[RFC] New kobject/kset/ktype documentation and example code

[Greg KH]

Right now I have about 80+ patches reworking the kset/ktype mess in the
-mm tree, cleaning things up and hopefully making it all easier for
people to both use, and understand.

So, while it is all relativly fresh in my mind, I thought it would be
good to also document the whole mess, and provide some solid example
code for others to use in the future.

Jonathan, I used your old lwn.net article about kobjects as the basis
for this document, I hope you don't mind (if you do, I'll be glad to
start over).  I've updated it to what is going on in the -mm tree, and
added new information.

This file should replace the existing Documentation/kobject.txt which is
woefully out of date and obsolete now.

I also have two example kernel modules showing how to use a simple
kobject and attributes, as well as a more complex kset/ktype/kobject
interaction.  I'll reply to this message with them as well, and I am
going to place them in the samples/kobject/ directory unless someone
really objects.

Any review comments that people might have on both the document, and the
two sample modules would be greatly appreciated.

thanks,

greg k-h

-----------------------------

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

Greg Kroah-Hartman <gregkh@xxxxxxx>

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 November 27, 2008


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 when a kobject is no longer referenced and the kobject's
   default representation in sysfs.

 - 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 (even unknown) 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.

So, for example, UIO code has a structure that defines the memory region
associated with a uio device:

struct uio_mem {
	struct kobject kobj;
	unsigned long addr;
	unsigned long size;
	int memtype;
	void __iomem *internal_addr;
};

If you have a struct uio_mem structure, finding its embedded kobject is just a
matter of using the kobj pointer.  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 given type. So, for example, a pointer to a struct kobject embedded
within a struct cdev called "kp" could be converted to a pointer to the
containing structure with:

    struct uio_mem *u_mem = container_of(kp, struct uio_mem, kobj);

Programmers will often define a simple macro for "back-casting" kobject
pointers to the containing type.


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);

Among other things, kobject_init() sets the kobject's reference count to
one.  Calling kobject_init() is not sufficient, however. Kobject users
must, at a minimum, set the name of the kobject; this is the name that will
be used in sysfs entries.  To set the name of a kobject properly, do not
attempt to manipulate the internal name field, but instead use:

    int kobject_set_name(struct kobject *kobj, const char *format, ...);

This function takes a printk-style variable argument list. Believe it or
not, it is actually possible for this operation to fail; conscientious code
should check the return value and react accordingly.

The other kobject fields which should be set, directly or indirectly, by
the creator are its ktype, kset, and parent. We will get to those shortly,
however please note that the ktype and kset must be set before the
kobject_init() function is called.



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. If, however, the kobject is
already in the process of being destroyed, the operation will fail and
kobject_get() will return NULL. This return value must always be tested, or
no end of unpleasant race conditions could result.

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 from the heap.  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.


Hooking into sysfs

An initialized kobject will perform reference counting without trouble, but
it will not appear in sysfs. To create sysfs entries, kernel code must pass
the object to kobject_add():

    int kobject_add(struct kobject *kobj);

As always, this operation can fail. The function:

    void kobject_del(struct kobject *kobj);

will remove the kobject from sysfs.

There is a kobject_register() function, which is really just the
combination of the calls to kobject_init() and kobject_add(). Similarly,
kobject_unregister() will call kobject_del(), then call kobject_put() to
release the initial reference created with kobject_register() (or really
kobject_init()).


Creating "simple" kobjects

Sometimes all that a developer wants is a way to create a simple directory
in the sysfs heirachy, and not have to mess with the whole complication of
ksets, show and store functions, and other details.  To create such an
entry, use the function:

    struct kobject *kobject_create_and_register(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_register() can be of type kobj_attribute, 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
predicatable object lifecycles become more complicated when sysfs is
brought in; user-space programs can keep a reference to a kobject (by
keeping one of its associated sysfs files open) for an arbitrary period of
time.

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.

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 merciously by the kobject
maintainer if you attempt this.

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 *);
	    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 can be placed in the
kobject's ktype field at initialization time, or (more likely) it can be
defined by the kobject's containing kset.

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.


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; in this way the device model
   hierarchy is constructed.

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

 - A kset can provide a set of default attributes that all kobjects that
   belong to it automatically inherit and have created whenever a kobject
   is registered belonging to the kset.

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 contained kobjects also have a pointer to the kset (or, strictly, its
embedded kobject) in their parent field.

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_register(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. There is currently
no other way to add a kobject to a kset without directly messing with the
list pointers.


Kobject initialization again

Now that we have covered all of that stuff, we can talk in detail about how
a kobject should be prepared for its existence in the kernel. Here are all
of the struct kobject fields which must be initialized somehow:

 - k_name - the name of the object. This fields should always be
   initialized with kobject_set_name(), or specified in the original call
   to kobject_create_and_register().

 - refcount is the kobject's reference count; it is initialized by kobject_init()

 - parent is the kobject's parent in whatever hierarchy it belongs to. It
   can be set explicitly by the creator. If parent is NULL when
   kobject_add() is called, it will be set to the kobject of the containing
   kset.

 - kset is a pointer to the kset which will contain this kobject; it should
   be set prior to calling kobject_init().

 - ktype is the type of the kobject; it should be set prior to calling
   kobject_init().

Often, much of the initialization of a kobject is handled by the layer that
manages the containing kset.  See the sample/kobject/kset-example.c for how
this is usually handled.

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