Re: [PATCH] Fix private_list handling

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 10 Jan 2008 16:55:13 +0100
Jan Kara <jack@suse.cz> wrote:

>   Hi,
> 
>   sorry for the previous empty email...
> 
>   Supriya noted in his testing that sometimes buffers removed by
> __remove_assoc_queue() don't have b_assoc_mapping set (and thus IO error
> won't be properly propagated). Actually, looking more into the code I found
> there are some more races. The patch below should fix them. It survived
> beating with LTP and fsstress on ext2 filesystem on my testing machine so
> it should be reasonably bugfree... Andrew, would you put the patch into
> -mm? Thanks.
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.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.

private_lock should prevent this race.

Which call to drop_buffers() is the culprit?  The first one in
try_to_free_buffers(), I assume?  The "can this still happen?" one?

If so, it can happen.  How?  Perhaps this is a bug.

> 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().

Maybe fsync_buffers_list should put the buffer back onto private_list if it
got dirtied again.