configfs - Userspace-driven kernel object configuration.
  
  Joel Becker <joel.becker@oracle.com>
  
  Updated: 31 March 2005
  
  Copyright (c) 2005 Oracle Corporation,
  	Joel Becker <joel.becker@oracle.com>
  
  
  [What is configfs?]
  
  configfs is a ram-based filesystem that provides the converse of
  sysfs's functionality.  Where sysfs is a filesystem-based view of
  kernel objects, configfs is a filesystem-based manager of kernel
  objects, or config_items.
  
  With sysfs, an object is created in kernel (for example, when a device
  is discovered) and it is registered with sysfs.  Its attributes then
  appear in sysfs, allowing userspace to read the attributes via
  readdir(3)/read(2).  It may allow some attributes to be modified via
  write(2).  The important point is that the object is created and
  destroyed in kernel, the kernel controls the lifecycle of the sysfs
  representation, and sysfs is merely a window on all this.
  
  A configfs config_item is created via an explicit userspace operation:
  mkdir(2).  It is destroyed via rmdir(2).  The attributes appear at
  mkdir(2) time, and can be read or modified via read(2) and write(2).
  As with sysfs, readdir(3) queries the list of items and/or attributes.
  symlink(2) can be used to group items together.  Unlike sysfs, the
  lifetime of the representation is completely driven by userspace.  The
  kernel modules backing the items must respond to this.
  
  Both sysfs and configfs can and should exist together on the same
  system.  One is not a replacement for the other.
  
  [Using configfs]
  
  configfs can be compiled as a module or into the kernel.  You can access
  it by doing
  
  	mount -t configfs none /config
  
  The configfs tree will be empty unless client modules are also loaded.
  These are modules that register their item types with configfs as
  subsystems.  Once a client subsystem is loaded, it will appear as a
  subdirectory (or more than one) under /config.  Like sysfs, the
  configfs tree is always there, whether mounted on /config or not.
  
  An item is created via mkdir(2).  The item's attributes will also
  appear at this time.  readdir(3) can determine what the attributes are,
  read(2) can query their default values, and write(2) can store new
  values.  Like sysfs, attributes should be ASCII text files, preferably
  with only one value per file.  The same efficiency caveats from sysfs
  apply.  Don't mix more than one attribute in one attribute file.
  
  Like sysfs, configfs expects write(2) to store the entire buffer at
  once.  When writing to configfs attributes, userspace processes should
  first read the entire file, modify the portions they wish to change, and
  then write the entire buffer back.  Attribute files have a maximum size
  of one page (PAGE_SIZE, 4096 on i386).
  
  When an item needs to be destroyed, remove it with rmdir(2).  An
  item cannot be destroyed if any other item has a link to it (via
  symlink(2)).  Links can be removed via unlink(2).
  
  [Configuring FakeNBD: an Example]
  
  Imagine there's a Network Block Device (NBD) driver that allows you to
  access remote block devices.  Call it FakeNBD.  FakeNBD uses configfs
  for its configuration.  Obviously, there will be a nice program that
  sysadmins use to configure FakeNBD, but somehow that program has to tell
  the driver about it.  Here's where configfs comes in.
  
  When the FakeNBD driver is loaded, it registers itself with configfs.
  readdir(3) sees this just fine:
  
  	# ls /config
  	fakenbd
  
  A fakenbd connection can be created with mkdir(2).  The name is
  arbitrary, but likely the tool will make some use of the name.  Perhaps
  it is a uuid or a disk name:
  
  	# mkdir /config/fakenbd/disk1
  	# ls /config/fakenbd/disk1
  	target device rw
  
  The target attribute contains the IP address of the server FakeNBD will
  connect to.  The device attribute is the device on the server.
  Predictably, the rw attribute determines whether the connection is
  read-only or read-write.
  
  	# echo 10.0.0.1 > /config/fakenbd/disk1/target
  	# echo /dev/sda1 > /config/fakenbd/disk1/device
  	# echo 1 > /config/fakenbd/disk1/rw
  
  That's it.  That's all there is.  Now the device is configured, via the
  shell no less.
  
