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![John de la Garza](/assets/img/avatar_default.png)
Signed-off-by: John de la Garza <john@jjdev.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
415 lines
18 KiB
Text
415 lines
18 KiB
Text
Everything you never wanted to know about kobjects, ksets, and ktypes
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Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Based on an original article by Jon Corbet for lwn.net written October 1,
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2003 and located at http://lwn.net/Articles/51437/
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Last updated December 19, 2007
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Part of the difficulty in understanding the driver model - and the kobject
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abstraction upon which it is built - is that there is no obvious starting
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place. Dealing with kobjects requires understanding a few different types,
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all of which make reference to each other. In an attempt to make things
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easier, we'll take a multi-pass approach, starting with vague terms and
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adding detail as we go. To that end, here are some quick definitions of
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some terms we will be working with.
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- A kobject is an object of type struct kobject. Kobjects have a name
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and a reference count. A kobject also has a parent pointer (allowing
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objects to be arranged into hierarchies), a specific type, and,
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usually, a representation in the sysfs virtual filesystem.
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Kobjects are generally not interesting on their own; instead, they are
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usually embedded within some other structure which contains the stuff
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the code is really interested in.
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No structure should EVER have more than one kobject embedded within it.
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If it does, the reference counting for the object is sure to be messed
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up and incorrect, and your code will be buggy. So do not do this.
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- A ktype is the type of object that embeds a kobject. Every structure
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that embeds a kobject needs a corresponding ktype. The ktype controls
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what happens to the kobject when it is created and destroyed.
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- A kset is a group of kobjects. These kobjects can be of the same ktype
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or belong to different ktypes. The kset is the basic container type for
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collections of kobjects. Ksets contain their own kobjects, but you can
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safely ignore that implementation detail as the kset core code handles
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this kobject automatically.
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When you see a sysfs directory full of other directories, generally each
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of those directories corresponds to a kobject in the same kset.
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We'll look at how to create and manipulate all of these types. A bottom-up
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approach will be taken, so we'll go back to kobjects.
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Embedding kobjects
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It is rare for kernel code to create a standalone kobject, with one major
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exception explained below. Instead, kobjects are used to control access to
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a larger, domain-specific object. To this end, kobjects will be found
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embedded in other structures. If you are used to thinking of things in
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object-oriented terms, kobjects can be seen as a top-level, abstract class
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from which other classes are derived. A kobject implements a set of
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capabilities which are not particularly useful by themselves, but which are
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nice to have in other objects. The C language does not allow for the
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direct expression of inheritance, so other techniques - such as structure
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embedding - must be used.
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(As an aside, for those familiar with the kernel linked list implementation,
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this is analogous as to how "list_head" structs are rarely useful on
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their own, but are invariably found embedded in the larger objects of
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interest.)
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So, for example, the UIO code in drivers/uio/uio.c has a structure that
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defines the memory region associated with a uio device:
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struct uio_map {
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struct kobject kobj;
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struct uio_mem *mem;
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};
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If you have a struct uio_map structure, finding its embedded kobject is
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just a matter of using the kobj member. Code that works with kobjects will
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often have the opposite problem, however: given a struct kobject pointer,
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what is the pointer to the containing structure? You must avoid tricks
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(such as assuming that the kobject is at the beginning of the structure)
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and, instead, use the container_of() macro, found in <linux/kernel.h>:
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container_of(pointer, type, member)
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where:
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* "pointer" is the pointer to the embedded kobject,
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* "type" is the type of the containing structure, and
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* "member" is the name of the structure field to which "pointer" points.
