Re: [RFC] kobject/kset/ktype documentation and example code updated

[Alan Stern]

On Wed, 19 Dec 2007, Greg KH wrote:

> Any review comments that people might have on both the document, and the
> two sample modules would be greatly appreciated.
 
Here are some things I noticed on a first reading:

> 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 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

I think the wording here is a little misleading.  It might be better to
say: "For every kind of structure with an embedded kobject, there
should be a corresponding ktype object.  In each structure of that kind
the embedded kobject should contain a pointer to that ktype."

Even that isn't great.  Maybe somebody can suggest something better.

>    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 for kernel code to create a standalone kobject; with one major

The ';' should be a ','.

> 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, the 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 structure.  Code that works with kobjects

"using the kobj member."

> 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 uio_mem 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, struct kobj_type *ktype);
> 
> The ktype is required for a kobject to be created properly, as every kobject
> must have an associated kobj_type.  Among other things, kobject_init() sets
> the kobject's reference count to one.  After calling kobject_init(), to

Setting the reference count to one is explained below, so the sentence
can be omitted here.

> 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, that
> assignment must be done before calling kobject_add().  If a kset is

"kobj->kset must be assigned" instead of "that assignment must be done"

> 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 will be placed under
> the kset itself.

"and then the kobject's parent will be set to the kset itself."

Kay, in my version of kobject_add() it says:

	if (kobj->kset) {
		kobj->kset = kset_get(kobj->kset);

		if (!parent) {
			parent = kobject_get(&kobj->kset->kobj);
			/*
			 * If the kset is our parent, get a second
			 * reference, we drop both the kset and the
			 * parent ref on cleanup
			 */
			kobject_get(parent);
		}

That last call to kobject_get(parent) doesn't make sense.  The code has
already taken two references to the kset: one in the assignment of
kobj->kset and the other in the assignment of parent.  The final
kobject_get() then acquires a third reference, which will never be
dropped.

> 
> 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);
> 
> 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 supprisingly 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, it needs to be
> announced to the world that it has been created.  This can be done with

"you need to announce"

> 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 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.
> 
> If all that you are wanting to use a kobject for is to provide a reference
> counter for your structure, please use the struct kref instead, a kobject

The second ',' in the line above should be a ';'.

> 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, 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 *);
> 	    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.

"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; in this way the device model
>    hierarchy is constructed.

"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.

Don't you want to include the definition of struct kset here?

> 
> 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.

"In almost all cases, the kobjects belonging to a kset have that kset 
(or, strictly, its embedded kobject) as 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(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.

This paragraph is unnecessary since it is already explained in the
section about kobject_add().

> 
> 
> Kobject cleaning up
> 
> 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_unregister(), or kobject_del() followed with a call to
> kobject_put().
> 
> Any initialization of the kobject that has been done will be automatically
> cleaned up by the kobject core.  If a KOBJ_ADD uevent has been sent for
> the object, a corrisponding KOBJ_REMOVE uevent will be sent, and any other
> sysfs housekeeping will be handled for the caller properly.
> 
> In the future, the kobject_unregister() call will be going away to help
> clean up the kobject api.

Will there be a kobject_del_and_put() routine, to complement 
kobject_init_and_add()?

> 
> 
> For a more complete example of using ksets and kobjects properly, see the
> sample/kobject/kset-example.c code.

Alan Stern

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