| 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 *); |
| char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen); |
| |
| locking rules: |
| none have BKL |
| dcache_lock rename_lock ->d_lock may block |
| d_revalidate: no no no yes |
| d_hash no no no yes |
| d_compare: no yes no no |
| d_delete: yes no yes no |
| d_release: no no no yes |
| d_iput: no no no yes |
| d_dname: no no no no |
| |
| --------------------------- inode_operations --------------------------- |
| prototypes: |
| int (*create) (struct inode *,struct dentry *,int, struct nameidata *); |
| struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid |
| ata *); |
| 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,dev_t); |
| int (*rename) (struct inode *, struct dentry *, |
| struct inode *, struct dentry *); |
| int (*readlink) (struct dentry *, char __user *,int); |
| int (*follow_link) (struct dentry *, struct nameidata *); |
| void (*truncate) (struct inode *); |
| int (*permission) (struct inode *, int, struct nameidata *); |
| int (*setattr) (struct dentry *, struct iattr *); |
| int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *); |
| int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); |
| ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); |
| ssize_t (*listxattr) (struct dentry *, char *, size_t); |
| int (*removexattr) (struct dentry *, const char *); |
| |
| locking rules: |
| all may block, none have BKL |
| i_mutex(inode) |
| lookup: yes |
| create: yes |
| link: yes (both) |
| mknod: yes |
| symlink: yes |
| mkdir: yes |
| unlink: yes (both) |
| rmdir: yes (both) (see below) |
| rename: yes (all) (see below) |
| readlink: no |
| follow_link: no |
| truncate: yes (see below) |
| setattr: yes |
| permission: no |
| getattr: no |
| setxattr: yes |
| getxattr: no |
| listxattr: no |
| removexattr: yes |
| Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on |
| victim. |
| cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem. |
| ->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). |
| |
| See Documentation/filesystems/directory-locking for more detailed discussion |
| of the locking scheme for directory operations. |
| |
| --------------------------- super_operations --------------------------- |
| prototypes: |
| struct inode *(*alloc_inode)(struct super_block *sb); |
| void (*destroy_inode)(struct inode *); |
| void (*dirty_inode) (struct inode *); |
| int (*write_inode) (struct inode *, int); |
| void (*drop_inode) (struct inode *); |
| void (*delete_inode) (struct inode *); |
| void (*put_super) (struct super_block *); |
| void (*write_super) (struct super_block *); |
| int (*sync_fs)(struct super_block *sb, int wait); |
| int (*freeze_fs) (struct super_block *); |
| int (*unfreeze_fs) (struct super_block *); |
| int (*statfs) (struct dentry *, struct kstatfs *); |
| int (*remount_fs) (struct super_block *, int *, char *); |
| void (*clear_inode) (struct inode *); |
| void (*umount_begin) (struct super_block *); |
| int (*show_options)(struct seq_file *, struct vfsmount *); |
| ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); |
| ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); |
| |
| locking rules: |
| All may block. |
| BKL s_lock s_umount |
| alloc_inode: no no no |
| destroy_inode: no |
| dirty_inode: no (must not sleep) |
| write_inode: no |
| drop_inode: no !!!inode_lock!!! |
| delete_inode: no |
| put_super: yes yes no |
| write_super: no yes read |
| sync_fs: no no read |
| freeze_fs: ? |
| unfreeze_fs: ? |
| statfs: no no no |
| remount_fs: yes yes maybe (see below) |
| clear_inode: no |
| umount_begin: yes no no |
| show_options: no (vfsmount->sem) |
| quota_read: no no no (see below) |
| quota_write: no no no (see below) |
| |
| ->remount_fs() will have the s_umount lock if it's already mounted. |
| When called from get_sb_single, it does NOT have the s_umount lock. |
| ->quota_read() and ->quota_write() functions are both guaranteed to |
| be the only ones operating on the quota file by the quota code (via |
| dqio_sem) (unless an admin really wants to screw up something and |
| writes to quota files with quotas on). For other details about locking |
