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  1Everything you never wanted to know about kobjects, ksets, and ktypes
  3Greg Kroah-Hartman <>
  5Based on an original article by Jon Corbet for written October 1,
  62003 and located at
  8Last updated December 19, 2007
 11Part of the difficulty in understanding the driver model - and the kobject
 12abstraction upon which it is built - is that there is no obvious starting
 13place. Dealing with kobjects requires understanding a few different types,
 14all of which make reference to each other. In an attempt to make things
 15easier, we'll take a multi-pass approach, starting with vague terms and
 16adding detail as we go. To that end, here are some quick definitions of
 17some terms we will be working with.
 19 - A kobject is an object of type struct kobject.  Kobjects have a name
 20   and a reference count.  A kobject also has a parent pointer (allowing
 21   objects to be arranged into hierarchies), a specific type, and,
 22   usually, a representation in the sysfs virtual filesystem.
 24   Kobjects are generally not interesting on their own; instead, they are
 25   usually embedded within some other structure which contains the stuff
 26   the code is really interested in.
 28   No structure should EVER have more than one kobject embedded within it.
 29   If it does, the reference counting for the object is sure to be messed
 30   up and incorrect, and your code will be buggy.  So do not do this.
 32 - A ktype is the type of object that embeds a kobject.  Every structure
 33   that embeds a kobject needs a corresponding ktype.  The ktype controls
 34   what happens to the kobject when it is created and destroyed.
 36 - A kset is a group of kobjects.  These kobjects can be of the same ktype
 37   or belong to different ktypes.  The kset is the basic container type for
 38   collections of kobjects. Ksets contain their own kobjects, but you can
 39   safely ignore that implementation detail as the kset core code handles
 40   this kobject automatically.
 42   When you see a sysfs directory full of other directories, generally each
 43   of those directories corresponds to a kobject in the same kset.
 45We'll look at how to create and manipulate all of these types. A bottom-up
 46approach will be taken, so we'll go back to kobjects.
 49Embedding kobjects
 51It is rare for kernel code to create a standalone kobject, with one major
 52exception explained below.  Instead, kobjects are used to control access to
 53a larger, domain-specific object.  To this end, kobjects will be found
 54embedded in other structures.  If you are used to thinking of things in
 55object-oriented terms, kobjects can be seen as a top-level, abstract class
 56from which other classes are derived.  A kobject implements a set of
 57capabilities which are not particularly useful by themselves, but which are
 58nice to have in other objects.  The C language does not allow for the
 59direct expression of inheritance, so other techniques - such as structure
 60embedding - must be used.
 62(As an aside, for those familiar with the kernel linked list implementation,
 63this is analogous as to how "list_head" structs are rarely useful on
 64their own, but are invariably found embedded in the larger objects of
 67So, for example, the UIO code in drivers/uio/uio.c has a structure that
 68defines the memory region associated with a uio device:
 70    struct uio_map {
 71	struct kobject kobj;
 72	struct uio_mem *mem;
 73    };
 75If you have a struct uio_map structure, finding its embedded kobject is
 76just a matter of using the kobj member.  Code that works with kobjects will
 77often have the opposite problem, however: given a struct kobject pointer,
 78what is the pointer to the containing structure?  You must avoid tricks
 79(such as assuming that the kobject is at the beginning of the structure)
 80and, instead, use the container_of() macro, found in <linux/kernel.h>:
 82    container_of(pointer, type, member)
 86  * "pointer" is the pointer to the embedded kobject,
 87  * "type" is the type of the containing structure, and
 88  * "member" is the name of the structure field to which "pointer" points.
 90The return value from container_of() is a pointer to the corresponding
 91container type. So, for example, a pointer "kp" to a struct kobject
 92embedded *within* a struct uio_map could be converted to a pointer to the
 93*containing* uio_map structure with:
 95    struct uio_map *u_map = container_of(kp, struct uio_map, kobj);
 97For convenience, programmers often define a simple macro for "back-casting"
 98kobject pointers to the containing type.  Exactly this happens in the
 99earlier drivers/uio/uio.c, as you can see here:
101    struct uio_map {
102        struct kobject kobj;
103        struct uio_mem *mem;
104    };
106    #define to_map(map) container_of(map, struct uio_map, kobj)
108where the macro argument "map" is a pointer to the struct kobject in
109question.  That macro is subsequently invoked with:
111    struct uio_map *map = to_map(kobj);
114Initialization of kobjects
116Code which creates a kobject must, of course, initialize that object. Some
117of the internal fields are setup with a (mandatory) call to kobject_init():
119    void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
121The ktype is required for a kobject to be created properly, as every kobject
122must have an associated kobj_type.  After calling kobject_init(), to
123register the kobject with sysfs, the function kobject_add() must be called:
125    int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
127This sets up the parent of the kobject and the name for the kobject
128properly.  If the kobject is to be associated with a specific kset,
129kobj->kset must be assigned before calling kobject_add().  If a kset is
130associated with a kobject, then the parent for the kobject can be set to
131NULL in the call to kobject_add() and then the kobject's parent will be the
132kset itself.
