670 lines
25 KiB
ReStructuredText
670 lines
25 KiB
ReStructuredText
=======
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Locking
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=======
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The text below describes the locking rules for VFS-related methods.
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It is (believed to be) up-to-date. *Please*, if you change anything in
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prototypes or locking protocols - update this file. And update the relevant
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instances in the tree, don't leave that to maintainers of filesystems/devices/
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etc. At the very least, put the list of dubious cases in the end of this file.
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Don't turn it into log - maintainers of out-of-the-tree code are supposed to
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be able to use diff(1).
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Thing currently missing here: socket operations. Alexey?
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dentry_operations
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=================
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prototypes::
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int (*d_revalidate)(struct dentry *, unsigned int);
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int (*d_weak_revalidate)(struct dentry *, unsigned int);
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int (*d_hash)(const struct dentry *, struct qstr *);
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int (*d_compare)(const struct dentry *,
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unsigned int, const char *, const struct qstr *);
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int (*d_delete)(struct dentry *);
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int (*d_init)(struct dentry *);
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void (*d_release)(struct dentry *);
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void (*d_iput)(struct dentry *, struct inode *);
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char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
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struct vfsmount *(*d_automount)(struct path *path);
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int (*d_manage)(const struct path *, bool);
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struct dentry *(*d_real)(struct dentry *, const struct inode *);
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locking rules:
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================== =========== ======== ============== ========
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ops rename_lock ->d_lock may block rcu-walk
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================== =========== ======== ============== ========
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d_revalidate: no no yes (ref-walk) maybe
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d_weak_revalidate: no no yes no
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d_hash no no no maybe
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d_compare: yes no no maybe
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d_delete: no yes no no
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d_init: no no yes no
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d_release: no no yes no
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d_prune: no yes no no
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d_iput: no no yes no
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d_dname: no no no no
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d_automount: no no yes no
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d_manage: no no yes (ref-walk) maybe
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d_real no no yes no
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================== =========== ======== ============== ========
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inode_operations
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================
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prototypes::
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int (*create) (struct inode *,struct dentry *,umode_t, bool);
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struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
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int (*link) (struct dentry *,struct inode *,struct dentry *);
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int (*unlink) (struct inode *,struct dentry *);
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int (*symlink) (struct inode *,struct dentry *,const char *);
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int (*mkdir) (struct inode *,struct dentry *,umode_t);
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int (*rmdir) (struct inode *,struct dentry *);
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int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
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int (*rename) (struct inode *, struct dentry *,
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struct inode *, struct dentry *, unsigned int);
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int (*readlink) (struct dentry *, char __user *,int);
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const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
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void (*truncate) (struct inode *);
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int (*permission) (struct inode *, int, unsigned int);
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int (*get_acl)(struct inode *, int);
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int (*setattr) (struct dentry *, struct iattr *);
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int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
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ssize_t (*listxattr) (struct dentry *, char *, size_t);
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int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
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void (*update_time)(struct inode *, struct timespec *, int);
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int (*atomic_open)(struct inode *, struct dentry *,
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struct file *, unsigned open_flag,
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umode_t create_mode);
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int (*tmpfile) (struct inode *, struct dentry *, umode_t);
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locking rules:
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all may block
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============ =============================================
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ops i_rwsem(inode)
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============ =============================================
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lookup: shared
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create: exclusive
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link: exclusive (both)
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mknod: exclusive
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symlink: exclusive
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mkdir: exclusive
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unlink: exclusive (both)
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rmdir: exclusive (both)(see below)
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rename: exclusive (all) (see below)
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readlink: no
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get_link: no
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setattr: exclusive
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permission: no (may not block if called in rcu-walk mode)
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get_acl: no
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getattr: no
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listxattr: no
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fiemap: no
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update_time: no
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atomic_open: shared (exclusive if O_CREAT is set in open flags)
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tmpfile: no
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============ =============================================
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Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
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exclusive on victim.
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cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
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See Documentation/filesystems/directory-locking.rst for more detailed discussion
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of the locking scheme for directory operations.