| see also dquot_operations section. |
| |
| --------------------------- file_system_type --------------------------- |
| prototypes: |
| int (*get_sb) (struct file_system_type *, int, |
| const char *, void *, struct vfsmount *); |
| void (*kill_sb) (struct super_block *); |
| locking rules: |
| may block BKL |
| get_sb yes no |
| kill_sb yes no |
| |
| ->get_sb() returns error or 0 with locked superblock attached to the vfsmount |
| (exclusive on ->s_umount). |
| ->kill_sb() takes a write-locked superblock, does all shutdown work on it, |
| unlocks and drops the reference. |
| |
| --------------------------- address_space_operations -------------------------- |
| prototypes: |
| int (*writepage)(struct page *page, struct writeback_control *wbc); |
| int (*readpage)(struct file *, struct page *); |
| int (*sync_page)(struct page *); |
| int (*writepages)(struct address_space *, struct writeback_control *); |
| int (*set_page_dirty)(struct page *page); |
| int (*readpages)(struct file *filp, struct address_space *mapping, |
| struct list_head *pages, unsigned nr_pages); |
| int (*write_begin)(struct file *, struct address_space *mapping, |
| loff_t pos, unsigned len, unsigned flags, |
| struct page **pagep, void **fsdata); |
| int (*write_end)(struct file *, struct address_space *mapping, |
| loff_t pos, unsigned len, unsigned copied, |
| struct page *page, void *fsdata); |
| sector_t (*bmap)(struct address_space *, sector_t); |
| int (*invalidatepage) (struct page *, unsigned long); |
| int (*releasepage) (struct page *, int); |
| int (*direct_IO)(int, struct kiocb *, const struct iovec *iov, |
| loff_t offset, unsigned long nr_segs); |
| int (*launder_page) (struct page *); |
| |
| locking rules: |
| All except set_page_dirty may block |
| |
| BKL PageLocked(page) i_sem |
| writepage: no yes, unlocks (see below) |
| readpage: no yes, unlocks |
| sync_page: no maybe |
| writepages: no |
| set_page_dirty no no |
| readpages: no |
| write_begin: no locks the page yes |
| write_end: no yes, unlocks yes |
| perform_write: no n/a yes |
| bmap: yes |
| invalidatepage: no yes |
| releasepage: no yes |
| direct_IO: no |
| launder_page: no yes |
| |
| ->write_begin(), ->write_end(), ->sync_page() and ->readpage() |
| may be called from the request handler (/dev/loop). |
| |
| ->readpage() unlocks the page, either synchronously or via I/O |
| completion. |
| |
| ->readpages() populates the pagecache with the passed pages and starts |
| I/O against them. They come unlocked upon I/O completion. |
| |
| ->writepage() is used for two purposes: for "memory cleansing" and for |
| "sync". These are quite different operations and the behaviour may differ |
| depending upon the mode. |
| |
| If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then |
| it *must* start I/O against the page, even if that would involve |
| blocking on in-progress I/O. |
| |
| If writepage is called for memory cleansing (sync_mode == |
| WBC_SYNC_NONE) then its role is to get as much writeout underway as |
| possible. So writepage should try to avoid blocking against |
| currently-in-progress I/O. |
| |
| If the filesystem is not called for "sync" and it determines that it |
| would need to block against in-progress I/O to be able to start new I/O |
| against the page the filesystem should redirty the page with |
| redirty_page_for_writepage(), then unlock the page and return zero. |
| This may also be done to avoid internal deadlocks, but rarely. |
| |
| If the filesystem is called for sync then it must wait on any |
| in-progress I/O and then start new I/O. |
| |
| The filesystem should unlock the page synchronously, before returning to the |
| caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE |
| value. WRITEPAGE_ACTIVATE means that page cannot really be written out |
| currently, and VM should stop calling ->writepage() on this page for some |
| time. VM does this by moving page to the head of the active list, hence the |
| name. |
| |
| Unless the filesystem is going to redirty_page_for_writepage(), unlock the page |
| and return zero, writepage *must* run set_page_writeback() against the page, |
