[PATCH] VFS: update documentation

This patch brings the now out-of-date Documentation/filesystems/vfs.txt
back to life.  Thanks to Carsten Otte, Trond Myklebust, and Anton
Altaparmakov for their help on updating this documentation.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Pekka J Enberg 2005-09-09 13:10:19 -07:00 committed by Linus Torvalds
parent 952b649272
commit 5ea626aac1

View File

@ -1,35 +1,27 @@
/* -*- auto-fill -*- */
Overview of the Virtual File System
Overview of the Linux Virtual File System
Richard Gooch <rgooch@atnf.csiro.au>
Original author: Richard Gooch <rgooch@atnf.csiro.au>
5-JUL-1999
Last updated on August 25, 2005
Copyright (C) 1999 Richard Gooch
Copyright (C) 2005 Pekka Enberg
This file is released under the GPLv2.
Conventions used in this document <section>
=================================
Each section in this document will have the string "<section>" at the
right-hand side of the section title. Each subsection will have
"<subsection>" at the right-hand side. These strings are meant to make
it easier to search through the document.
NOTE that the master copy of this document is available online at:
http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt
What is it? <section>
What is it?
===========
The Virtual File System (otherwise known as the Virtual Filesystem
Switch) is the software layer in the kernel that provides the
filesystem interface to userspace programs. It also provides an
abstraction within the kernel which allows different filesystem
implementations to co-exist.
implementations to coexist.
A Quick Look At How It Works <section>
A Quick Look At How It Works
============================
In this section I'll briefly describe how things work, before
@ -38,7 +30,8 @@ when user programs open and manipulate files, and then look from the
other view which is how a filesystem is supported and subsequently
mounted.
Opening a File <subsection>
Opening a File
--------------
The VFS implements the open(2), stat(2), chmod(2) and similar system
@ -77,7 +70,7 @@ back to userspace.
Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file
descriptors). The freshly allocated file structure is initialised with
descriptors). The freshly allocated file structure is initialized with
a pointer to the dentry and a set of file operation member functions.
These are taken from the inode data. The open() file method is then
called so the specific filesystem implementation can do it's work. You
@ -102,7 +95,8 @@ filesystem or driver code at the same time, on different
processors. You should ensure that access to shared resources is
protected by appropriate locks.
Registering and Mounting a Filesystem <subsection>
Registering and Mounting a Filesystem
-------------------------------------
If you want to support a new kind of filesystem in the kernel, all you
@ -123,17 +117,21 @@ updated to point to the root inode for the new filesystem.
It's now time to look at things in more detail.
struct file_system_type <section>
struct file_system_type
=======================
This describes the filesystem. As of kernel 2.1.99, the following
This describes the filesystem. As of kernel 2.6.13, the following
members are defined:
struct file_system_type {
const char *name;
int fs_flags;
struct super_block *(*read_super) (struct super_block *, void *, int);
struct file_system_type * next;
struct super_block *(*get_sb) (struct file_system_type *, int,
const char *, void *);
void (*kill_sb) (struct super_block *);
struct module *owner;
struct file_system_type * next;
struct list_head fs_supers;
};
name: the name of the filesystem type, such as "ext2", "iso9660",
@ -141,51 +139,97 @@ struct file_system_type {
fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
read_super: the method to call when a new instance of this
get_sb: the method to call when a new instance of this
filesystem should be mounted
next: for internal VFS use: you should initialise this to NULL
kill_sb: the method to call when an instance of this filesystem
should be unmounted
The read_super() method has the following arguments:
owner: for internal VFS use: you should initialize this to THIS_MODULE in
most cases.
next: for internal VFS use: you should initialize this to NULL
The get_sb() method has the following arguments:
struct super_block *sb: the superblock structure. This is partially
initialised by the VFS and the rest must be initialised by the
read_super() method
initialized by the VFS and the rest must be initialized by the
get_sb() method
int flags: mount flags
const char *dev_name: the device name we are mounting.
void *data: arbitrary mount options, usually comes as an ASCII
string
int silent: whether or not to be silent on error
The read_super() method must determine if the block device specified
The get_sb() method must determine if the block device specified
in the superblock contains a filesystem of the type the method
supports. On success the method returns the superblock pointer, on
failure it returns NULL.
The most interesting member of the superblock structure that the
read_super() method fills in is the "s_op" field. This is a pointer to
get_sb() method fills in is the "s_op" field. This is a pointer to
a "struct super_operations" which describes the next level of the
filesystem implementation.
