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