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The return value from container_of() is a pointer to the corresponding
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container type. So, for example, a pointer "kp" to a struct kobject
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embedded *within* a struct uio_map could be converted to a pointer to the
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*containing* uio_map structure with:
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struct uio_map *u_map = container_of(kp, struct uio_map, kobj);
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For convenience, programmers often define a simple macro for "back-casting"
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kobject pointers to the containing type. Exactly this happens in the
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earlier drivers/uio/uio.c, as you can see here:
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struct uio_map {
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struct kobject kobj;
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struct uio_mem *mem;
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};
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#define to_map(map) container_of(map, struct uio_map, kobj)
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where the macro argument "map" is a pointer to the struct kobject in
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question. That macro is subsequently invoked with:
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struct uio_map *map = to_map(kobj);
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Initialization of kobjects
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Code which creates a kobject must, of course, initialize that object. Some
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of the internal fields are setup with a (mandatory) call to kobject_init():
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void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
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The ktype is required for a kobject to be created properly, as every kobject
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must have an associated kobj_type. After calling kobject_init(), to
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register the kobject with sysfs, the function kobject_add() must be called:
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int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
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This sets up the parent of the kobject and the name for the kobject
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properly. If the kobject is to be associated with a specific kset,
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kobj->kset must be assigned before calling kobject_add(). If a kset is
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associated with a kobject, then the parent for the kobject can be set to
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NULL in the call to kobject_add() and then the kobject's parent will be the
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kset itself.
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As the name of the kobject is set when it is added to the kernel, the name
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of the kobject should never be manipulated directly. If you must change
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the name of the kobject, call kobject_rename():
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int kobject_rename(struct kobject *kobj, const char *new_name);
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kobject_rename does not perform any locking or have a solid notion of
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what names are valid so the caller must provide their own sanity checking
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and serialization.
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There is a function called kobject_set_name() but that is legacy cruft and
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is being removed. If your code needs to call this function, it is
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incorrect and needs to be fixed.
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To properly access the name of the kobject, use the function
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kobject_name():
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const char *kobject_name(const struct kobject * kobj);
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There is a helper function to both initialize and add the kobject to the
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kernel at the same time, called surprisingly enough kobject_init_and_add():
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int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
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struct kobject *parent, const char *fmt, ...);
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The arguments are the same as the individual kobject_init() and
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kobject_add() functions described above.
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Uevents
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After a kobject has been registered with the kobject core, you need to
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announce to the world that it has been created. This can be done with a
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call to kobject_uevent():
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int kobject_uevent(struct kobject *kobj, enum kobject_action action);
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Use the KOBJ_ADD action for when the kobject is first added to the kernel.
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This should be done only after any attributes or children of the kobject
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have been initialized properly, as userspace will instantly start to look
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for them when this call happens.
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When the kobject is removed from the kernel (details on how to do that are
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below), the uevent for KOBJ_REMOVE will be automatically created by the
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kobject core, so the caller does not have to worry about doing that by
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hand.
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Reference counts
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One of the key functions of a kobject is to serve as a reference counter
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for the object in which it is embedded. As long as references to the object
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exist, the object (and the code which supports it) must continue to exist.
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The low-level functions for manipulating a kobject's reference counts are:
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struct kobject *kobject_get(struct kobject *kobj);
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void kobject_put(struct kobject *kobj);
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A successful call to kobject_get() will increment the kobject's reference
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counter and return the pointer to the kobject.
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When a reference is released, the call to kobject_put() will decrement the
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reference count and, possibly, free the object. Note that kobject_init()
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sets the reference count to one, so the code which sets up the kobject will
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need to do a kobject_put() eventually to release that reference.
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Because kobjects are dynamic, they must not be declared statically or on
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the stack, but instead, always allocated dynamically. Future versions of
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the kernel will contain a run-time check for kobjects that are created
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statically and will warn the developer of this improper usage.
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If all that you want to use a kobject for is to provide a reference counter
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for your structure, please use the struct kref instead; a kobject would be
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overkill. For more information on how to use struct kref, please see the
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file Documentation/kref.txt in the Linux kernel source tree.
