ehea: fix skb_frag_size typo

[linux-btrfs-devel.git] / Documentation / kobject.txt

Everything you never wanted to know about kobjects, ksets, and ktypes

Greg Kroah-Hartman <gregkh@suse.de>

Based on an original article by Jon Corbet for lwn.net written October 1,
2003 and located at http://lwn.net/Articles/51437/

Last updated December 19, 2007


Part of the difficulty in understanding the driver model - and the kobject
abstraction upon which it is built - is that there is no obvious starting
place. Dealing with kobjects requires understanding a few different types,
all of which make reference to each other. In an attempt to make things
easier, we'll take a multi-pass approach, starting with vague terms and
adding detail as we go. To that end, here are some quick definitions of
some terms we will be working with.

 - A kobject is an object of type struct kobject.  Kobjects have a name
   and a reference count.  A kobject also has a parent pointer (allowing
   objects to be arranged into hierarchies), a specific type, and,
   usually, a representation in the sysfs virtual filesystem.

   Kobjects are generally not interesting on their own; instead, they are
   usually embedded within some other structure which contains the stuff
   the code is really interested in.

   No structure should EVER have more than one kobject embedded within it.
   If it does, the reference counting for the object is sure to be messed
   up and incorrect, and your code will be buggy.  So do not do this.

 - A ktype is the type of object that embeds a kobject.  Every structure
   that embeds a kobject needs a corresponding ktype.  The ktype controls
   what happens to the kobject when it is created and destroyed.

 - A kset is a group of kobjects.  These kobjects can be of the same ktype
   or belong to different ktypes.  The kset is the basic container type for
   collections of kobjects. Ksets contain their own kobjects, but you can
   safely ignore that implementation detail as the kset core code handles
   this kobject automatically.

   When you see a sysfs directory full of other directories, generally each
   of those directories corresponds to a kobject in the same kset.

We'll look at how to create and manipulate all of these types. A bottom-up
approach will be taken, so we'll go back to kobjects.


Embedding kobjects

It is rare for kernel code to create a standalone kobject, with one major
exception explained below.  Instead, kobjects are used to control access to
a larger, domain-specific object.  To this end, kobjects will be found
embedded in other structures.  If you are used to thinking of things in
object-oriented terms, kobjects can be seen as a top-level, abstract class
from which other classes are derived.  A kobject implements a set of
capabilities which are not particularly useful by themselves, but which are
nice to have in other objects.  The C language does not allow for the
direct expression of inheritance, so other techniques - such as structure
embedding - must be used.

(As an aside, for those familiar with the kernel linked list implementation,
this is analogous as to how "list_head" structs are rarely useful on
their own, but are invariably found embedded in the larger objects of
interest.)

So, for example, the UIO code in drivers/uio/uio.c has a structure that
defines the memory region associated with a uio device:

    struct uio_map {
        struct kobject kobj;
        struct uio_mem *mem;
    };

If you have a struct uio_map structure, finding its embedded kobject is
just a matter of using the kobj member.  Code that works with kobjects will
often have the opposite problem, however: given a struct kobject pointer,
what is the pointer to the containing structure?  You must avoid tricks
(such as assuming that the kobject is at the beginning of the structure)
and, instead, use the container_of() macro, found in <linux/kernel.h>:

    container_of(pointer, type, member)

where:

  * "pointer" is the pointer to the embedded kobject,
  * "type" is the type of the containing structure, and
  * "member" is the name of the structure field to which "pointer" points.

The return value from container_of() is a pointer to the corresponding
container type. So, for example, a pointer "kp" to a struct kobject
embedded *within* a struct uio_map could be converted to a pointer to the
*containing* uio_map structure with:

    struct uio_map *u_map = container_of(kp, struct uio_map, kobj);

For convenience, programmers often define a simple macro for "back-casting"
kobject pointers to the containing type.  Exactly this happens in the
earlier drivers/uio/uio.c, as you can see here:

    struct uio_map {
        struct kobject kobj;
        struct uio_mem *mem;
    };

    #define to_map(map) container_of(map, struct uio_map, kobj)

where the macro argument "map" is a pointer to the struct kobject in
question.  That macro is subsequently invoked with:

    struct uio_map *map = to_map(kobj);


Initialization of kobjects

Code which creates a kobject must, of course, initialize that object. Some
of the internal fields are setup with a (mandatory) call to kobject_init():

    void kobject_init(struct kobject *kobj, struct kobj_type *ktype);

The ktype is required for a kobject to be created properly, as every kobject
must have an associated kobj_type.  After calling kobject_init(), to
register the kobject with sysfs, the function kobject_add() must be called:

    int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);

This sets up the parent of the kobject and the name for the kobject
properly.  If the kobject is to be associated with a specific kset,
kobj->kset must be assigned before calling kobject_add().  If a kset is
associated with a kobject, then the parent for the kobject can be set to
NULL in the call to kobject_add() and then the kobject's parent will be the
kset itself.

