get() it. That violates rule 3 because you are not already
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holding a valid pointer. You must add locks or semaphores. For
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instance:
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static DECLARE_MUTEX(sem);
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static LIST_HEAD(q);
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struct my_data
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{
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struct kref refcount;
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struct list_head link;
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};
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static struct my_data *get_entry()
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{
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struct my_data *entry = NULL;
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down(&sem);
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if (!list_empty(&q)) {
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entry = container_of(q.next, struct my_q_entry, link);
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kref_get(&entry->refcount);
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}
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up(&sem);
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return entry;
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}
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static void release_entry(struct kref *ref)
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{
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struct my_data *entry = container_of(ref, struct my_data, refcount);
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list_del(&entry->link);
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kfree(entry);
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}
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static void put_entry(struct my_data *entry)
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{
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down(&sem);
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kref_put(&entry->refcount, release_entry);
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up(&sem);
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}
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The kref_put() return value is useful if you do not want to hold the
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lock during the whole release operation. Say you didn't want to call
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kfree() with the lock held in the example above (since it is kind of
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pointless to do so). You could use kref_put() as follows:
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static void release_entry(struct kref *ref)
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{
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/* All work is done after the return from kref_put(). */
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}
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static void put_entry(struct my_data *entry)
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{
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down(&sem);
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if (kref_put(&entry->refcount, release_entry)) {
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list_del(&entry->link);
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up(&sem);
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kfree(entry);
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} else
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up(&sem);
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}
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This is really more useful if you have to call other routines as part
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of the free operations that could take a long time or might claim the
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same lock. Note that doing everything in the release routine is still
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preferred as it is a little neater.
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Corey Minyard <minyard@acm.org>
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A lot of this was lifted from Greg KH's OLS presentation on krefs.
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