Usually, a filesystem uses generic one of the generic get_sb()
implementations and provides a fill_super() method instead. The
generic methods are:
struct super_operations <section>
get_sb_bdev: mount a filesystem residing on a block device
get_sb_nodev: mount a filesystem that is not backed by a device
get_sb_single: mount a filesystem which shares the instance between
all mounts
A fill_super() method implementation has the following arguments:
struct super_block *sb: the superblock structure. The method fill_super()
must initialize this properly.
void *data: arbitrary mount options, usually comes as an ASCII
string
int silent: whether or not to be silent on error
struct super_operations
=======================
This describes how the VFS can manipulate the superblock of your
filesystem. As of kernel 2.1.99, the following members are defined:
filesystem. As of kernel 2.6.13, the following members are defined:
struct super_operations {
void (*read_inode) (struct inode *);
int (*write_inode) (struct inode *, int);
void (*put_inode) (struct inode *);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
int (*notify_change) (struct dentry *, struct iattr *);
void (*put_super) (struct super_block *);
void (*write_super) (struct super_block *);
int (*statfs) (struct super_block *, struct statfs *, int);
int (*remount_fs) (struct super_block *, int *, char *);
void (*clear_inode) (struct inode *);
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);
void (*read_inode) (struct inode *);
void (*dirty_inode) (struct inode *);
int (*write_inode) (struct inode *, int);
void (*put_inode) (struct inode *);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
void (*put_super) (struct super_block *);
void (*write_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
void (*write_super_lockfs) (struct super_block *);
void (*unlockfs) (struct super_block *);
int (*statfs) (struct super_block *, struct kstatfs *);
int (*remount_fs) (struct super_block *, int *, char *);
void (*clear_inode) (struct inode *);
void (*umount_begin) (struct super_block *);
void (*sync_inodes) (struct super_block *sb,
struct writeback_control *wbc);
int (*show_options)(struct seq_file *, struct vfsmount *);
ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
};
All methods are called without any locks being held, unless otherwise
@ -193,43 +237,62 @@ noted. This means that most methods can block safely. All methods are
only called from a process context (i.e. not from an interrupt handler
or bottom half).
alloc_inode: this method is called by inode_alloc() to allocate memory
for struct inode and initialize it.
destroy_inode: this method is called by destroy_inode() to release
resources allocated for struct inode.
read_inode: this method is called to read a specific inode from the
mounted filesystem. The "i_ino" member in the "struct inode"
will be initialised by the VFS to indicate which inode to
read. Other members are filled in by this method
mounted filesystem. The i_ino member in the struct inode is
initialized by the VFS to indicate which inode to read. Other
members are filled in by this method.
You can set this to NULL and use iget5_locked() instead of iget()
to read inodes. This is necessary for filesystems for which the
inode number is not sufficient to identify an inode.
dirty_inode: this method is called by the VFS to mark an inode dirty.
write_inode: this method is called when the VFS needs to write an
inode to disc. The second parameter indicates whether the write
should be synchronous or not, not all filesystems check this flag.
put_inode: called when the VFS inode is removed from the inode
cache. This method is optional
cache.
drop_inode: called when the last access to the inode is dropped,
with the inode_lock spinlock held.
This method should be either NULL (normal unix filesystem
This method should be either NULL (normal UNIX filesystem
semantics) or "generic_delete_inode" (for filesystems that do not
want to cache inodes - causing "delete_inode" to always be
called regardless of the value of i_nlink)
The "generic_delete_inode()" behaviour is equivalent to the
The "generic_delete_inode()" behavior is equivalent to the
old practice of using "force_delete" in the put_inode() case,
but does not have the races that the "force_delete()" approach
had.
delete_inode: called when the VFS wants to delete an inode
notify_change: called when VFS inode attributes are changed. If this
is NULL the VFS falls back to the write_inode() method. This
is called with the kernel lock held
put_super: called when the VFS wishes to free the superblock
(i.e. unmount). This is called with the superblock lock held
write_super: called when the VFS superblock needs to be written to
disc. This method is optional
sync_fs: called when VFS is writing out all dirty data associated with
a superblock. The second parameter indicates whether the method
should wait until the write out has been completed. Optional.
write_super_lockfs: called when VFS is locking a filesystem and forcing
it into a consistent state. This function is currently used by the
Logical Volume Manager (LVM).
unlockfs: called when VFS is unlocking a filesystem and making it writable
again.
statfs: called when the VFS needs to get filesystem statistics. This
is called with the kernel lock held
@ -238,21 +301,31 @@ or bottom half).
clear_inode: called then the VFS clears the inode. Optional
umount_begin: called when the VFS is unmounting a filesystem.
sync_inodes: called when the VFS is writing out dirty data associated with
a superblock.
show_options: called by the VFS to show mount options for /proc/<pid>/mounts.
quota_read: called by the VFS to read from filesystem quota file.
quota_write: called by the VFS to write to filesystem quota file.