As the name of the kobject is set when it is added to the kernel, the name
of the kobject should never be manipulated directly.  If you must change
the name of the kobject, call kobject_rename():

    int kobject_rename(struct kobject *kobj, const char *new_name);

kobject_rename does not perform any locking or have a solid notion of
what names are valid so the caller must provide their own sanity checking
and serialization.

There is a function called kobject_set_name() but that is legacy cruft and
is being removed.  If your code needs to call this function, it is
incorrect and needs to be fixed.

To properly access the name of the kobject, use the function
kobject_name():

    const char *kobject_name(const struct kobject * kobj);

There is a helper function to both initialize and add the kobject to the
kernel at the same time, called surprisingly enough kobject_init_and_add():

    int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
                             struct kobject *parent, const char *fmt, ...);

The arguments are the same as the individual kobject_init() and
kobject_add() functions described above.


Uevents

After a kobject has been registered with the kobject core, you need to
announce to the world that it has been created.  This can be done with a
call to kobject_uevent():

    int kobject_uevent(struct kobject *kobj, enum kobject_action action);

Use the KOBJ_ADD action for when the kobject is first added to the kernel.
This should be done only after any attributes or children of the kobject
have been initialized properly, as userspace will instantly start to look
for them when this call happens.

When the kobject is removed from the kernel (details on how to do that is
below), the uevent for KOBJ_REMOVE will be automatically created by the
kobject core, so the caller does not have to worry about doing that by
hand.


Reference counts

One of the key functions of a kobject is to serve as a reference counter
for the object in which it is embedded. As long as references to the object
exist, the object (and the code which supports it) must continue to exist.
The low-level functions for manipulating a kobject's reference counts are:

    struct kobject *kobject_get(struct kobject *kobj);
    void kobject_put(struct kobject *kobj);

A successful call to kobject_get() will increment the kobject's reference
counter and return the pointer to the kobject.

When a reference is released, the call to kobject_put() will decrement the
reference count and, possibly, free the object. Note that kobject_init()
sets the reference count to one, so the code which sets up the kobject will
need to do a kobject_put() eventually to release that reference.

Because kobjects are dynamic, they must not be declared statically or on
the stack, but instead, always allocated dynamically.  Future versions of
the kernel will contain a run-time check for kobjects that are created
statically and will warn the developer of this improper usage.

If all that you want to use a kobject for is to provide a reference counter
for your structure, please use the struct kref instead; a kobject would be
overkill.  For more information on how to use struct kref, please see the
file Documentation/kref.txt in the Linux kernel source tree.


Creating "simple" kobjects

Sometimes all that a developer wants is a way to create a simple directory
in the sysfs hierarchy, and not have to mess with the whole complication of
ksets, show and store functions, and other details.  This is the one
exception where a single kobject should be created.  To create such an
entry, use the function:

    struct kobject *kobject_create_and_add(char *name, struct kobject *parent);

This function will create a kobject and place it in sysfs in the location
underneath the specified parent kobject.  To create simple attributes
associated with this kobject, use:

    int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
or
    int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);

Both types of attributes used here, with a kobject that has been created
with the kobject_create_and_add(), can be of type kobj_attribute, so no
special custom attribute is needed to be created.

See the example module, samples/kobject/kobject-example.c for an
implementation of a simple kobject and attributes.



ktypes and release methods

One important thing still missing from the discussion is what happens to a
kobject when its reference count reaches zero. The code which created the
kobject generally does not know when that will happen; if it did, there
would be little point in using a kobject in the first place. Even
predictable object lifecycles become more complicated when sysfs is brought
in as other portions of the kernel can get a reference on any kobject that
is registered in the system.

The end result is that a structure protected by a kobject cannot be freed
before its reference count goes to zero. The reference count is not under
the direct control of the code which created the kobject. So that code must
be notified asynchronously whenever the last reference to one of its
kobjects goes away.

Once you registered your kobject via kobject_add(), you must never use
kfree() to free it directly. The only safe way is to use kobject_put(). It
is good practice to always use kobject_put() after kobject_init() to avoid
errors creeping in.

This notification is done through a kobject's release() method. Usually
such a method has a form like:

    void my_object_release(struct kobject *kobj)
    {
            struct my_object *mine = container_of(kobj, struct my_object, kobj);

            /* Perform any additional cleanup on this object, then... */
            kfree(mine);
    }

One important point cannot be overstated: every kobject must have a
release() method, and the kobject must persist (in a consistent state)
until that method is called. If these constraints are not met, the code is
flawed.  Note that the kernel will warn you if you forget to provide a
release() method.  Do not try to get rid of this warning by providing an
"empty" release function; you will be mocked mercilessly by the kobject
maintainer if you attempt this.

