541 lines
21 KiB
Plaintext
541 lines
21 KiB
Plaintext
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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prototypes:
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int (*d_revalidate)(struct dentry *, int);
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int (*d_hash) (struct dentry *, struct qstr *);
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int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
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int (*d_delete)(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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locking rules:
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none have BKL
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dcache_lock rename_lock ->d_lock may block
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d_revalidate: no no no yes
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d_hash no no no yes
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d_compare: no yes no no
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d_delete: yes no yes no
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d_release: no no no yes
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d_iput: no no no yes
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d_dname: no no no no
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--------------------------- inode_operations ---------------------------
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prototypes:
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int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
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struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
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ata *);
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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 *,int);
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int (*rmdir) (struct inode *,struct dentry *);
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int (*mknod) (struct inode *,struct dentry *,int,dev_t);
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int (*rename) (struct inode *, struct dentry *,
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struct inode *, struct dentry *);
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int (*readlink) (struct dentry *, char __user *,int);
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int (*follow_link) (struct dentry *, struct nameidata *);
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void (*truncate) (struct inode *);
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int (*permission) (struct inode *, int, struct nameidata *);
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int (*setattr) (struct dentry *, struct iattr *);
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int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
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int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
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ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
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ssize_t (*listxattr) (struct dentry *, char *, size_t);
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int (*removexattr) (struct dentry *, const char *);
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locking rules:
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all may block, none have BKL
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i_mutex(inode)
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lookup: yes
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create: yes
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link: yes (both)
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mknod: yes
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symlink: yes
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mkdir: yes
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unlink: yes (both)
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rmdir: yes (both) (see below)
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rename: yes (all) (see below)
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readlink: no
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follow_link: no
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truncate: yes (see below)
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setattr: yes
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permission: no
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getattr: no
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setxattr: yes
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getxattr: no
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listxattr: no
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removexattr: yes
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Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
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victim.
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cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
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->truncate() is never called directly - it's a callback, not a
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method. It's called by vmtruncate() - library function normally used by
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->setattr(). Locking information above applies to that call (i.e. is
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inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
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passed).
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See Documentation/filesystems/directory-locking for more detailed discussion
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of the locking scheme for directory operations.
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--------------------------- super_operations ---------------------------
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prototypes:
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struct inode *(*alloc_inode)(struct super_block *sb);
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void (*destroy_inode)(struct inode *);
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void (*dirty_inode) (struct inode *);
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int (*write_inode) (struct inode *, int);
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void (*put_inode) (struct inode *);
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void (*drop_inode) (struct inode *);
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void (*delete_inode) (struct inode *);
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void (*put_super) (struct super_block *);
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void (*write_super) (struct super_block *);
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int (*sync_fs)(struct super_block *sb, int wait);
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void (*write_super_lockfs) (struct super_block *);
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void (*unlockfs) (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 (*clear_inode) (struct inode *);
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void (*umount_begin) (struct super_block *);
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int (*show_options)(struct seq_file *, struct vfsmount *);
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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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locking rules:
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All may block.
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BKL s_lock s_umount
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alloc_inode: no no no
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destroy_inode: no
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dirty_inode: no (must not sleep)
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write_inode: no
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put_inode: no
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drop_inode: no !!!inode_lock!!!
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delete_inode: no
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put_super: yes yes no
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write_super: no yes read
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sync_fs: no no read
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write_super_lockfs: ?
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unlockfs: ?
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statfs: no no no
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remount_fs: yes yes maybe (see below)
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clear_inode: no
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umount_begin: yes no no
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show_options: no (vfsmount->sem)
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quota_read: no no no (see below)
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quota_write: no no no (see below)
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->remount_fs() will have the s_umount lock if it's already mounted.
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When called from get_sb_single, it does NOT have the s_umount lock.
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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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--------------------------- file_system_type ---------------------------
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prototypes:
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int (*get_sb) (struct file_system_type *, int,
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const char *, void *, struct vfsmount *);
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void (*kill_sb) (struct super_block *);
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locking rules:
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may block BKL
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get_sb yes yes
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kill_sb yes yes
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->get_sb() returns error or 0 with locked superblock attached to the vfsmount
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(exclusive on ->s_umount).