The read_inode() method is responsible for filling in the "i_op"
field. This is a pointer to a "struct inode_operations" which
describes the methods that can be performed on individual inodes.
struct inode_operations <section>
struct inode_operations
=======================
This describes how the VFS can manipulate an inode in your
filesystem. As of kernel 2.1.99, the following members are defined:
filesystem. As of kernel 2.6.13, the following members are defined:
struct inode_operations {
struct file_operations * default_file_ops;
int (*create) (struct inode *,struct dentry *,int);
int (*lookup) (struct inode *,struct dentry *);
int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
int (*link) (struct dentry *,struct inode *,struct dentry *);
int (*unlink) (struct inode *,struct dentry *);
int (*symlink) (struct inode *,struct dentry *,const char *);
@ -261,25 +334,22 @@ struct inode_operations {
int (*mknod) (struct inode *,struct dentry *,int,dev_t);
int (*rename) (struct inode *, struct dentry *,
struct inode *, struct dentry *);
int (*readlink) (struct dentry *, char *,int);
struct dentry * (*follow_link) (struct dentry *, struct dentry *);
int (*readpage) (struct file *, struct page *);
int (*writepage) (struct page *page, struct writeback_control *wbc);
int (*bmap) (struct inode *,int);
int (*readlink) (struct dentry *, char __user *,int);
void * (*follow_link) (struct dentry *, struct nameidata *);
void (*put_link) (struct dentry *, struct nameidata *, void *);
void (*truncate) (struct inode *);
int (*permission) (struct inode *, int);
int (*smap) (struct inode *,int);
int (*updatepage) (struct file *, struct page *, const char *,
unsigned long, unsigned int, int);
int (*revalidate) (struct dentry *);
int (*permission) (struct inode *, int, struct nameidata *);
int (*setattr) (struct dentry *, struct iattr *);
int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
ssize_t (*listxattr) (struct dentry *, char *, size_t);
int (*removexattr) (struct dentry *, const char *);
};
Again, all methods are called without any locks being held, unless
otherwise noted.
default_file_ops: this is a pointer to a "struct file_operations"
which describes how to open and then manipulate open files
create: called by the open(2) and creat(2) system calls. Only
required if you want to support regular files. The dentry you
get should not have an inode (i.e. it should be a negative
@ -328,31 +398,143 @@ otherwise noted.
you want to support reading symbolic links
follow_link: called by the VFS to follow a symbolic link to the
inode it points to. Only required if you want to support
symbolic links
inode it points to. Only required if you want to support
symbolic links. This function returns a void pointer cookie
that is passed to put_link().
put_link: called by the VFS to release resources allocated by
follow_link(). The cookie returned by follow_link() is passed to
to this function as the last parameter. It is used by filesystems
such as NFS where page cache is not stable (i.e. page that was
installed when the symbolic link walk started might not be in the
page cache at the end of the walk).
truncate: called by the VFS to change the size of a file. The i_size
field of the inode is set to the desired size by the VFS before
this function is called. This function is called by the truncate(2)
system call and related functionality.
permission: called by the VFS to check for access rights on a POSIX-like
filesystem.
setattr: called by the VFS to set attributes for a file. This function is
called by chmod(2) and related system calls.
getattr: called by the VFS to get attributes of a file. This function is
called by stat(2) and related system calls.
setxattr: called by the VFS to set an extended attribute for a file.
Extended attribute is a name:value pair associated with an inode. This
function is called by setxattr(2) system call.
getxattr: called by the VFS to retrieve the value of an extended attribute
name. This function is called by getxattr(2) function call.
listxattr: called by the VFS to list all extended attributes for a given
file. This function is called by listxattr(2) system call.
removexattr: called by the VFS to remove an extended attribute from a file.
