[PATCH] kref: add documentation

Add some documentation for krefs.
Signed-off-by: NCorey Minyard <minyard@acm.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/kref.txt

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