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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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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 (*sync_page)(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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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 (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
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int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
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sector_t (*bmap)(struct address_space *, sector_t);
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int (*invalidatepage) (struct page *, unsigned long);
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int (*releasepage) (struct page *, int);
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int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
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loff_t offset, unsigned long nr_segs);
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int (*launder_page) (struct page *);
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locking rules:
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All except set_page_dirty may block
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BKL PageLocked(page) i_sem
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writepage: no yes, unlocks (see below)
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readpage: no yes, unlocks
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sync_page: no maybe
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writepages: no
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set_page_dirty no no
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readpages: no
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prepare_write: no yes yes
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commit_write: no yes yes
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write_begin: no locks the page yes
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write_end: no yes, unlocks yes
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perform_write: no n/a yes
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bmap: yes
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invalidatepage: no yes
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releasepage: no yes
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direct_IO: no
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launder_page: no yes
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->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
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may be called from 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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->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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->sync_page() locking rules are not well-defined - usually it is called
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with lock on page, but that is not guaranteed. Considering the currently
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existing instances of this method ->sync_page() itself doesn't look
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well-defined...
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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 which is
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written. The address_space implementation may write more (or less) pages
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than *nr_to_write asks for, but it should try to be reasonably close. If
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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. All
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instances do not actually need the BKL. Please, keep it that way and don't
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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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->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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Note: currently almost all instances of address_space methods are
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using BKL for internal serialization and that's one of the worst sources
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of contention. Normally they are calling library functions (in fs/buffer.c)
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and pass foo_get_block() as a callback (on local block-based filesystems,
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indeed). BKL is not needed for library stuff and is usually taken by
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foo_get_block(). It's an overkill, since block bitmaps can be protected by
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internal fs locking and real critical areas are much smaller than the areas
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filesystems protect now.
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----------------------- file_lock_operations ------------------------------
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prototypes:
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void (*fl_insert)(struct file_lock *); /* lock insertion callback */
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void (*fl_remove)(struct file_lock *); /* lock removal callback */
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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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BKL may block
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fl_insert: yes no
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fl_remove: yes no
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fl_copy_lock: yes no
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fl_release_private: yes yes
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----------------------- lock_manager_operations ---------------------------
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prototypes:
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int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
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void (*fl_notify)(struct file_lock *); /* unblock callback */
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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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void (*fl_break)(struct file_lock *); /* break_lease callback */
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locking rules:
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BKL may block
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fl_compare_owner: yes no
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fl_notify: yes no
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fl_copy_lock: yes no
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fl_release_private: yes yes
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fl_break: yes no
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Currently only NFSD and NLM provide instances of this class. None of the
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them block. If you have out-of-tree instances - please, show up. Locking
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in that area will change.
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--------------------------- buffer_head -----------------------------------
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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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prototypes:
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int (*open) (struct inode *, struct file *);
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int (*release) (struct inode *, struct file *);
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int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
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int (*media_changed) (struct gendisk *);
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int (*revalidate_disk) (struct gendisk *);
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locking rules:
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BKL bd_sem
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open: yes yes
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release: yes yes
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ioctl: yes no
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media_changed: no no
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revalidate_disk: no no
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The last two are called only from check_disk_change().
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--------------------------- file_operations -------------------------------
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prototypes:
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loff_t (*llseek) (struct file *, loff_t, int);
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ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
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ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
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ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
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ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
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int (*readdir) (struct file *, void *, filldir_t);
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unsigned int (*poll) (struct file *, struct poll_table_struct *);
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int (*ioctl) (struct inode *, struct file *, unsigned int,
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unsigned long);
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long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
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long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
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int (*mmap) (struct file *, struct vm_area_struct *);
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int (*open) (struct inode *, struct file *);
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int (*flush) (struct file *);
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int (*release) (struct inode *, struct file *);
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int (*fsync) (struct file *, struct dentry *, int datasync);
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int (*aio_fsync) (struct kiocb *, int datasync);
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int (*fasync) (int, struct file *, int);
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int (*lock) (struct file *, int, struct file_lock *);
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ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
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loff_t *);
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ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
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loff_t *);
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ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
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void __user *);
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ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
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loff_t *, int);
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unsigned long (*get_unmapped_area)(struct file *, unsigned long,
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unsigned long, unsigned long, unsigned long);
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int (*check_flags)(int);
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int (*dir_notify)(struct file *, unsigned long);
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};
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locking rules:
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All except ->poll() may block.
