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-
-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.
-
-