  [Coding With configfs]
  
  Every object in configfs is a config_item.  A config_item reflects an
  object in the subsystem.  It has attributes that match values on that
  object.  configfs handles the filesystem representation of that object
  and its attributes, allowing the subsystem to ignore all but the
  basic show/store interaction.
  
  Items are created and destroyed inside a config_group.  A group is a
  collection of items that share the same attributes and operations.
  Items are created by mkdir(2) and removed by rmdir(2), but configfs
  handles that.  The group has a set of operations to perform these tasks
  
  A subsystem is the top level of a client module.  During initialization,
  the client module registers the subsystem with configfs, the subsystem
  appears as a directory at the top of the configfs filesystem.  A
  subsystem is also a config_group, and can do everything a config_group
  can.
  
  [struct config_item]
  
  	struct config_item {
  		char                    *ci_name;
  		char                    ci_namebuf[UOBJ_NAME_LEN];
  		struct kref             ci_kref;
  		struct list_head        ci_entry;
  		struct config_item      *ci_parent;
  		struct config_group     *ci_group;
  		struct config_item_type *ci_type;
  		struct dentry           *ci_dentry;
  	};
  
  	void config_item_init(struct config_item *);
  	void config_item_init_type_name(struct config_item *,
  					const char *name,
  					struct config_item_type *type);
  	struct config_item *config_item_get(struct config_item *);
  	void config_item_put(struct config_item *);
  
  Generally, struct config_item is embedded in a container structure, a
  structure that actually represents what the subsystem is doing.  The
  config_item portion of that structure is how the object interacts with
  configfs.
  
  Whether statically defined in a source file or created by a parent
  config_group, a config_item must have one of the _init() functions
  called on it.  This initializes the reference count and sets up the
  appropriate fields.
  
  All users of a config_item should have a reference on it via
  config_item_get(), and drop the reference when they are done via
  config_item_put().
  
  By itself, a config_item cannot do much more than appear in configfs.
  Usually a subsystem wants the item to display and/or store attributes,
  among other things.  For that, it needs a type.
  
  [struct config_item_type]
  
  	struct configfs_item_operations {
  		void (*release)(struct config_item *);
  		ssize_t (*show_attribute)(struct config_item *,
  					  struct configfs_attribute *,
  					  char *);
  		ssize_t (*store_attribute)(struct config_item *,
  					   struct configfs_attribute *,
  					   const char *, size_t);
  		int (*allow_link)(struct config_item *src,
  				  struct config_item *target);
  		int (*drop_link)(struct config_item *src,
  				 struct config_item *target);
  	};
  
  	struct config_item_type {
  		struct module                           *ct_owner;
  		struct configfs_item_operations         *ct_item_ops;
  		struct configfs_group_operations        *ct_group_ops;
  		struct configfs_attribute               **ct_attrs;
  	};
  
  The most basic function of a config_item_type is to define what
  operations can be performed on a config_item.  All items that have been
  allocated dynamically will need to provide the ct_item_ops->release()
  method.  This method is called when the config_item's reference count
  reaches zero.  Items that wish to display an attribute need to provide
  the ct_item_ops->show_attribute() method.  Similarly, storing a new
  attribute value uses the store_attribute() method.
  
  [struct configfs_attribute]
  
  	struct configfs_attribute {
  		char                    *ca_name;
  		struct module           *ca_owner;
  		umode_t                  ca_mode;
  	};
  
  When a config_item wants an attribute to appear as a file in the item's
  configfs directory, it must define a configfs_attribute describing it.
  It then adds the attribute to the NULL-terminated array
  config_item_type->ct_attrs.  When the item appears in configfs, the
  attribute file will appear with the configfs_attribute->ca_name
  filename.  configfs_attribute->ca_mode specifies the file permissions.
  