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BKL
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llseek: no (see below)
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read: no
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aio_read: no
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write: no
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aio_write: no
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readdir: no
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poll: no
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ioctl: yes (see below)
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unlocked_ioctl: no (see below)
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compat_ioctl: no
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mmap: no
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open: maybe (see below)
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flush: no
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release: no
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fsync: no (see below)
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aio_fsync: no
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fasync: yes (see below)
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lock: yes
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readv: no
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writev: no
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sendfile: no
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sendpage: no
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get_unmapped_area: no
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check_flags: no
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dir_notify: no
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->llseek() locking has moved from llseek to the individual llseek
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implementations. If your fs is not using generic_file_llseek, you
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need to acquire and release the appropriate locks in your ->llseek().
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For many filesystems, it is probably safe to acquire the inode
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semaphore. Note some filesystems (i.e. remote ones) provide no
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protection for i_size so you will need to use the BKL.
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->open() locking is in-transit: big lock partially moved into the methods.
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The only exception is ->open() in the instances of file_operations that never
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end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
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(chrdev_open() takes lock before replacing ->f_op and calling the secondary
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method. As soon as we fix the handling of module reference counters all
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instances of ->open() will be called without the BKL.
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Note: ext2_release() was *the* source of contention on fs-intensive
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loads and dropping BKL on ->release() helps to get rid of that (we still
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grab BKL for cases when we close a file that had been opened r/w, but that
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can and should be done using the internal locking with smaller critical areas).
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Current worst offender is ext2_get_block()...
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->fasync() is a mess. This area needs a big cleanup and that will probably
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affect locking.
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->readdir() and ->ioctl() on directories must be changed. Ideally we would
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move ->readdir() to inode_operations and use a separate method for directory
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->ioctl() or kill the latter completely. One of the problems is that for
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anything that resembles union-mount we won't have a struct file for all
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components. And there are other reasons why the current interface is a mess...
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->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
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doesn't take the BKL.
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->read on directories probably must go away - we should just enforce -EISDIR
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in sys_read() and friends.
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->fsync() has i_mutex on inode.
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--------------------------- dquot_operations -------------------------------
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prototypes:
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int (*initialize) (struct inode *, int);
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int (*drop) (struct inode *);
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int (*alloc_space) (struct inode *, qsize_t, int);
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int (*alloc_inode) (const struct inode *, unsigned long);
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int (*free_space) (struct inode *, qsize_t);
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int (*free_inode) (const struct inode *, unsigned long);
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int (*transfer) (struct inode *, struct iattr *);
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int (*write_dquot) (struct dquot *);
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int (*acquire_dquot) (struct dquot *);
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int (*release_dquot) (struct dquot *);
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int (*mark_dirty) (struct dquot *);
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int (*write_info) (struct super_block *, int);
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These operations are intended to be more or less wrapping functions that ensure
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a proper locking wrt the filesystem and call the generic quota operations.
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What filesystem should expect from the generic quota functions:
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FS recursion Held locks when called
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initialize: yes maybe dqonoff_sem
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drop: yes -
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alloc_space: ->mark_dirty() -
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alloc_inode: ->mark_dirty() -
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free_space: ->mark_dirty() -
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free_inode: ->mark_dirty() -
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transfer: yes -
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write_dquot: yes dqonoff_sem or dqptr_sem
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acquire_dquot: yes dqonoff_sem or dqptr_sem
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release_dquot: yes dqonoff_sem or dqptr_sem
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mark_dirty: no -
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write_info: yes dqonoff_sem
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FS recursion means calling ->quota_read() and ->quota_write() from superblock
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operations.
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->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
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only directly by the filesystem and do not call any fs functions only
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the ->mark_dirty() operation.
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More details about quota locking can be found in fs/dquot.c.
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--------------------------- vm_operations_struct -----------------------------
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prototypes:
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void (*open)(struct vm_area_struct*);
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void (*close)(struct vm_area_struct*);
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int (*fault)(struct vm_area_struct*, struct vm_fault *);
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struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
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int (*page_mkwrite)(struct vm_area_struct *, struct page *);
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locking rules:
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BKL mmap_sem PageLocked(page)
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open: no yes
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close: no yes
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fault: no yes
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nopage: no yes
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page_mkwrite: no yes no
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->page_mkwrite() is called when a previously read-only page is
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about to become writeable. The file system is responsible for
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protecting against truncate races. Once appropriate action has been
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taking to lock out truncate, the page range should be verified to be
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within i_size. The page mapping should also be checked that it is not
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NULL.
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================================================================================
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Dubious stuff
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(if you break something or notice that it is broken and do not fix it yourself
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- at least put it here)
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ipc/shm.c::shm_delete() - may need BKL.
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->read() and ->write() in many drivers are (probably) missing BKL.
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drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.
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