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Creating "simple" kobjects
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Sometimes all that a developer wants is a way to create a simple directory
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in the sysfs hierarchy, and not have to mess with the whole complication of
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ksets, show and store functions, and other details. This is the one
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exception where a single kobject should be created. To create such an
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entry, use the function:
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struct kobject *kobject_create_and_add(char *name, struct kobject *parent);
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This function will create a kobject and place it in sysfs in the location
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underneath the specified parent kobject. To create simple attributes
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associated with this kobject, use:
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int sysfs_create_file(struct kobject *kobj, struct attribute *attr);
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or
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int sysfs_create_group(struct kobject *kobj, struct attribute_group *grp);
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Both types of attributes used here, with a kobject that has been created
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with the kobject_create_and_add(), can be of type kobj_attribute, so no
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special custom attribute is needed to be created.
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See the example module, samples/kobject/kobject-example.c for an
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implementation of a simple kobject and attributes.
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ktypes and release methods
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One important thing still missing from the discussion is what happens to a
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kobject when its reference count reaches zero. The code which created the
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kobject generally does not know when that will happen; if it did, there
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would be little point in using a kobject in the first place. Even
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predictable object lifecycles become more complicated when sysfs is brought
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in as other portions of the kernel can get a reference on any kobject that
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is registered in the system.
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The end result is that a structure protected by a kobject cannot be freed
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before its reference count goes to zero. The reference count is not under
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the direct control of the code which created the kobject. So that code must
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be notified asynchronously whenever the last reference to one of its
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kobjects goes away.
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Once you registered your kobject via kobject_add(), you must never use
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kfree() to free it directly. The only safe way is to use kobject_put(). It
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is good practice to always use kobject_put() after kobject_init() to avoid
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errors creeping in.
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This notification is done through a kobject's release() method. Usually
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such a method has a form like:
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void my_object_release(struct kobject *kobj)
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{
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struct my_object *mine = container_of(kobj, struct my_object, kobj);
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/* Perform any additional cleanup on this object, then... */
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kfree(mine);
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}
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One important point cannot be overstated: every kobject must have a
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release() method, and the kobject must persist (in a consistent state)
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until that method is called. If these constraints are not met, the code is
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flawed. Note that the kernel will warn you if you forget to provide a
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release() method. Do not try to get rid of this warning by providing an
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"empty" release function; you will be mocked mercilessly by the kobject
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maintainer if you attempt this.
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Note, the name of the kobject is available in the release function, but it
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must NOT be changed within this callback. Otherwise there will be a memory
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leak in the kobject core, which makes people unhappy.
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Interestingly, the release() method is not stored in the kobject itself;
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instead, it is associated with the ktype. So let us introduce struct
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kobj_type:
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struct kobj_type {
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void (*release)(struct kobject *kobj);
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const struct sysfs_ops *sysfs_ops;
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struct attribute **default_attrs;
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const struct kobj_ns_type_operations *(*child_ns_type)(struct kobject *kobj);
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const void *(*namespace)(struct kobject *kobj);
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};
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This structure is used to describe a particular type of kobject (or, more
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correctly, of containing object). Every kobject needs to have an associated
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kobj_type structure; a pointer to that structure must be specified when you
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call kobject_init() or kobject_init_and_add().
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The release field in struct kobj_type is, of course, a pointer to the
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release() method for this type of kobject. The other two fields (sysfs_ops
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and default_attrs) control how objects of this type are represented in
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sysfs; they are beyond the scope of this document.
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The default_attrs pointer is a list of default attributes that will be
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automatically created for any kobject that is registered with this ktype.
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ksets
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A kset is merely a collection of kobjects that want to be associated with
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each other. There is no restriction that they be of the same ktype, but be
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very careful if they are not.
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A kset serves these functions:
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- It serves as a bag containing a group of objects. A kset can be used by
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the kernel to track "all block devices" or "all PCI device drivers."
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- A kset is also a subdirectory in sysfs, where the associated kobjects
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with the kset can show up. Every kset contains a kobject which can be
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set up to be the parent of other kobjects; the top-level directories of
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the sysfs hierarchy are constructed in this way.