| followed by unlocking it. Once set_page_writeback() has been run against the |
| page, write I/O can be submitted and the write I/O completion handler must run |
| end_page_writeback() once the I/O is complete. If no I/O is submitted, the |
| filesystem must run end_page_writeback() against the page before returning from |
| writepage. |
| |
| That is: after 2.5.12, pages which are under writeout are *not* locked. Note, |
| if the filesystem needs the page to be locked during writeout, that is ok, too, |
| the page is allowed to be unlocked at any point in time between the calls to |
| set_page_writeback() and end_page_writeback(). |
| |
| Note, failure to run either redirty_page_for_writepage() or the combination of |
| set_page_writeback()/end_page_writeback() on a page submitted to writepage |
| will leave the page itself marked clean but it will be tagged as dirty in the |
| radix tree. This incoherency can lead to all sorts of hard-to-debug problems |
| in the filesystem like having dirty inodes at umount and losing written data. |
| |
| ->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... |
| |
| ->writepages() is used for periodic writeback and for syscall-initiated |
| sync operations. The address_space should start I/O against at least |
| *nr_to_write pages. *nr_to_write must be decremented for each page which is |
| written. The address_space implementation may write more (or less) pages |
| than *nr_to_write asks for, but it should try to be reasonably close. If |
| nr_to_write is NULL, all dirty pages must be written. |
| |
| writepages should _only_ write pages which are present on |
| mapping->io_pages. |
| |
| ->set_page_dirty() is called from various places in the kernel |
| when the target page is marked as needing writeback. It may be called |
| under spinlock (it cannot block) and is sometimes called with the page |
| not locked. |
| |
| ->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. |
| |
| ->invalidatepage() 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 ->invalidatepage is zero, the kernel uses |
| block_invalidatepage() 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. |
| |
| ->launder_page() may be called prior to releasing a page if |
| it is still found to be dirty. It returns zero if the page was successfully |
| cleaned, or an error value if not. Note that in order to prevent the page |
| getting mapped back in and redirtied, it needs to be kept locked |
| across the entire operation. |
| |
| 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_operations ------------------------------ |
| prototypes: |
| void (*fl_insert)(struct file_lock *); /* lock insertion callback */ |
| void (*fl_remove)(struct file_lock *); /* lock removal callback */ |
| void (*fl_copy_lock)(struct file_lock *, struct file_lock *); |
| void (*fl_release_private)(struct file_lock *); |
| |
| |
| locking rules: |
| BKL may block |
| fl_insert: yes no |
| fl_remove: yes no |
| fl_copy_lock: yes no |
| fl_release_private: yes yes |
| |
| ----------------------- lock_manager_operations --------------------------- |
| prototypes: |
| int (*fl_compare_owner)(struct file_lock *, struct file_lock *); |
| void (*fl_notify)(struct file_lock *); /* unblock callback */ |
| void (*fl_copy_lock)(struct file_lock *, struct file_lock *); |
| void (*fl_release_private)(struct file_lock *); |
| void (*fl_break)(struct file_lock *); /* break_lease callback */ |
| |
| locking rules: |
| BKL may block |
| fl_compare_owner: yes no |
| fl_notify: yes no |
| fl_copy_lock: yes no |
| fl_release_private: yes yes |
| fl_break: yes no |
| |
| Currently only NFSD and NLM provide 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, fs/buffer.c, and fs/ntfs/aops.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 (*media_changed) (struct gendisk *); |
| int (*revalidate_disk) (struct gendisk *); |
| |
| locking rules: |
| BKL bd_sem |
| open: yes yes |
| release: yes yes |
| ioctl: yes no |
| media_changed: no no |
| revalidate_disk: no no |
| |
| The last two are called only from check_disk_change(). |
| |