Note, the name of the kobject is available in the release function, but it
must NOT be changed within this callback.  Otherwise there will be a memory
leak in the kobject core, which makes people unhappy.

Interestingly, the release() method is not stored in the kobject itself;
instead, it is associated with the ktype. So let us introduce struct
kobj_type:

    struct kobj_type {
            void (*release)(struct kobject *);
            const struct sysfs_ops *sysfs_ops;
            struct attribute    **default_attrs;
    };

This structure is used to describe a particular type of kobject (or, more
correctly, of containing object). Every kobject needs to have an associated
kobj_type structure; a pointer to that structure must be specified when you
call kobject_init() or kobject_init_and_add().

The release field in struct kobj_type is, of course, a pointer to the
release() method for this type of kobject. The other two fields (sysfs_ops
and default_attrs) control how objects of this type are represented in
sysfs; they are beyond the scope of this document.

The default_attrs pointer is a list of default attributes that will be
automatically created for any kobject that is registered with this ktype.


ksets

A kset is merely a collection of kobjects that want to be associated with
each other.  There is no restriction that they be of the same ktype, but be
very careful if they are not.

A kset serves these functions:

 - It serves as a bag containing a group of objects. A kset can be used by
   the kernel to track "all block devices" or "all PCI device drivers."

 - A kset is also a subdirectory in sysfs, where the associated kobjects
   with the kset can show up.  Every kset contains a kobject which can be
   set up to be the parent of other kobjects; the top-level directories of
   the sysfs hierarchy are constructed in this way.

 - Ksets can support the "hotplugging" of kobjects and influence how
   uevent events are reported to user space.

In object-oriented terms, "kset" is the top-level container class; ksets
contain their own kobject, but that kobject is managed by the kset code and
should not be manipulated by any other user.

A kset keeps its children in a standard kernel linked list.  Kobjects point
back to their containing kset via their kset field. In almost all cases,
the kobjects belonging to a kset have that kset (or, strictly, its embedded
kobject) in their parent.

As a kset contains a kobject within it, it should always be dynamically
created and never declared statically or on the stack.  To create a new
kset use:
  struct kset *kset_create_and_add(const char *name,
                                   struct kset_uevent_ops *u,
                                   struct kobject *parent);

When you are finished with the kset, call:
  void kset_unregister(struct kset *kset);
to destroy it.

An example of using a kset can be seen in the
samples/kobject/kset-example.c file in the kernel tree.

If a kset wishes to control the uevent operations of the kobjects
associated with it, it can use the struct kset_uevent_ops to handle it:

struct kset_uevent_ops {
        int (*filter)(struct kset *kset, struct kobject *kobj);
        const char *(*name)(struct kset *kset, struct kobject *kobj);
        int (*uevent)(struct kset *kset, struct kobject *kobj,
                      struct kobj_uevent_env *env);
};


The filter function allows a kset to prevent a uevent from being emitted to
userspace for a specific kobject.  If the function returns 0, the uevent
will not be emitted.

The name function will be called to override the default name of the kset
that the uevent sends to userspace.  By default, the name will be the same
as the kset itself, but this function, if present, can override that name.

The uevent function will be called when the uevent is about to be sent to
userspace to allow more environment variables to be added to the uevent.

One might ask how, exactly, a kobject is added to a kset, given that no
functions which perform that function have been presented.  The answer is
that this task is handled by kobject_add().  When a kobject is passed to
kobject_add(), its kset member should point to the kset to which the
kobject will belong.  kobject_add() will handle the rest.

If the kobject belonging to a kset has no parent kobject set, it will be
added to the kset's directory.  Not all members of a kset do necessarily
live in the kset directory.  If an explicit parent kobject is assigned
before the kobject is added, the kobject is registered with the kset, but
added below the parent kobject.


Kobject removal

After a kobject has been registered with the kobject core successfully, it
must be cleaned up when the code is finished with it.  To do that, call
kobject_put().  By doing this, the kobject core will automatically clean up
all of the memory allocated by this kobject.  If a KOBJ_ADD uevent has been
sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
any other sysfs housekeeping will be handled for the caller properly.

If you need to do a two-stage delete of the kobject (say you are not
allowed to sleep when you need to destroy the object), then call
kobject_del() which will unregister the kobject from sysfs.  This makes the
kobject "invisible", but it is not cleaned up, and the reference count of
the object is still the same.  At a later time call kobject_put() to finish
the cleanup of the memory associated with the kobject.

kobject_del() can be used to drop the reference to the parent object, if
circular references are constructed.  It is valid in some cases, that a
parent objects references a child.  Circular references _must_ be broken
with an explicit call to kobject_del(), so that a release functions will be
called, and the objects in the former circle release each other.


Example code to copy from

For a more complete example of using ksets and kobjects properly, see the
example programs samples/kobject/{kobject-example.c,kset-example.c},
which will be built as loadable modules if you select CONFIG_SAMPLE_KOBJECT.