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- Ksets can support the "hotplugging" of kobjects and influence how
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uevent events are reported to user space.
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In object-oriented terms, "kset" is the top-level container class; ksets
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contain their own kobject, but that kobject is managed by the kset code and
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should not be manipulated by any other user.
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A kset keeps its children in a standard kernel linked list. Kobjects point
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back to their containing kset via their kset field. In almost all cases,
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the kobjects belonging to a kset have that kset (or, strictly, its embedded
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kobject) in their parent.
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As a kset contains a kobject within it, it should always be dynamically
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created and never declared statically or on the stack. To create a new
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kset use:
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struct kset *kset_create_and_add(const char *name,
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struct kset_uevent_ops *u,
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struct kobject *parent);
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When you are finished with the kset, call:
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void kset_unregister(struct kset *kset);
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to destroy it. This removes the kset from sysfs and decrements its reference
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count. When the reference count goes to zero, the kset will be released.
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Because other references to the kset may still exist, the release may happen
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after kset_unregister() returns.
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An example of using a kset can be seen in the
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samples/kobject/kset-example.c file in the kernel tree.
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If a kset wishes to control the uevent operations of the kobjects
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associated with it, it can use the struct kset_uevent_ops to handle it:
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struct kset_uevent_ops {
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int (*filter)(struct kset *kset, struct kobject *kobj);
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const char *(*name)(struct kset *kset, struct kobject *kobj);
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int (*uevent)(struct kset *kset, struct kobject *kobj,
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struct kobj_uevent_env *env);
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};
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The filter function allows a kset to prevent a uevent from being emitted to
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userspace for a specific kobject. If the function returns 0, the uevent
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will not be emitted.
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The name function will be called to override the default name of the kset
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that the uevent sends to userspace. By default, the name will be the same
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as the kset itself, but this function, if present, can override that name.
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The uevent function will be called when the uevent is about to be sent to
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userspace to allow more environment variables to be added to the uevent.
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One might ask how, exactly, a kobject is added to a kset, given that no
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functions which perform that function have been presented. The answer is
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that this task is handled by kobject_add(). When a kobject is passed to
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kobject_add(), its kset member should point to the kset to which the
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kobject will belong. kobject_add() will handle the rest.
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If the kobject belonging to a kset has no parent kobject set, it will be
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added to the kset's directory. Not all members of a kset do necessarily
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live in the kset directory. If an explicit parent kobject is assigned
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before the kobject is added, the kobject is registered with the kset, but
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added below the parent kobject.
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Kobject removal
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After a kobject has been registered with the kobject core successfully, it
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must be cleaned up when the code is finished with it. To do that, call
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kobject_put(). By doing this, the kobject core will automatically clean up
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all of the memory allocated by this kobject. If a KOBJ_ADD uevent has been
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sent for the object, a corresponding KOBJ_REMOVE uevent will be sent, and
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any other sysfs housekeeping will be handled for the caller properly.
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If you need to do a two-stage delete of the kobject (say you are not
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allowed to sleep when you need to destroy the object), then call
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kobject_del() which will unregister the kobject from sysfs. This makes the
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kobject "invisible", but it is not cleaned up, and the reference count of
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the object is still the same. At a later time call kobject_put() to finish
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the cleanup of the memory associated with the kobject.
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kobject_del() can be used to drop the reference to the parent object, if
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circular references are constructed. It is valid in some cases, that a
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parent objects references a child. Circular references _must_ be broken
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with an explicit call to kobject_del(), so that a release functions will be
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called, and the objects in the former circle release each other.
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Example code to copy from
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For a more complete example of using ksets and kobjects properly, see the
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example programs samples/kobject/{kobject-example.c,kset-example.c},
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which will be built as loadable modules if you select CONFIG_SAMPLE_KOBJECT.
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