  If an attribute is readable and the config_item provides a
  ct_item_ops->show_attribute() method, that method will be called
  whenever userspace asks for a read(2) on the attribute.  The converse
  will happen for write(2).
  
  [struct config_group]
  
  A config_item cannot live in a vacuum.  The only way one can be created
  is via mkdir(2) on a config_group.  This will trigger creation of a
  child item.
  
  	struct config_group {
  		struct config_item		cg_item;
  		struct list_head		cg_children;
  		struct configfs_subsystem 	*cg_subsys;
  		struct config_group		**default_groups;
  	};
  
  	void config_group_init(struct config_group *group);
  	void config_group_init_type_name(struct config_group *group,
  					 const char *name,
  					 struct config_item_type *type);
  
  
  The config_group structure contains a config_item.  Properly configuring
  that item means that a group can behave as an item in its own right.
  However, it can do more: it can create child items or groups.  This is
  accomplished via the group operations specified on the group's
  config_item_type.
  
  	struct configfs_group_operations {
  		struct config_item *(*make_item)(struct config_group *group,
  						 const char *name);
  		struct config_group *(*make_group)(struct config_group *group,
  						   const char *name);
  		int (*commit_item)(struct config_item *item);
  		void (*disconnect_notify)(struct config_group *group,
  					  struct config_item *item);
  		void (*drop_item)(struct config_group *group,
  				  struct config_item *item);
  	};
  
  A group creates child items by providing the
  ct_group_ops->make_item() method.  If provided, this method is called from mkdir(2) in the group's directory.  The subsystem allocates a new
  config_item (or more likely, its container structure), initializes it,
  and returns it to configfs.  Configfs will then populate the filesystem
  tree to reflect the new item.
  
  If the subsystem wants the child to be a group itself, the subsystem
  provides ct_group_ops->make_group().  Everything else behaves the same,
  using the group _init() functions on the group.
  
  Finally, when userspace calls rmdir(2) on the item or group,
  ct_group_ops->drop_item() is called.  As a config_group is also a
  config_item, it is not necessary for a separate drop_group() method.
  The subsystem must config_item_put() the reference that was initialized
  upon item allocation.  If a subsystem has no work to do, it may omit
  the ct_group_ops->drop_item() method, and configfs will call
  config_item_put() on the item on behalf of the subsystem.
  
  IMPORTANT: drop_item() is void, and as such cannot fail.  When rmdir(2)
  is called, configfs WILL remove the item from the filesystem tree
  (assuming that it has no children to keep it busy).  The subsystem is
  responsible for responding to this.  If the subsystem has references to
  the item in other threads, the memory is safe.  It may take some time
  for the item to actually disappear from the subsystem's usage.  But it
  is gone from configfs.
  
  When drop_item() is called, the item's linkage has already been torn
  down.  It no longer has a reference on its parent and has no place in
  the item hierarchy.  If a client needs to do some cleanup before this
  teardown happens, the subsystem can implement the
  ct_group_ops->disconnect_notify() method.  The method is called after
  configfs has removed the item from the filesystem view but before the
  item is removed from its parent group.  Like drop_item(),
  disconnect_notify() is void and cannot fail.  Client subsystems should
  not drop any references here, as they still must do it in drop_item().
  
  A config_group cannot be removed while it still has child items.  This
  is implemented in the configfs rmdir(2) code.  ->drop_item() will not be
  called, as the item has not been dropped.  rmdir(2) will fail, as the
  directory is not empty.
  
  [struct configfs_subsystem]
  
  A subsystem must register itself, usually at module_init time.  This
  tells configfs to make the subsystem appear in the file tree.
  
  	struct configfs_subsystem {
  		struct config_group	su_group;
  		struct mutex		su_mutex;
  	};
  
  	int configfs_register_subsystem(struct configfs_subsystem *subsys);
  	void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
  
  	A subsystem consists of a toplevel config_group and a mutex.
  The group is where child config_items are created.  For a subsystem,
  this group is usually defined statically.  Before calling
  configfs_register_subsystem(), the subsystem must have initialized the
  group via the usual group _init() functions, and it must also have
  initialized the mutex.
  	When the register call returns, the subsystem is live, and it
  will be visible via configfs.  At that point, mkdir(2) can be called and
  the subsystem must be ready for it.
  
