[PATCH] i_dirty_buffers locking fix

This fixes a race between try_to_free_buffers' call to
__remove_inode_queue() and other users of b_inode_buffers
(fsync_inode_buffers and mark_buffer_dirty_inode()).  They are
presently taking different locks.

The patch relocates and redefines and clarifies(?) the role of
inode.i_dirty_buffers.

The 2.4 definition of i_dirty_buffers is "a list of random buffers
which is protected by a kernel-wide lock".  This definition needs to be
narrowed in the 2.5 context.  It is now

"a list of buffers from a different mapping, protected by a lock within
that mapping".  This list of buffers is specifically for fsync().

As this is a "data plane" operation, all the structures have been moved
out of the inode and into the address_space.  So address_space now has:

list_head private_list;

     A list, available to the address_space for any purpose.  If
     that address_space chooses to use the helper functions
     mark_buffer_dirty_inode and sync_mapping_buffers() then this list
     will contain buffer_heads, attached via
     buffer_head.b_assoc_buffers.

     If the address_space does not call those helper functions
     then the list is free for other usage.  The only requirement is
     that the list be list_empty() at destroy_inode() time.

     At least, this is the objective.  At present,
     generic_file_write() will call generic_osync_inode(), which
     expects that list to contain buffer_heads.  So private_list isn't
     useful for anything else yet.

spinlock_t private_lock;

     A spinlock, available to the address_space.

     If the address_space is using try_to_free_buffers(),
     mark_inode_dirty_buffers() and fsync_inode_buffers() then this
     lock is used to protect the private_list of *other* mappings which
     have listed buffers from *this* mapping onto themselves.

     That is: for buffer_heads, mapping_A->private_lock does not
     protect mapping_A->private_list!  It protects the b_assoc_buffers
     list from buffers which are backed by mapping_A and it protects
     mapping_B->private_list, mapping_C->private_list, ...

     So what we have here is a cross-mapping association.  S_ISREG
     mappings maintain a list of buffers from the blockdev's
     address_space which they need to know about for a successful
     fsync().  The locking follows the buffers: the lock in in the
     blockdev's mapping, not in the S_ISREG file's mapping.

     For address_spaces which use try_to_free_buffers,
     private_lock is also (and quite unrelatedly) used for protection
     of the buffer ring at page->private.  Exclusion between
     try_to_free_buffers(), __get_hash_table() and
     __set_page_dirty_buffers().  This is in fact its major use.

address_space *assoc_mapping

    Sigh.  This is the address of the mapping which backs the
    buffers which are attached to private_list.  It's here so that
    generic_osync_inode() can locate the lock which protects this
    mapping's private_list.  Will probably go away.


A consequence of all the above is that:

    a) All the buffers at a mapping_A's ->private_list must come
       from the same mapping, mapping_B.  There is no requirement that
       mapping_B be a blockdev mapping, but that's how it's used.

       There is a BUG() check in mark_buffer_dirty_inode() for this.

    b) blockdev mappings never have any buffers on ->private_list.
       It just never happens, and doesn't make a lot of sense.

reiserfs is using b_inode_buffers for attaching dependent buffers to its
journal and that caused a few problems.  Fixed in reiserfs_releasepage.patch

1 files changed, 4 insertions, 5 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4b858f90c6fe..25578c7a5e62 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -306,6 +306,7 @@ struct address_space_operations {
 };
 
 struct address_space {
+	struct inode		*host;		/* owner: inode, block_device */
 	struct radix_tree_root	page_tree;	/* radix tree of all pages */
 	rwlock_t		page_lock;	/* and rwlock protecting it */
 	struct list_head	clean_pages;	/* list of clean pages */
@@ -314,13 +315,15 @@ struct address_space {
 	struct list_head	io_pages;	/* being prepared for I/O */
 	unsigned long		nrpages;	/* number of total pages */
 	struct address_space_operations *a_ops;	/* methods */
-	struct inode		*host;		/* owner: inode, block_device */
 	list_t			i_mmap;		/* list of private mappings */
 	list_t			i_mmap_shared;	/* list of private mappings */
 	spinlock_t		i_shared_lock;  /* and spinlock protecting it */
 	unsigned long		dirtied_when;	/* jiffies of first page dirtying */
 	int			gfp_mask;	/* how to allocate the pages */
 	unsigned long 		*ra_pages;	/* device readahead */
+	spinlock_t		private_lock;	/* for use by the address_space */
+	struct list_head	private_list;	/* ditto */
+	struct address_space	*assoc_mapping;	/* ditto */
 };
 
 struct char_device {
@@ -350,10 +353,6 @@ struct inode {
 	struct list_head	i_hash;
 	struct list_head	i_list;
 	struct list_head	i_dentry;
-
-	struct list_head	i_dirty_buffers;   /* uses i_bufferlist_lock */
-	spinlock_t		i_bufferlist_lock;
-
 	unsigned long		i_ino;
 	atomic_t		i_count;
 	kdev_t			i_dev;