[RFC] sample kobject implementation

[Greg KH <greg@kroah.com>]

/*
 * Sample kobject implementation
 *
 * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
 * Copyright (C) 2007 Novell Inc.
 *
 * Released under the GPL version 2 only.
 *
 */
#include <linux/kobject.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <linux/module.h>
#include <linux/init.h>

/*
 * This module shows how to create a simple subdirectory in sysfs called
 * /sys/kernel/kobject-example  In that directory, 3 files are created:
 * "foo", "baz", and "bar".  If an integer is written to these files, it can be
 * later read out of it.
 */

static int foo;
static int baz;
static int bar;

/*
 * The "foo" file where a static variable is read from and written to.
 */
static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
			char *buf)
{
	return sprintf(buf, "%d\n", foo);
}

static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
			 const char *buf, size_t count)
{
	sscanf(buf, "%du", &foo);
	return count;
}

static struct kobj_attribute foo_attribute =
	__ATTR(foo, 0666, foo_show, foo_store);

/*
 * More complex function where we determine which varible is being accessed by
 * looking at the attribute for the "baz" and "bar" files.
 */
static ssize_t b_show(struct kobject *kobj, struct kobj_attribute *attr,
		      char *buf)
{
	int var;

	if (strcmp(attr->attr.name, "baz") == 0)
		var = baz;
	else
		var = bar;
	return sprintf(buf, "%d\n", var);
}

static ssize_t b_store(struct kobject *kobj, struct kobj_attribute *attr,
		       const char *buf, size_t count)
{
	int var;

	sscanf(buf, "%du", &var);
	if (strcmp(attr->attr.name, "baz") == 0)
		baz = var;
	else
		bar = var;
	return count;
}

static struct kobj_attribute baz_attribute =
	__ATTR(baz, 0666, b_show, b_store);
static struct kobj_attribute bar_attribute =
	__ATTR(bar, 0666, b_show, b_store);


/*
 * Create a group of attributes so that we can create and destory them all
 * at once.
 */
static struct attribute *attrs[] = {
	&foo_attribute.attr,
	&baz_attribute.attr,
	&bar_attribute.attr,
	NULL,	/* need to NULL terminate the list of attributes */
};

/*
 * An unnamed attribute group will put all of the attributes directly in
 * the kobject directory.  If we specify a name, a subdirectory will be
 * created for the attributes with the directory being the name of the
 * attribute group.
 */
static struct attribute_group attr_group = {
	.attrs = attrs,
};

static struct kobject *example_kobj;

static int example_init(void)
{
	int retval;

	/*
	 * Create a simple kobject with the name of "kobject_example",
	 * located under /sys/kernel/
	 */
	example_kobj = kobject_create_and_register("kobject_example",
						   kernel_kobj);
	if (!example_kobj)
		return -ENOMEM;

	/*
	 * Note, these files will be created _after_ the kobject above is
	 * created.  This can cause userspace to be looking around in sysfs
	 * for these files before they are really created.  If you are
	 * worried about something like this, perhaps you really need to
	 * create your own kset and have a default attribute group for your
	 * kobject.
	 */
	retval = sysfs_create_group(example_kobj, &attr_group);
	if (retval)
		kobject_unregister(example_kobj);

	return retval;
}

static void example_exit(void)
{
	kobject_unregister(example_kobj);
}

module_init(example_init);
module_exit(example_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");
On Tue, Nov 27, 2007 at 03:02:52PM -0800, Greg KH wrote:
> 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@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 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.
>