  [An Example]
  
  The best example of these basic concepts is the simple_children
  subsystem/group and the simple_child item in configfs_example_explicit.c
  and configfs_example_macros.c.  It shows a trivial object displaying and
  storing an attribute, and a simple group creating and destroying these
  children.
  
  The only difference between configfs_example_explicit.c and
  configfs_example_macros.c is how the attributes of the childless item
  are defined.  The childless item has extended attributes, each with
  their own show()/store() operation.  This follows a convention commonly
  used in sysfs.  configfs_example_explicit.c creates these attributes
  by explicitly defining the structures involved.  Conversely
  configfs_example_macros.c uses some convenience macros from configfs.h
  to define the attributes.  These macros are similar to their sysfs
  counterparts.
  
  [Hierarchy Navigation and the Subsystem Mutex]
  
  There is an extra bonus that configfs provides.  The config_groups and
  config_items are arranged in a hierarchy due to the fact that they
  appear in a filesystem.  A subsystem is NEVER to touch the filesystem
  parts, but the subsystem might be interested in this hierarchy.  For
  this reason, the hierarchy is mirrored via the config_group->cg_children
  and config_item->ci_parent structure members.
  
  A subsystem can navigate the cg_children list and the ci_parent pointer
  to see the tree created by the subsystem.  This can race with configfs'
  management of the hierarchy, so configfs uses the subsystem mutex to
  protect modifications.  Whenever a subsystem wants to navigate the
  hierarchy, it must do so under the protection of the subsystem
  mutex.
  
  A subsystem will be prevented from acquiring the mutex while a newly
  allocated item has not been linked into this hierarchy.   Similarly, it
  will not be able to acquire the mutex while a dropping item has not
  yet been unlinked.  This means that an item's ci_parent pointer will
  never be NULL while the item is in configfs, and that an item will only
  be in its parent's cg_children list for the same duration.  This allows
  a subsystem to trust ci_parent and cg_children while they hold the
  mutex.
  
  [Item Aggregation Via symlink(2)]
  
  configfs provides a simple group via the group->item parent/child
  relationship.  Often, however, a larger environment requires aggregation
  outside of the parent/child connection.  This is implemented via
  symlink(2).
  
  A config_item may provide the ct_item_ops->allow_link() and
  ct_item_ops->drop_link() methods.  If the ->allow_link() method exists,
  symlink(2) may be called with the config_item as the source of the link.
  These links are only allowed between configfs config_items.  Any
  symlink(2) attempt outside the configfs filesystem will be denied.
  
  When symlink(2) is called, the source config_item's ->allow_link()
  method is called with itself and a target item.  If the source item
  allows linking to target item, it returns 0.  A source item may wish to
  reject a link if it only wants links to a certain type of object (say,
  in its own subsystem).
  
  When unlink(2) is called on the symbolic link, the source item is
  notified via the ->drop_link() method.  Like the ->drop_item() method,
  this is a void function and cannot return failure.  The subsystem is
  responsible for responding to the change.
  
  A config_item cannot be removed while it links to any other item, nor
  can it be removed while an item links to it.  Dangling symlinks are not
  allowed in configfs.
  
  [Automatically Created Subgroups]
  
  A new config_group may want to have two types of child config_items.
  While this could be codified by magic names in ->make_item(), it is much
  more explicit to have a method whereby userspace sees this divergence.
  
  Rather than have a group where some items behave differently than
  others, configfs provides a method whereby one or many subgroups are
  automatically created inside the parent at its creation.  Thus,
  mkdir("parent") results in "parent", "parent/subgroup1", up through
  "parent/subgroupN".  Items of type 1 can now be created in
  "parent/subgroup1", and items of type N can be created in
  "parent/subgroupN".
  