This function is called by removexattr(2) system call.
struct file_operations <section>
struct address_space_operations
===============================
This describes how the VFS can manipulate mapping of a file to page cache in
your filesystem. As of kernel 2.6.13, the following members are defined:
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*readpage)(struct file *, struct page *);
int (*sync_page)(struct page *);
int (*writepages)(struct address_space *, struct writeback_control *);
int (*set_page_dirty)(struct page *page);
int (*readpages)(struct file *filp, struct address_space *mapping,
struct list_head *pages, unsigned nr_pages);
int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
sector_t (*bmap)(struct address_space *, sector_t);
int (*invalidatepage) (struct page *, unsigned long);
int (*releasepage) (struct page *, int);
ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
loff_t offset, unsigned long nr_segs);
struct page* (*get_xip_page)(struct address_space *, sector_t,
int);
};
writepage: called by the VM write a dirty page to backing store.
readpage: called by the VM to read a page from backing store.
sync_page: called by the VM to notify the backing store to perform all
queued I/O operations for a page. I/O operations for other pages
associated with this address_space object may also be performed.
writepages: called by the VM to write out pages associated with the
address_space object.
set_page_dirty: called by the VM to set a page dirty.
readpages: called by the VM to read pages associated with the address_space
object.
prepare_write: called by the generic write path in VM to set up a write
request for a page.
commit_write: called by the generic write path in VM to write page to
its backing store.
bmap: called by the VFS to map a logical block offset within object to
physical block number. This method is use by for the legacy FIBMAP
ioctl. Other uses are discouraged.
invalidatepage: called by the VM on truncate to disassociate a page from its
address_space mapping.
releasepage: called by the VFS to release filesystem specific metadata from
a page.
direct_IO: called by the VM for direct I/O writes and reads.
get_xip_page: called by the VM to translate a block number to a page.
The page is valid until the corresponding filesystem is unmounted.
Filesystems that want to use execute-in-place (XIP) need to implement
it. An example implementation can be found in fs/ext2/xip.c.
struct file_operations
======================
This describes how the VFS can manipulate an open file. As of kernel
2.1.99, the following members are defined:
2.6.13, the following members are defined:
struct file_operations {
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);
int (*readdir) (struct file *, void *, filldir_t);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
int (*fsync) (struct file *, struct dentry *);
int (*fasync) (struct file *, int);
int (*check_media_change) (kdev_t dev);
int (*revalidate) (kdev_t dev);
int (*fsync) (struct file *, struct dentry *, int datasync);
int (*aio_fsync) (struct kiocb *, int datasync);
int (*fasync) (int, struct file *, int);
int (*lock) (struct file *, int, struct file_lock *);
ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
int (*check_flags)(int);
int (*dir_notify)(struct file *filp, unsigned long arg);
int (*flock) (struct file *, int, struct file_lock *);
};
Again, all methods are called without any locks being held, unless
@ -362,8 +544,12 @@ otherwise noted.
read: called by read(2) and related system calls
aio_read: called by io_submit(2) and other asynchronous I/O operations
write: called by write(2) and related system calls
aio_write: called by io_submit(2) and other asynchronous I/O operations
readdir: called when the VFS needs to read the directory contents
poll: called by the VFS when a process wants to check if there is
@ -372,18 +558,25 @@ otherwise noted.
ioctl: called by the ioctl(2) system call
unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not
require the BKL should use this method instead of the ioctl() above.
compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
are used on 64 bit kernels.
mmap: called by the mmap(2) system call
open: called by the VFS when an inode should be opened. When the VFS
opens a file, it creates a new "struct file" and initialises
the "f_op" file operations member with the "default_file_ops"
field in the inode structure. It then calls the open method
for the newly allocated file structure. You might think that
the open method really belongs in "struct inode_operations",
and you may be right. I think it's done the way it is because
it makes filesystems simpler to implement. The open() method
is a good place to initialise the "private_data" member in the
file structure if you want to point to a device structure
opens a file, it creates a new "struct file". It then calls the
open method for the newly allocated file structure. You might
think that the open method really belongs in
"struct inode_operations", and you may be right. I think it's
done the way it is because it makes filesystems simpler to
implement. The open() method is a good place to initialize the
"private_data" member in the file structure if you want to point
to a device structure
flush: called by the close(2) system call to flush a file
release: called when the last reference to an open file is closed
@ -392,6 +585,23 @@ otherwise noted.
fasync: called by the fcntl(2) system call when asynchronous
(non-blocking) mode is enabled for a file
lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
commands
readv: called by the readv(2) system call
writev: called by the writev(2) system call
sendfile: called by the sendfile(2) system call
get_unmapped_area: called by the mmap(2) system call
check_flags: called by the fcntl(2) system call for F_SETFL command
dir_notify: called by the fcntl(2) system call for F_NOTIFY command
flock: called by the flock(2) system call
Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node
(character or block special) most filesystems will call special
@ -400,29 +610,28 @@ driver information. These support routines replace the filesystem file
operations with those for the device driver, and then proceed to call
the new open() method for the file. This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
method. Note the devfs (the Device FileSystem) has a more direct path
from device node to device driver (this is an unofficial kernel
patch).
method.
