locking

linux 内核源代码

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);
	int (*dir_notify)(struct file *, unsigned long);
};

locking rules:
	All except ->poll() 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:			maybe	(see below)
flush:			no
release:		no
fsync:			no	(see below)
aio_fsync:		no
fasync:			yes	(see below)
lock:			yes
readv:			no
writev:			no
sendfile:		no
sendpage:		no
get_unmapped_area:	no
check_flags:		no
dir_notify:		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.

->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...

->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 *);
	struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
	int (*page_mkwrite)(struct vm_area_struct *, struct page *);

locking rules:
		BKL	mmap_sem	PageLocked(page)
open:		no	yes
close:		no	yes
fault:		no	yes
nopage:		no	yes
page_mkwrite:	no	yes		no

	->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.

================================================================================
			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.