  These automatic subgroups, or default groups, do not preclude other
  children of the parent group.  If ct_group_ops->make_group() exists,
  other child groups can be created on the parent group directly.
  
  A configfs subsystem specifies default groups by filling in the
  NULL-terminated array default_groups on the config_group structure.
  Each group in that array is populated in the configfs tree at the same
  time as the parent group.  Similarly, they are removed at the same time
  as the parent.  No extra notification is provided.  When a ->drop_item()
  method call notifies the subsystem the parent group is going away, it
  also means every default group child associated with that parent group.
  
  As a consequence of this, default_groups cannot be removed directly via
  rmdir(2).  They also are not considered when rmdir(2) on the parent
  group is checking for children.
  
  [Dependent Subsystems]
  
  Sometimes other drivers depend on particular configfs items.  For
  example, ocfs2 mounts depend on a heartbeat region item.  If that
  region item is removed with rmdir(2), the ocfs2 mount must BUG or go
  readonly.  Not happy.
  
  configfs provides two additional API calls: configfs_depend_item() and
  configfs_undepend_item().  A client driver can call
  configfs_depend_item() on an existing item to tell configfs that it is
  depended on.  configfs will then return -EBUSY from rmdir(2) for that
  item.  When the item is no longer depended on, the client driver calls
  configfs_undepend_item() on it.
  
  These API cannot be called underneath any configfs callbacks, as
  they will conflict.  They can block and allocate.  A client driver
  probably shouldn't calling them of its own gumption.  Rather it should
  be providing an API that external subsystems call.
  
  How does this work?  Imagine the ocfs2 mount process.  When it mounts,
  it asks for a heartbeat region item.  This is done via a call into the
  heartbeat code.  Inside the heartbeat code, the region item is looked
  up.  Here, the heartbeat code calls configfs_depend_item().  If it
  succeeds, then heartbeat knows the region is safe to give to ocfs2.
  If it fails, it was being torn down anyway, and heartbeat can gracefully
  pass up an error.
  
  [Committable Items]
  
  NOTE: Committable items are currently unimplemented.
  
  Some config_items cannot have a valid initial state.  That is, no
  default values can be specified for the item's attributes such that the
  item can do its work.  Userspace must configure one or more attributes,
  after which the subsystem can start whatever entity this item
  represents.
  
  Consider the FakeNBD device from above.  Without a target address *and*
  a target device, the subsystem has no idea what block device to import.
  The simple example assumes that the subsystem merely waits until all the
  appropriate attributes are configured, and then connects.  This will,
  indeed, work, but now every attribute store must check if the attributes
  are initialized.  Every attribute store must fire off the connection if
  that condition is met.
  
  Far better would be an explicit action notifying the subsystem that the
  config_item is ready to go.  More importantly, an explicit action allows
  the subsystem to provide feedback as to whether the attributes are
  initialized in a way that makes sense.  configfs provides this as
  committable items.
  
  configfs still uses only normal filesystem operations.  An item is
  committed via rename(2).  The item is moved from a directory where it
  can be modified to a directory where it cannot.
  
  Any group that provides the ct_group_ops->commit_item() method has
  committable items.  When this group appears in configfs, mkdir(2) will
  not work directly in the group.  Instead, the group will have two
  subdirectories: "live" and "pending".  The "live" directory does not
  support mkdir(2) or rmdir(2) either.  It only allows rename(2).  The
  "pending" directory does allow mkdir(2) and rmdir(2).  An item is
  created in the "pending" directory.  Its attributes can be modified at
  will.  Userspace commits the item by renaming it into the "live"
  directory.  At this point, the subsystem receives the ->commit_item()
  callback.  If all required attributes are filled to satisfaction, the
  method returns zero and the item is moved to the "live" directory.
  
  As rmdir(2) does not work in the "live" directory, an item must be
  shutdown, or "uncommitted".  Again, this is done via rename(2), this
  time from the "live" directory back to the "pending" one.  The subsystem
  is notified by the ct_group_ops->uncommit_object() method.