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Date: Mon, 28 Jan 2008 23:16:56 +0100
From: Jan Kara <jack@...e.cz>
To: Nick Piggin <nickpiggin@...oo.com.au>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RESEND] Minimal fix for private_list handling races
On Fri 25-01-08 19:34:07, Nick Piggin wrote:
> > > But let's see... there must be a memory ordering problem here in existing
> > > code anyway, because I don't see any barriers. Between b_assoc_buffers
> > > and b_state (via buffer_dirty); fsync_buffers_list vs
> > > mark_buffer_dirty_inode, right?
> >
> > I'm not sure. What exactly to you mean? BTW: spin_lock is a memory
> > barrier, isn't it?
>
> In existing code:
>
> mark_buffer_dirty_inode(): fsync_buffers_list():
> test_set_buffer_dirty(bh); list_del_init(&bh->b_assoc_buffers);
> if (list_empty(&bh->b_assoc_buffers)) if (buffer_dirty(bh)) {
> ... list_add(&bh->b_assoc_buffers, );
>
> These two code sequences can run concurrently because only fsync_buffers_list
> takes the lock.
>
> So fsync_buffers_list can speculatively load bh->b_state before
> its stores to clear b_assoc_buffers propagate to the CPU running
> mark_buffer_dirty_inode.
>
> So if there is a !dirty buffer on the list, then fsync_buffers_list will
> remove it from the list, but mark_buffer_dirty_inode won't see it has been
> removed from the list and won't re-add it. I think.
>
> This is actually even possible to hit on x86 because they reorder loads
> past stores. It needs a smp_mb() before if (buffer_dirty(bh) {}.
>
> Actually I very much dislike testing list entries locklessly, because they
> are not trivially atomic operations like single stores... which is another
> reason why I like your first patch.
OK, Nick, how do you like the patch below?
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
---
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.
Signed-off-by: Jan Kara <jack@...e.cz>
diff --git a/fs/buffer.c b/fs/buffer.c
index 7249e01..76e1ab4 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -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);
@@ -3037,7 +3052,7 @@ 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);
--
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