| --------------------------- 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 (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
| ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
| int (*readdir) (struct file *, void *, filldir_t); |
| unsigned int (*poll) (struct file *, struct poll_table_struct *); |
| int (*ioctl) (struct inode *, struct file *, unsigned int, |
| unsigned long); |
| long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
| long (*compat_ioctl) (struct file *, unsigned int, unsigned long); |
| int (*mmap) (struct file *, struct vm_area_struct *); |
| int (*open) (struct inode *, struct file *); |
| int (*flush) (struct file *); |
| int (*release) (struct inode *, struct file *); |
| int (*fsync) (struct file *, struct dentry *, int datasync); |
| int (*aio_fsync) (struct kiocb *, int datasync); |
| int (*fasync) (int, struct file *, int); |
| int (*lock) (struct file *, int, struct file_lock *); |
| ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, |
| loff_t *); |
| ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, |
| loff_t *); |
| ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, |
| void __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); |
| }; |
| |
| locking rules: |
| All may block. |
| BKL |
| llseek: no (see below) |
| read: no |
| aio_read: no |
| write: no |
| aio_write: no |
| readdir: no |
| poll: no |
| ioctl: yes (see below) |
| unlocked_ioctl: no (see below) |
| compat_ioctl: no |
| mmap: no |
| open: no |
| flush: no |
| release: no |
| fsync: no (see below) |
| aio_fsync: no |
| fasync: no |
| lock: yes |
| readv: no |
| writev: no |
| sendfile: no |
| sendpage: no |
| get_unmapped_area: no |
| check_flags: no |
| |
| ->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 |
| semaphore. Note some filesystems (i.e. remote ones) provide no |
| protection for i_size so you will need to use 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... |
| |
| ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that |
| doesn't take the BKL. |
| |
| ->read on directories probably must go away - we should just enforce -EISDIR |
| in sys_read() and friends. |
| |
| ->fsync() has i_mutex on inode. |
| |
| --------------------------- dquot_operations ------------------------------- |
| prototypes: |
| int (*initialize) (struct inode *, int); |
| int (*drop) (struct inode *); |
| int (*alloc_space) (struct inode *, qsize_t, int); |
| int (*alloc_inode) (const struct inode *, unsigned long); |
| int (*free_space) (struct inode *, qsize_t); |
| int (*free_inode) (const struct inode *, unsigned long); |
| int (*transfer) (struct inode *, struct iattr *); |
| 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: |
| |
| FS recursion Held locks when called |
| initialize: yes maybe dqonoff_sem |
| drop: yes - |
| alloc_space: ->mark_dirty() - |
| alloc_inode: ->mark_dirty() - |
| free_space: ->mark_dirty() - |
| free_inode: ->mark_dirty() - |
| transfer: yes - |
| 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. |
| |
| ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called |
| only directly by the filesystem and do not call any fs functions only |
| the ->mark_dirty() operation. |
| |
| 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*); |
| int (*fault)(struct vm_area_struct*, struct vm_fault *); |
| int (*page_mkwrite)(struct vm_area_struct *, struct page *); |
| int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); |
| |
| locking rules: |
| BKL mmap_sem PageLocked(page) |
| open: no yes |
| close: no yes |
| fault: no yes |
| page_mkwrite: no yes no |
| access: no yes |
| |
| ->page_mkwrite() is called when a previously read-only page is |
| about to become writeable. The file system is responsible for |
| protecting against truncate races. Once appropriate action has been |
| taking to lock out truncate, the page range should be verified to be |
| within i_size. The page mapping should also be checked that it is not |
| NULL. |
| |
| ->access() is called when get_user_pages() fails in |
| acces_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) |
| |
| ipc/shm.c::shm_delete() - may need BKL. |
| ->read() and ->write() in many drivers are (probably) missing BKL. |