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xattr_handler operations
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========================
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prototypes::
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bool (*list)(struct dentry *dentry);
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int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
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struct inode *inode, const char *name, void *buffer,
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size_t size);
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int (*set)(const struct xattr_handler *handler, struct dentry *dentry,
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struct inode *inode, const char *name, const void *buffer,
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size_t size, int flags);
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locking rules:
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all may block
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===== ==============
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ops i_rwsem(inode)
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===== ==============
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list: no
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get: no
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set: exclusive
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===== ==============
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super_operations
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================
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prototypes::
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struct inode *(*alloc_inode)(struct super_block *sb);
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void (*free_inode)(struct inode *);
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void (*destroy_inode)(struct inode *);
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void (*dirty_inode) (struct inode *, int flags);
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int (*write_inode) (struct inode *, struct writeback_control *wbc);
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int (*drop_inode) (struct inode *);
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void (*evict_inode) (struct inode *);
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void (*put_super) (struct super_block *);
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int (*sync_fs)(struct super_block *sb, int wait);
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int (*freeze_fs) (struct super_block *);
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int (*unfreeze_fs) (struct super_block *);
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int (*statfs) (struct dentry *, struct kstatfs *);
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int (*remount_fs) (struct super_block *, int *, char *);
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void (*umount_begin) (struct super_block *);
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int (*show_options)(struct seq_file *, struct dentry *);
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ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
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ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
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int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
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locking rules:
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All may block [not true, see below]
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====================== ============ ========================
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ops s_umount note
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====================== ============ ========================
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alloc_inode:
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free_inode: called from RCU callback
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destroy_inode:
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dirty_inode:
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write_inode:
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drop_inode: !!!inode->i_lock!!!
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evict_inode:
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put_super: write
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sync_fs: read
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freeze_fs: write
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unfreeze_fs: write
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statfs: maybe(read) (see below)
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remount_fs: write
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umount_begin: no
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show_options: no (namespace_sem)
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quota_read: no (see below)
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quota_write: no (see below)
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bdev_try_to_free_page: no (see below)
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====================== ============ ========================
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->statfs() has s_umount (shared) when called by ustat(2) (native or
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compat), but that's an accident of bad API; s_umount is used to pin
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the superblock down when we only have dev_t given us by userland to
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identify the superblock. Everything else (statfs(), fstatfs(), etc.)
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doesn't hold it when calling ->statfs() - superblock is pinned down
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by resolving the pathname passed to syscall.
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->quota_read() and ->quota_write() functions are both guaranteed to
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be the only ones operating on the quota file by the quota code (via
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dqio_sem) (unless an admin really wants to screw up something and
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writes to quota files with quotas on). For other details about locking
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see also dquot_operations section.
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->bdev_try_to_free_page is called from the ->releasepage handler of
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the block device inode. See there for more details.
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file_system_type
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================
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prototypes::
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struct dentry *(*mount) (struct file_system_type *, int,
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const char *, void *);
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void (*kill_sb) (struct super_block *);
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locking rules:
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======= =========
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ops may block
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======= =========
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mount yes
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kill_sb yes
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======= =========
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->mount() returns ERR_PTR or the root dentry; its superblock should be locked
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on return.
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->kill_sb() takes a write-locked superblock, does all shutdown work on it,
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unlocks and drops the reference.
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address_space_operations
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========================
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prototypes::
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int (*writepage)(struct page *page, struct writeback_control *wbc);
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int (*readpage)(struct file *, struct page *);
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int (*writepages)(struct address_space *, struct writeback_control *);
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int (*set_page_dirty)(struct page *page);
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void (*readahead)(struct readahead_control *);
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int (*readpages)(struct file *filp, struct address_space *mapping,
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struct list_head *pages, unsigned nr_pages);
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int (*write_begin)(struct file *, struct address_space *mapping,
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loff_t pos, unsigned len, unsigned flags,
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struct page **pagep, void **fsdata);
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int (*write_end)(struct file *, struct address_space *mapping,
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loff_t pos, unsigned len, unsigned copied,
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struct page *page, void *fsdata);
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sector_t (*bmap)(struct address_space *, sector_t);
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void (*invalidatepage) (struct page *, unsigned int, unsigned int);
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int (*releasepage) (struct page *, int);
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void (*freepage)(struct page *);
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int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
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bool (*isolate_page) (struct page *, isolate_mode_t);
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int (*migratepage)(struct address_space *, struct page *, struct page *);
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void (*putback_page) (struct page *);
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int (*launder_page)(struct page *);
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int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
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int (*error_remove_page)(struct address_space *, struct page *);
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int (*swap_activate)(struct file *);
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int (*swap_deactivate)(struct file *);
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locking rules:
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All except set_page_dirty and freepage may block
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====================== ======================== =========
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ops PageLocked(page) i_rwsem
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====================== ======================== =========
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writepage: yes, unlocks (see below)
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readpage: yes, unlocks
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writepages:
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set_page_dirty no
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readahead: yes, unlocks
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readpages: no
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write_begin: locks the page exclusive
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write_end: yes, unlocks exclusive
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bmap:
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invalidatepage: yes
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releasepage: yes
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freepage: yes
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direct_IO:
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isolate_page: yes
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migratepage: yes (both)
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putback_page: yes
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launder_page: yes
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is_partially_uptodate: yes
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error_remove_page: yes
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swap_activate: no
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swap_deactivate: no
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====================== ======================== =========
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->write_begin(), ->write_end() and ->readpage() may be called from
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the request handler (/dev/loop).