Directory Entry Cache (dcache) <section>
------------------------------
Directory Entry Cache (dcache)
==============================
struct dentry_operations
========================
------------------------
This describes how a filesystem can overload the standard dentry
operations. Dentries and the dcache are the domain of the VFS and the
individual filesystem implementations. Device drivers have no business
here. These methods may be set to NULL, as they are either optional or
the VFS uses a default. As of kernel 2.1.99, the following members are
the VFS uses a default. As of kernel 2.6.13, the following members are
defined:
struct dentry_operations {
int (*d_revalidate)(struct dentry *);
int (*d_revalidate)(struct dentry *, struct nameidata *);
int (*d_hash) (struct dentry *, struct qstr *);
int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
void (*d_delete)(struct dentry *);
int (*d_delete)(struct dentry *);
void (*d_release)(struct dentry *);
void (*d_iput)(struct dentry *, struct inode *);
};
@ -451,6 +660,7 @@ Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.
Directory Entry Cache APIs
--------------------------
@ -471,7 +681,7 @@ manipulate dentries:
"d_delete" method is called
d_drop: this unhashes a dentry from its parents hash list. A
subsequent call to dput() will dellocate the dentry if its
subsequent call to dput() will deallocate the dentry if its
usage count drops to 0
d_delete: delete a dentry. If there are no other open references to
@ -507,16 +717,16 @@ up by walking the tree starting with the first component
of the pathname and using that dentry along with the next
component to look up the next level and so on. Since it
is a frequent operation for workloads like multiuser
environments and webservers, it is important to optimize
environments and web servers, it is important to optimize
this path.
Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus
in every component during path look-up. Since 2.5.10 onwards,
fastwalk algorithm changed this by holding the dcache_lock
fast-walk algorithm changed this by holding the dcache_lock
at the beginning and walking as many cached path component
dentries as possible. This signficantly decreases the number
dentries as possible. This significantly decreases the number
of acquisition of dcache_lock. However it also increases the
lock hold time signficantly and affects performance in large
lock hold time significantly and affects performance in large
SMP machines. Since 2.5.62 kernel, dcache has been using
a new locking model that uses RCU to make dcache look-up
lock-free.
@ -527,7 +737,7 @@ protected the hash chain, d_child, d_alias, d_lru lists as well
as d_inode and several other things like mount look-up. RCU-based
changes affect only the way the hash chain is protected. For everything
else the dcache_lock must be taken for both traversing as well as
updating. The hash chain updations too take the dcache_lock.
updating. The hash chain updates too take the dcache_lock.
The significant change is the way d_lookup traverses the hash chain,
it doesn't acquire the dcache_lock for this and rely on RCU to
ensure that the dentry has not been *freed*.
@ -535,14 +745,15 @@ ensure that the dentry has not been *freed*.
Dcache locking details
----------------------
For many multi-user workloads, open() and stat() on files are
very frequently occurring operations. Both involve walking
of path names to find the dentry corresponding to the
concerned file. In 2.4 kernel, dcache_lock was held
during look-up of each path component. Contention and
cacheline bouncing of this global lock caused significant
cache-line bouncing of this global lock caused significant
scalability problems. With the introduction of RCU
in linux kernel, this was worked around by making
in Linux kernel, this was worked around by making
the look-up of path components during path walking lock-free.
@ -562,7 +773,7 @@ Some of the important changes are :
2. Insertion of a dentry into the hash table is done using
hlist_add_head_rcu() which take care of ordering the writes -
the writes to the dentry must be visible before the dentry
is inserted. This works in conjuction with hlist_for_each_rcu()
is inserted. This works in conjunction with hlist_for_each_rcu()
while walking the hash chain. The only requirement is that
all initialization to the dentry must be done before hlist_add_head_rcu()
since we don't have dcache_lock protection while traversing
@ -584,7 +795,7 @@ Some of the important changes are :
the same. In some sense, dcache_rcu path walking looks like
the pre-2.5.10 version.
5. All dentry hash chain updations must take the dcache_lock as well as
5. All dentry hash chain updates must take the dcache_lock as well as
the per-dentry lock in that order. dput() does this to ensure
that a dentry that has just been looked up in another CPU
doesn't get deleted before dget() can be done on it.
@ -640,10 +851,10 @@ handled as described below :
Since we redo the d_parent check and compare name while holding
d_lock, lock-free look-up will not race against d_move().
4. There can be a theoritical race when a dentry keeps coming back
4. There can be a theoretical race when a dentry keeps coming back
to original bucket due to double moves. Due to this look-up may
consider that it has never moved and can end up in a infinite loop.
But this is not any worse that theoritical livelocks we already
But this is not any worse that theoretical livelocks we already
have in the kernel.