134As the name of the kobject is set when it is added to the kernel, the name
135of the kobject should never be manipulated directly.  If you must change
136the name of the kobject, call kobject_rename():
138    int kobject_rename(struct kobject *kobj, const char *new_name);
140kobject_rename does not perform any locking or have a solid notion of
141what names are valid so the caller must provide their own sanity checking
142and serialization.
144There is a function called kobject_set_name() but that is legacy cruft and
145is being removed.  If your code needs to call this function, it is
146incorrect and needs to be fixed.
148To properly access the name of the kobject, use the function
151    const char *kobject_name(const struct kobject * kobj);
153There is a helper function to both initialize and add the kobject to the
154kernel at the same time, called surprisingly enough kobject_init_and_add():
156    int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
157                             struct kobject *parent, const char *fmt, ...);
159The arguments are the same as the individual kobject_init() and
160kobject_add() functions described above.
165After a kobject has been registered with the kobject core, you need to
166announce to the world that it has been created.  This can be done with a
167call to kobject_uevent():
169    int kobject_uevent(struct kobject *kobj, enum kobject_action action);
171Use the KOBJ_ADD action for when the kobject is first added to the kernel.
172This should be done only after any attributes or children of the kobject
173have been initialized properly, as userspace will instantly start to look
174for them when this call happens.
176When the kobject is removed from the kernel (details on how to do that is
177below), the uevent for KOBJ_REMOVE will be automatically created by the
178kobject core, so the caller does not have to worry about doing that by
182Reference counts
184One of the key functions of a kobject is to serve as a reference counter
185for the object in which it is embedded. As long as references to the object
186exist, the object (and the code which supports it) must continue to exist.
187The low-level functions for manipulating a kobject's reference counts are:
189    struct kobject *kobject_get(struct kobject *kobj);
190    void kobject_put(struct kobject *kobj);
192A successful call to kobject_get() will increment the kobject's reference
193counter and return the pointer to the kobject.
195When a reference is released, the call to kobject_put() will decrement the
196reference count and, possibly, free the object. Note that kobject_init()
197sets the reference count to one, so the code which sets up the kobject will
198need to do a kobject_put() eventually to release that reference.
200Because kobjects are dynamic, they must not be declared statically or on
201the stack, but instead, always allocated dynamically.  Future versions of
202the kernel will contain a run-time check for kobjects that are created
203statically and will warn the developer of this improper usage.
205If all that you want to use a kobject for is to provide a reference counter
206for your structure, please use the struct kref instead; a kobject would be
207overkill.  For more information on how to use struct kref, please see the
208file Documentation/kref.txt in the Linux kernel source tree.
211Creating "simple" kobjects
213Sometimes all that a developer wants is a way to create a simple directory
214in the sysfs hierarchy, and not have to mess with the whole complication of
215ksets, show and store functions, and other details.  This is the one
216exception where a single kobject should be created.  To create such an
217entry, use the function:
219    struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
221This function will create a kobject and place it in sysfs in the location
222underneath the specified parent kobject.  To create simple attributes
223associated with this kobject, use:
225    int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
227    int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
229Both types of attributes used here, with a kobject that has been created
230with the kobject_create_and_add(), can be of type kobj_attribute, so no
231special custom attribute is needed to be created.
233See the example module, samples/kobject/kobject-example.c for an
234implementation of a simple kobject and attributes.
238ktypes and release methods
240One important thing still missing from the discussion is what happens to a
241kobject when its reference count reaches zero. The code which created the
242kobject generally does not know when that will happen; if it did, there
243would be little point in using a kobject in the first place. Even
244predictable object lifecycles become more complicated when sysfs is brought
245in as other portions of the kernel can get a reference on any kobject that
246is registered in the system.
248The end result is that a structure protected by a kobject cannot be freed
249before its reference count goes to zero. The reference count is not under
250the direct control of the code which created the kobject. So that code must
251be notified asynchronously whenever the last reference to one of its
252kobjects goes away.
254Once you registered your kobject via kobject_add(), you must never use
255kfree() to free it directly. The only safe way is to use kobject_put(). It
256is good practice to always use kobject_put() after kobject_init() to avoid
257errors creeping in.
259This notification is done through a kobject's release() method. Usually
260such a method has a form like:
262    void my_object_release(struct kobject *kobj)
263    {
264    	    struct my_object *mine = container_of(kobj, struct my_object, kobj);
266	    /* Perform any additional cleanup on this object, then... */
267	    kfree(mine);
268    }
270One important point cannot be overstated: every kobject must have a
271release() method, and the kobject must persist (in a consistent state)
272until that method is called. If these constraints are not met, the code is
273flawed.  Note that the kernel will warn you if you forget to provide a
274release() method.  Do not try to get rid of this warning by providing an
275"empty" release function; you will be mocked mercilessly by the kobject
276maintainer if you attempt this.
278Note, the name of the kobject is available in the release function, but it
279must NOT be changed within this callback.  Otherwise there will be a memory
280leak in the kobject core, which makes people unhappy.