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->readpage() unlocks the page, either synchronously or via I/O
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completion.
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->readahead() unlocks the pages that I/O is attempted on like ->readpage().
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->readpages() populates the pagecache with the passed pages and starts
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I/O against them. They come unlocked upon I/O completion.
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->writepage() is used for two purposes: for "memory cleansing" and for
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"sync". These are quite different operations and the behaviour may differ
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depending upon the mode.
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If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
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it *must* start I/O against the page, even if that would involve
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blocking on in-progress I/O.
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If writepage is called for memory cleansing (sync_mode ==
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WBC_SYNC_NONE) then its role is to get as much writeout underway as
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possible. So writepage should try to avoid blocking against
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currently-in-progress I/O.
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If the filesystem is not called for "sync" and it determines that it
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would need to block against in-progress I/O to be able to start new I/O
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against the page the filesystem should redirty the page with
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redirty_page_for_writepage(), then unlock the page and return zero.
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This may also be done to avoid internal deadlocks, but rarely.
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If the filesystem is called for sync then it must wait on any
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in-progress I/O and then start new I/O.
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The filesystem should unlock the page synchronously, before returning to the
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caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
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value. WRITEPAGE_ACTIVATE means that page cannot really be written out
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currently, and VM should stop calling ->writepage() on this page for some
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time. VM does this by moving page to the head of the active list, hence the
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name.
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Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
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and return zero, writepage *must* run set_page_writeback() against the page,
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followed by unlocking it. Once set_page_writeback() has been run against the
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page, write I/O can be submitted and the write I/O completion handler must run
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end_page_writeback() once the I/O is complete. If no I/O is submitted, the
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filesystem must run end_page_writeback() against the page before returning from
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writepage.
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That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
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if the filesystem needs the page to be locked during writeout, that is ok, too,
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the page is allowed to be unlocked at any point in time between the calls to
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set_page_writeback() and end_page_writeback().
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Note, failure to run either redirty_page_for_writepage() or the combination of
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set_page_writeback()/end_page_writeback() on a page submitted to writepage
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will leave the page itself marked clean but it will be tagged as dirty in the
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radix tree. This incoherency can lead to all sorts of hard-to-debug problems
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in the filesystem like having dirty inodes at umount and losing written data.
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->writepages() is used for periodic writeback and for syscall-initiated
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sync operations. The address_space should start I/O against at least
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``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page
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which is written. The address_space implementation may write more (or less)
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pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
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If nr_to_write is NULL, all dirty pages must be written.
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writepages should _only_ write pages which are present on
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mapping->io_pages.
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->set_page_dirty() is called from various places in the kernel
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when the target page is marked as needing writeback. It may be called
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under spinlock (it cannot block) and is sometimes called with the page
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not locked.
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->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
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filesystems and by the swapper. The latter will eventually go away. Please,
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keep it that way and don't breed new callers.
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->invalidatepage() is called when the filesystem must attempt to drop
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some or all of the buffers from the page when it is being truncated. It
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returns zero on success. If ->invalidatepage is zero, the kernel uses
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block_invalidatepage() instead.
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->releasepage() is called when the kernel is about to try to drop the
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buffers from the page in preparation for freeing it. It returns zero to
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indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
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the kernel assumes that the fs has no private interest in the buffers.
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->freepage() is called when the kernel is done dropping the page
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from the page cache.
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->launder_page() may be called prior to releasing a page if
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it is still found to be dirty. It returns zero if the page was successfully
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cleaned, or an error value if not. Note that in order to prevent the page
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getting mapped back in and redirtied, it needs to be kept locked
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across the entire operation.
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->swap_activate will be called with a non-zero argument on
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files backing (non block device backed) swapfiles. A return value
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of zero indicates success, in which case this file can be used for
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backing swapspace. The swapspace operations will be proxied to the
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address space operations.
