linux-ext4 - Re: [PATCH] jbd2: Fix a race between checkpointing code and journal_get_write

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Date:	Sun, 5 Jul 2009 22:53:19 -0400
From:	Theodore Tso <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] jbd2: Fix a race between checkpointing code and
	journal_get_write_access()

On Wed, Jun 24, 2009 at 06:02:40PM +0200, Jan Kara wrote:
> The following race can happen:
> 
>   CPU1                          CPU2
>                                 checkpointing code checks the buffer, adds
>                                   it to an array for writeback
> do_get_write_access()
>   ...
>   lock_buffer()
>   unlock_buffer()
>                                   flush_batch() submits the buffer for IO
>   __jbd2_journal_file_buffer()
> 
>   So a buffer under writeout is returned from do_get_write_access(). Since
> the filesystem code relies on the fact that journaled buffers cannot be
> written out, it does not take the buffer lock and so it can modify buffer
> while it is under writeout. That can lead to a filesystem corruption
> if we crash at the right moment.
>   We fix the problem by clearing the buffer dirty bit under buffer_lock
> even if the buffer is on BJ_None list. Actually, we clear the dirty bit
> regardless the list the buffer is in and warn about the fact if
> the buffer is already journalled.
> 
> Thanks for spotting the problem goes to dingdinghua <dingdinghua85@...il.com>.
> 
> Reported-by: dingdinghua <dingdinghua85@...il.com>
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied to the ext4 patch queue.

						- Ted
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