[Ocfs2-devel] [PATCH] ocfs2: Support nested transactions

[Joel Becker <Joel.Becker@oracle.com>]

On Mon, Oct 20, 2008 at 07:23:52PM +0200, Jan Kara wrote:
> OCFS2 can easily support nested transactions. We just have to
> take care and not spoil statistics acquire semaphore unnecessarily.

	I'm guessing this is required for the quota code, because it
wants to start its own transaction underneath the transaction ocfs2 is
doing?  Is there a way to have ocfs2 just pass the handle to the quota
op (I'm guessing not because it's in the VFS)?
	This just scares me a little.

> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ocfs2/journal.c |   14 +++++++-------
>  1 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
> index c47bc2a..b3e24f9 100644
> --- a/fs/ocfs2/journal.c
> +++ b/fs/ocfs2/journal.c
> @@ -256,11 +256,9 @@ handle_t *ocfs2_start_trans(struct ocfs2_super *osb, int max_buffs)
>  	BUG_ON(osb->journal->j_state == OCFS2_JOURNAL_FREE);
>  	BUG_ON(max_buffs <= 0);
>  
> -	/* JBD might support this, but our journalling code doesn't yet. */
> -	if (journal_current_handle()) {
> -		mlog(ML_ERROR, "Recursive transaction attempted!\n");
> -		BUG();
> -	}
> +	/* Nested transaction? Just return the handle... */
> +	if (journal_current_handle())
> +		return journal_start(journal, max_buffs);
>  
>  	down_read(&osb->journal->j_trans_barrier);
>  
> @@ -285,16 +283,18 @@ handle_t *ocfs2_start_trans(struct ocfs2_super *osb, int max_buffs)
>  int ocfs2_commit_trans(struct ocfs2_super *osb,
>  		       handle_t *handle)
>  {
> -	int ret;
> +	int ret, nested;
>  	struct ocfs2_journal *journal = osb->journal;
>  
>  	BUG_ON(!handle);
>  
> +	nested = handle->h_ref > 1;
>  	ret = journal_stop(handle);
>  	if (ret < 0)
>  		mlog_errno(ret);
>  
> -	up_read(&journal->j_trans_barrier);
> +	if (!nested)
> +		up_read(&journal->j_trans_barrier);
>  
>  	return ret;
>  }

	If we're doing to do this, I'd like to see a check in
ocfs2_commit_trans() to make sure we're running ABBA nested and not ABAB
overlapping.  Perhaps a flag on osb->journal somehow?

Joel

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Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127