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->swap_deactivate() will be called in the sys_swapoff()
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path after ->swap_activate() returned success.
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file_lock_operations
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====================
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prototypes::
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void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
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void (*fl_release_private)(struct file_lock *);
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locking rules:
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=================== ============= =========
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ops inode->i_lock may block
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=================== ============= =========
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fl_copy_lock: yes no
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fl_release_private: maybe maybe[1]_
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=================== ============= =========
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.. [1]:
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->fl_release_private for flock or POSIX locks is currently allowed
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to block. Leases however can still be freed while the i_lock is held and
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so fl_release_private called on a lease should not block.
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lock_manager_operations
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=======================
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prototypes::
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void (*lm_notify)(struct file_lock *); /* unblock callback */
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int (*lm_grant)(struct file_lock *, struct file_lock *, int);
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void (*lm_break)(struct file_lock *); /* break_lease callback */
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int (*lm_change)(struct file_lock **, int);
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bool (*lm_breaker_owns_lease)(struct file_lock *);
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locking rules:
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====================== ============= ================= =========
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ops inode->i_lock blocked_lock_lock may block
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====================== ============= ================= =========
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lm_notify: yes yes no
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lm_grant: no no no
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lm_break: yes no no
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lm_change yes no no
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lm_breaker_owns_lease: no no no
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====================== ============= ================= =========
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buffer_head
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===========
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prototypes::
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void (*b_end_io)(struct buffer_head *bh, int uptodate);
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locking rules:
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called from interrupts. In other words, extreme care is needed here.
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bh is locked, but that's all warranties we have here. Currently only RAID1,
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highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
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call this method upon the IO completion.
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block_device_operations
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=======================
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prototypes::
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int (*open) (struct block_device *, fmode_t);
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int (*release) (struct gendisk *, fmode_t);
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int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
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int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
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int (*direct_access) (struct block_device *, sector_t, void **,
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unsigned long *);
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void (*unlock_native_capacity) (struct gendisk *);
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int (*revalidate_disk) (struct gendisk *);
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int (*getgeo)(struct block_device *, struct hd_geometry *);
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void (*swap_slot_free_notify) (struct block_device *, unsigned long);
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locking rules:
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======================= ===================
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ops bd_mutex
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======================= ===================
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open: yes
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release: yes
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ioctl: no
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compat_ioctl: no
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direct_access: no
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unlock_native_capacity: no
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revalidate_disk: no
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getgeo: no
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swap_slot_free_notify: no (see below)
|
|
======================= ===================
|
|
|
|
unlock_native_capacity and revalidate_disk are called only from
|
|
check_disk_change().
|
|
|
|
swap_slot_free_notify is called with swap_lock and sometimes the page lock
|
|
held.
|
|
|
|
|
|
file_operations
|
|
===============
|
|
|
|
prototypes::
|
|
|
|
loff_t (*llseek) (struct file *, loff_t, int);
|
|
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
|
|
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
|
|
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
|
|
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
|
|
int (*iterate) (struct file *, struct dir_context *);
|
|
int (*iterate_shared) (struct file *, struct dir_context *);
|
|
__poll_t (*poll) (struct file *, struct poll_table_struct *);
|
|
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 *, loff_t start, loff_t end, 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 __user *);
|
|
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 (*flock) (struct file *, int, struct file_lock *);
|
|
ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
|
|
size_t, unsigned int);
|
|
ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
|
|
size_t, unsigned int);
|
|
int (*setlease)(struct file *, long, struct file_lock **, void **);
|
|
long (*fallocate)(struct file *, int, loff_t, loff_t);
|
|
|
|
locking rules:
|
|
All may block.
|
|
|
|
->llseek() locking has moved from llseek to the individual llseek
|
|
implementations. If your fs is not using generic_file_llseek, you
|
|
need to acquire and release the appropriate locks in your ->llseek().
|
|
For many filesystems, it is probably safe to acquire the inode
|
|
mutex or just to use i_size_read() instead.
|
|
Note: this does not protect the file->f_pos against concurrent modifications
|
|
since this is something the userspace has to take care about.
|
|
|
|
->iterate() is called with i_rwsem exclusive.
|
|
|
|
->iterate_shared() is called with i_rwsem at least shared.
|
|
|
|
->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
|
|
Most instances call fasync_helper(), which does that maintenance, so it's
|
|
not normally something one needs to worry about. Return values > 0 will be
|
|
mapped to zero in the VFS layer.