282Interestingly, the release() method is not stored in the kobject itself;
283instead, it is associated with the ktype. So let us introduce struct
286    struct kobj_type {
287	    void (*release)(struct kobject *);
288	    const struct sysfs_ops *sysfs_ops;
289	    struct attribute	**default_attrs;
290    };
292This structure is used to describe a particular type of kobject (or, more
293correctly, of containing object). Every kobject needs to have an associated
294kobj_type structure; a pointer to that structure must be specified when you
295call kobject_init() or kobject_init_and_add().
297The release field in struct kobj_type is, of course, a pointer to the
298release() method for this type of kobject. The other two fields (sysfs_ops
299and default_attrs) control how objects of this type are represented in
300sysfs; they are beyond the scope of this document.
302The default_attrs pointer is a list of default attributes that will be
303automatically created for any kobject that is registered with this ktype.
308A kset is merely a collection of kobjects that want to be associated with
309each other.  There is no restriction that they be of the same ktype, but be
310very careful if they are not.
312A kset serves these functions:
314 - It serves as a bag containing a group of objects. A kset can be used by
315   the kernel to track "all block devices" or "all PCI device drivers."
317 - A kset is also a subdirectory in sysfs, where the associated kobjects
318   with the kset can show up.  Every kset contains a kobject which can be
319   set up to be the parent of other kobjects; the top-level directories of
320   the sysfs hierarchy are constructed in this way.
322 - Ksets can support the "hotplugging" of kobjects and influence how
323   uevent events are reported to user space.
325In object-oriented terms, "kset" is the top-level container class; ksets
326contain their own kobject, but that kobject is managed by the kset code and
327should not be manipulated by any other user.
329A kset keeps its children in a standard kernel linked list.  Kobjects point
330back to their containing kset via their kset field. In almost all cases,
331the kobjects belonging to a kset have that kset (or, strictly, its embedded
332kobject) in their parent.
334As a kset contains a kobject within it, it should always be dynamically
335created and never declared statically or on the stack.  To create a new
336kset use:
337  struct kset *kset_create_and_add(const char *name,
338				   struct kset_uevent_ops *u,
339				   struct kobject *parent);
341When you are finished with the kset, call:
342  void kset_unregister(struct kset *kset);
343to destroy it.
345An example of using a kset can be seen in the
346samples/kobject/kset-example.c file in the kernel tree.
348If a kset wishes to control the uevent operations of the kobjects
349associated with it, it can use the struct kset_uevent_ops to handle it:
351struct kset_uevent_ops {
352        int (*filter)(struct kset *kset, struct kobject *kobj);
353        const char *(*name)(struct kset *kset, struct kobject *kobj);
354        int (*uevent)(struct kset *kset, struct kobject *kobj,
355                      struct kobj_uevent_env *env);
359The filter function allows a kset to prevent a uevent from being emitted to
360userspace for a specific kobject.  If the function returns 0, the uevent
361will not be emitted.
363The name function will be called to override the default name of the kset
364that the uevent sends to userspace.  By default, the name will be the same
365as the kset itself, but this function, if present, can override that name.
367The uevent function will be called when the uevent is about to be sent to
368userspace to allow more environment variables to be added to the uevent.
370One might ask how, exactly, a kobject is added to a kset, given that no
371functions which perform that function have been presented.  The answer is
372that this task is handled by kobject_add().  When a kobject is passed to
373kobject_add(), its kset member should point to the kset to which the
374kobject will belong.  kobject_add() will handle the rest.
376If the kobject belonging to a kset has no parent kobject set, it will be
377added to the kset's directory.  Not all members of a kset do necessarily
378live in the kset directory.  If an explicit parent kobject is assigned
379before the kobject is added, the kobject is registered with the kset, but
380added below the parent kobject.
383Kobject removal
385After a kobject has been registered with the kobject core successfully, it
386must be cleaned up when the code is finished with it.  To do that, call
387kobject_put().  By doing this, the kobject core will automatically clean up
388all of the memory allocated by this kobject.  If a KOBJ_ADD uevent has been
389sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
390any other sysfs housekeeping will be handled for the caller properly.
392If you need to do a two-stage delete of the kobject (say you are not
393allowed to sleep when you need to destroy the object), then call
394kobject_del() which will unregister the kobject from sysfs.  This makes the
395kobject "invisible", but it is not cleaned up, and the reference count of
396the object is still the same.  At a later time call kobject_put() to finish
397the cleanup of the memory associated with the kobject.
399kobject_del() can be used to drop the reference to the parent object, if
400circular references are constructed.  It is valid in some cases, that a
401parent objects references a child.  Circular references _must_ be broken
402with an explicit call to kobject_del(), so that a release functions will be
403called, and the objects in the former circle release each other.
406Example code to copy from
408For a more complete example of using ksets and kobjects properly, see the
409example programs samples/kobject/{kobject-example.c,kset-example.c},
410which will be built as loadable modules if you select CONFIG_SAMPLE_KOBJECT.