The text below describes the locking rules for VFS-related methods. It is (believed to be) up-to-date. *Please*, if you change anything in prototypes or locking protocols - update this file. And update the relevant instances in the tree, don't leave that to maintainers of filesystems/devices/ etc. At the very least, put the list of dubious cases in the end of this file. Don't turn it into log - maintainers of out-of-the-tree code are supposed to be able to use diff(1). Thing currently missing here: socket operations. Alexey? --------------------------- dentry_operations -------------------------- prototypes: int (*d_revalidate)(struct dentry *, int); int (*d_hash) (struct dentry *, struct qstr *); int (*d_compare) (struct dentry *, struct qstr *, struct qstr *); int (*d_delete)(struct dentry *); void (*d_release)(struct dentry *); void (*d_iput)(struct dentry *, struct inode *); locking rules: none have BKL dcache_lock may block d_revalidate: no yes d_hash no yes d_compare: yes no d_delete: yes no d_release: no yes d_iput: no yes --------------------------- inode_operations --------------------------- prototypes: int (*create) (struct inode *,struct dentry *,int); struct dentry * (*lookup) (struct inode *,struct dentry *); int (*link) (struct dentry *,struct inode *,struct dentry *); int (*unlink) (struct inode *,struct dentry *); int (*symlink) (struct inode *,struct dentry *,const char *); int (*mkdir) (struct inode *,struct dentry *,int); int (*rmdir) (struct inode *,struct dentry *); int (*mknod) (struct inode *,struct dentry *,int,int); int (*rename) (struct inode *, struct dentry *, struct inode *, struct dentry *); int (*readlink) (struct dentry *, char *,int); int (*follow_link) (struct dentry *, struct nameidata *); void (*truncate) (struct inode *); int (*permission) (struct inode *, int); int (*revalidate) (struct dentry *); int (*setattr) (struct dentry *, struct iattr *); int (*getattr) (struct dentry *, struct iattr *); locking rules: all may block BKL i_sem(inode) i_zombie(inode) lookup: yes yes no create: yes yes yes link: yes yes yes mknod: yes yes yes mkdir: yes yes yes unlink: yes yes yes rmdir: yes yes yes (see below) rename: yes yes (both) yes (both) (see below) readlink: no no no follow_link: no no no truncate: yes yes no (see below) setattr: yes if ATTR_SIZE no permssion: yes no no getattr: (see below) revalidate: no (see below) Additionally, ->rmdir() has i_zombie on victim and so does ->rename() in case when target exists and is a directory. ->rename() on directories has (per-superblock) ->s_vfs_rename_sem. ->revalidate(), it may be called both with and without the i_sem on dentry->d_inode. VFS never calls it with i_zombie on dentry->d_inode, but watch for other methods directly calling this one... ->truncate() is never called directly - it's a callback, not a method. It's called by vmtruncate() - library function normally used by ->setattr(). Locking information above applies to that call (i.e. is inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been passed). ->getattr() is currently unused. --------------------------- super_operations --------------------------- prototypes: void (*read_inode) (struct inode *); void (*write_inode) (struct inode *, int); void (*put_inode) (struct inode *); void (*delete_inode) (struct inode *); void (*put_super) (struct super_block *); void (*write_super) (struct super_block *); int (*statfs) (struct super_block *, struct statfs *); int (*remount_fs) (struct super_block *, int *, char *); void (*clear_inode) (struct inode *); void (*umount_begin) (struct super_block *); locking rules: All may block. BKL s_lock mount_sem read_inode: yes (see below) write_inode: no put_inode: no delete_inode: no clear_inode: no put_super: yes yes maybe (see below) write_super: yes yes maybe (see below) statfs: yes no no remount_fs: yes yes maybe (see below) umount_begin: yes no maybe (see below) ->read_inode() is not a method - it's a callback used in iget()/iget4(). rules for mount_sem are not too nice - it is going to die and be replaced by better scheme anyway. --------------------------- file_system_type --------------------------- prototypes: struct super_block *(*read_super) (struct super_block *, void *, int); locking rules: may block BKL ->s_lock mount_sem yes yes yes maybe --------------------------- address_space_operations -------------------------- prototypes: int (*writepage)(struct page *); int (*readpage)(struct file *, struct page *); int (*sync_page)(struct page *); int (*prepare_write)(struct file *, struct page *, unsigned, unsigned); int (*commit_write)(struct file *, struct page *, unsigned, unsigned); int (*bmap)(struct address_space *, long); int (*flushpage) (struct page *, unsigned long); int (*releasepage) (struct page *, int); int (*direct_IO)(int, struct inode *, struct kiobuf *, unsigned long, int); locking rules: All may block BKL PageLocked(page) writepage: no yes, unlocks readpage: no yes, unlocks sync_page: no maybe prepare_write: no yes commit_write: no yes bmap: yes flushpage: no yes releasepage: no yes ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage() may be called from the request handler (/dev/loop). ->readpage() and ->writepage() unlock the page. ->sync_page() locking rules are not well-defined - usually it is called with lock on page, but that is not guaranteed. Considering the currently existing instances of this method ->sync_page() itself doesn't look well-defined... ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some filesystems and by the swapper. The latter will eventually go away. All instances do not actually need the BKL. Please, keep it that way and don't breed new callers. ->flushpage() is called when the filesystem must attempt to drop some or all of the buffers from the page when it is being truncated. It returns zero on success. If ->flushpage is zero, the kernel uses block_flushpage() instead. ->releasepage() is called when the kernel is about to try to drop the buffers from the page in preparation for freeing it. It returns zero to indicate that the buffers are (or may be) freeable. If ->releasepage is zero, the kernel assumes that the fs has no private interest in the buffers. Note: currently almost all instances of address_space methods are using BKL for internal serialization and that's one of the worst sources of contention. Normally they are calling library functions (in fs/buffer.c) and pass foo_get_block() as a callback (on local block-based filesystems, indeed). BKL is not needed for library stuff and is usually taken by foo_get_block(). It's an overkill, since block bitmaps can be protected by internal fs locking and real critical areas are much smaller than the areas filesystems protect now. --------------------------- file_lock ------------------------------------ prototypes: void (*fl_notify)(struct file_lock *); /* unblock callback */ void (*fl_insert)(struct file_lock *); /* lock insertion callback */ void (*fl_remove)(struct file_lock *); /* lock removal callback */ locking rules: BKL may block fl_notify: yes no fl_insert: yes maybe fl_remove: yes maybe Currently only NLM provides instances of this class. None of the them block. If you have out-of-tree instances - please, show up. Locking in that area will change. --------------------------- buffer_head ----------------------------------- prototypes: void (*b_end_io)(struct buffer_head *bh, int uptodate); locking rules: called from interrupts. In other words, extreme care is needed here. bh is locked, but that's all warranties we have here. Currently only RAID1, highmem and fs/buffer.c are providing these. Block devices call this method upon the IO completion. --------------------------- block_device_operations ----------------------- prototypes: int (*open) (struct inode *, struct file *); int (*release) (struct inode *, struct file *); int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long); int (*check_media_change) (kdev_t); int (*revalidate) (kdev_t); locking rules: BKL bd_sem open: yes yes release: yes yes ioctl: yes no check_media_change: yes no revalidate: yes no The last two are called only from check_disk_change(). Prototypes are very bad - as soon as we'll get disk_struct they will change (and methods will become per-disk instead of per-partition). --------------------------- file_operations ------------------------------- prototypes: loff_t (*llseek) (struct file *, loff_t, int); ssize_t (*read) (struct file *, char *, size_t, loff_t *); ssize_t (*write) (struct file *, const char *, size_t, loff_t *); int (*readdir) (struct file *, void *, filldir_t); unsigned int (*poll) (struct file *, struct poll_table_struct *); int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long); int (*mmap) (struct file *, struct vm_area_struct *); int (*open) (struct inode *, struct file *); int (*flush) (struct file *); int (*release) (struct inode *, struct file *); int (*fsync) (struct file *, struct dentry *, int 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 *); }; locking rules: All except ->poll() may block. BKL llseek: yes read: no write: no readdir: yes (see below) poll: no ioctl: yes (see below) mmap: no open: maybe (see below) flush: yes release: no fsync: yes (see below) fasync: yes (see below) lock: yes readv: no writev: no ->open() locking is in-transit: big lock partially moved into the methods. The only exception is ->open() in the instances of file_operations that never end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices (chrdev_open() takes lock before replacing ->f_op and calling the secondary method. As soon as we fix the handling of module reference counters all instances of ->open() will be called without the BKL. Note: ext2_release() was *the* source of contention on fs-intensive loads and dropping BKL on ->release() helps to get rid of that (we still grab BKL for cases when we close a file that had been opened r/w, but that can and should be done using the internal locking with smaller critical areas). Current worst offender is ext2_get_block()... ->fasync() is a mess. This area needs a big cleanup and that will probably affect locking. ->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. ->fsync() has i_sem on inode. --------------------------- dquot_operations ------------------------------- prototypes: void (*initialize) (struct inode *, short); void (*drop) (struct inode *); int (*alloc_block) (const struct inode *, unsigned long, char); int (*alloc_inode) (const struct inode *, unsigned long); void (*free_block) (const struct inode *, unsigned long); void (*free_inode) (const struct inode *, unsigned long); int (*transfer) (struct dentry *, struct iattr *); locking rules: BKL initialize: no drop: no alloc_block: yes alloc_inode: yes free_block: yes free_inode: yes transfer: no --------------------------- vm_operations_struct ----------------------------- prototypes: void (*open)(struct vm_area_struct*); void (*close)(struct vm_area_struct*); struct page *(*nopage)(struct vm_area_struct*, unsigned long, int); locking rules: BKL mmap_sem open: no yes close: no yes nopage: no yes ================================================================================ Dubious stuff (if you break something or notice that it is broken and do not fix it yourself - at least put it here) ipc/shm.c::shm_delete() - may need BKL. ->read() and ->write() in many drivers are (probably) missing BKL. drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.