|
|
|
|
->readdir() and ->ioctl() on directories must be changed. Ideally we would
|
|
move ->readdir() to inode_operations and use a separate method for directory
|
|
->ioctl() or kill the latter completely. One of the problems is that for
|
|
anything that resembles union-mount we won't have a struct file for all
|
|
components. And there are other reasons why the current interface is a mess...
|
|
|
|
->read on directories probably must go away - we should just enforce -EISDIR
|
|
in sys_read() and friends.
|
|
|
|
->setlease operations should call generic_setlease() before or after setting
|
|
the lease within the individual filesystem to record the result of the
|
|
operation
|
|
|
|
dquot_operations
|
|
================
|
|
|
|
prototypes::
|
|
|
|
int (*write_dquot) (struct dquot *);
|
|
int (*acquire_dquot) (struct dquot *);
|
|
int (*release_dquot) (struct dquot *);
|
|
int (*mark_dirty) (struct dquot *);
|
|
int (*write_info) (struct super_block *, int);
|
|
|
|
These operations are intended to be more or less wrapping functions that ensure
|
|
a proper locking wrt the filesystem and call the generic quota operations.
|
|
|
|
What filesystem should expect from the generic quota functions:
|
|
|
|
============== ============ =========================
|
|
ops FS recursion Held locks when called
|
|
============== ============ =========================
|
|
write_dquot: yes dqonoff_sem or dqptr_sem
|
|
acquire_dquot: yes dqonoff_sem or dqptr_sem
|
|
release_dquot: yes dqonoff_sem or dqptr_sem
|
|
mark_dirty: no -
|
|
write_info: yes dqonoff_sem
|
|
============== ============ =========================
|
|
|
|
FS recursion means calling ->quota_read() and ->quota_write() from superblock
|
|
operations.
|
|
|
|
More details about quota locking can be found in fs/dquot.c.
|
|
|
|
vm_operations_struct
|
|
====================
|
|
|
|
prototypes::
|
|
|
|
void (*open)(struct vm_area_struct*);
|
|
void (*close)(struct vm_area_struct*);
|
|
vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
|
|
vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
|
|
vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
|
|
int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
|
|
|
|
locking rules:
|
|
|
|
============= ========= ===========================
|
|
ops mmap_lock PageLocked(page)
|
|
============= ========= ===========================
|
|
open: yes
|
|
close: yes
|
|
fault: yes can return with page locked
|
|
map_pages: yes
|
|
page_mkwrite: yes can return with page locked
|
|
pfn_mkwrite: yes
|
|
access: yes
|
|
============= ========= ===========================
|
|
|
|
->fault() is called when a previously not present pte is about
|
|
to be faulted in. The filesystem must find and return the page associated
|
|
with the passed in "pgoff" in the vm_fault structure. If it is possible that
|
|
the page may be truncated and/or invalidated, then the filesystem must lock
|
|
the page, then ensure it is not already truncated (the page lock will block
|
|
subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
|
|
locked. The VM will unlock the page.
|
|
|
|
->map_pages() is called when VM asks to map easy accessible pages.
|
|
Filesystem should find and map pages associated with offsets from "start_pgoff"
|
|
till "end_pgoff". ->map_pages() is called with page table locked and must
|
|
not block. If it's not possible to reach a page without blocking,
|
|
filesystem should skip it. Filesystem should use do_set_pte() to setup
|
|
page table entry. Pointer to entry associated with the page is passed in
|
|
"pte" field in vm_fault structure. Pointers to entries for other offsets
|
|
should be calculated relative to "pte".
|
|
|
|
->page_mkwrite() is called when a previously read-only pte is
|
|
about to become writeable. The filesystem again must ensure that there are
|
|
no truncate/invalidate races, and then return with the page locked. If
|
|
the page has been truncated, the filesystem should not look up a new page
|
|
like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
|
|
will cause the VM to retry the fault.
|
|
|
|
->pfn_mkwrite() is the same as page_mkwrite but when the pte is
|
|
VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
|
|
VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
|
|
after this call is to make the pte read-write, unless pfn_mkwrite returns
|
|
an error.
|
|
|
|
->access() is called when get_user_pages() fails in
|
|
access_process_vm(), typically used to debug a process through
|
|
/proc/pid/mem or ptrace. This function is needed only for
|
|
VM_IO | VM_PFNMAP VMAs.
|
|
|
|
--------------------------------------------------------------------------------
|
|
|
|
Dubious stuff
|
|
|
|
(if you break something or notice that it is broken and do not fix it yourself
|
|
- at least put it here)
|