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buffer_head: fix private_list handling 

There are two possible races in handling of private_list in buffer cache.

1) When fsync_buffers_list() processes a private_list, it clears
   b_assoc_mapping and moves buffer to its private list.  Now
   drop_buffers() comes, sees a buffer is on list so it calls
   __remove_assoc_queue() which complains about b_assoc_mapping being
   cleared (as it cannot propagate possible IO error).  This race has been
   actually observed in the wild.

2) When fsync_buffers_list() processes a private_list,
   mark_buffer_dirty_inode() can be called on bh which is already on the
   private list of fsync_buffers_list().  As buffer is on some list (note
   that the check is performed without private_lock), it is not readded to
   the mapping's private_list and after fsync_buffers_list() finishes, we
   have a dirty buffer which should be on private_list but it isn't.  This
   race has not been reported, probably because most (but not all) callers
   of mark_buffer_dirty_inode() hold i_mutex and thus are serialized with
   fsync().

Fix these issues by not clearing b_assoc_map when fsync_buffers_list()
moves buffer to a dedicated list and by reinserting buffer in private_list
when it is found dirty after we have submitted buffer for IO.  We also
change the tests whether a buffer is on a private list from
!list_empty(&bh->b_assoc_buffers) to bh->b_assoc_map so that they are
single word reads and hence lockless checks are safe.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jan Kara 2008-02-08 04:21:59 -08:00 committed by Linus Torvalds
parent f6f21c8146
commit 535ee2fbf7
1 changed files with 19 additions and 4 deletions
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23
fs/buffer.c
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@ -678,7 +678,7 @@ void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
} else {
BUG_ON(mapping->assoc_mapping != buffer_mapping);
}
if (list_empty(&bh->b_assoc_buffers)) {
if (!bh->b_assoc_map) {
spin_lock(&buffer_mapping->private_lock);
list_move_tail(&bh->b_assoc_buffers,
&mapping->private_list);
@ -794,6 +794,7 @@ static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
{
struct buffer_head *bh;
struct list_head tmp;
struct address_space *mapping;
int err = 0, err2;
INIT_LIST_HEAD(&tmp);
@ -801,9 +802,14 @@ static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
spin_lock(lock);
while (!list_empty(list)) {
bh = BH_ENTRY(list->next);
mapping = bh->b_assoc_map;
__remove_assoc_queue(bh);
/* Avoid race with mark_buffer_dirty_inode() which does
* a lockless check and we rely on seeing the dirty bit */
smp_mb();
if (buffer_dirty(bh) || buffer_locked(bh)) {
list_add(&bh->b_assoc_buffers, &tmp);
bh->b_assoc_map = mapping;
if (buffer_dirty(bh)) {
get_bh(bh);
spin_unlock(lock);
@ -822,8 +828,17 @@ static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
while (!list_empty(&tmp)) {
bh = BH_ENTRY(tmp.prev);
list_del_init(&bh->b_assoc_buffers);
get_bh(bh);
mapping = bh->b_assoc_map;
__remove_assoc_queue(bh);
/* Avoid race with mark_buffer_dirty_inode() which does
* a lockless check and we rely on seeing the dirty bit */
smp_mb();
if (buffer_dirty(bh)) {
list_add(&bh->b_assoc_buffers,
&bh->b_assoc_map->private_list);
bh->b_assoc_map = mapping;
}
spin_unlock(lock);
wait_on_buffer(bh);
if (!buffer_uptodate(bh))
@ -1195,7 +1210,7 @@ void __brelse(struct buffer_head * buf)
void __bforget(struct buffer_head *bh)
{
clear_buffer_dirty(bh);
if (!list_empty(&bh->b_assoc_buffers)) {
if (bh->b_assoc_map) {
struct address_space *buffer_mapping = bh->b_page->mapping;
spin_lock(&buffer_mapping->private_lock);
@ -3022,7 +3037,7 @@ drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
do {
struct buffer_head *next = bh->b_this_page;
if (!list_empty(&bh->b_assoc_buffers))
if (bh->b_assoc_map)
__remove_assoc_queue(bh);
bh = next;
} while (bh != head);