general protection fault: from release_blocks_on_commit

On Mon, Oct 27, 2008 at 07:08:37PM -0500, Eric Sandeen wrote:
> Theodore Tso wrote:
> > On Mon, Oct 27, 2008 at 05:26:47PM -0500, Eric Sandeen wrote:
> >> Ted, you probably need some slab debugging on to hit it.
> > 
> > I had slab debugging enabled, but haven't been able to replicate it
> > yet.  I'll do some more work to try to replicate it.
> > 
> >> I think the problem is that jbd2_journal_commit_transaction may call
> >> __jbd2_journal_drop_transaction(journal, commit_transaction) if the
> >> checkpoint lists are NULL, and this frees the commit_transaction.
> > 
> > I think you're right.  I would probably change the patch around so
> > that after calling __jbd2_jurnal_drop_transaction(), we set
> > commit_transaction to NULL, and then adding an "if
> > (commit_transaction)" to the lines in questions; that way we keep the
> > commit callback outside of the j_list_lock() spinlock.
> 
> Ok, I thought about that; sounds good.  will resend.

I looked at this some more, and at least in theory it could happen
that we could not have any buffers that need to be checkpointed (so
t_checkpoint_list and t_checkpoint_io_list are NULL), but there are
still blocks to be released (or some other users of the jbd2 layer
still wants to have the callback be called).  So I'm currently testing
this patch (see below).

							- Ted

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Theodore Tso wrote:

> I looked at this some more, and at least in theory it could happen
> that we could not have any buffers that need to be checkpointed (so
> t_checkpoint_list and t_checkpoint_io_list are NULL), but there are
> still blocks to be released (or some other users of the jbd2 layer
> still wants to have the callback be called).  So I'm currently testing
> this patch (see below).
> 
> 							- Ted

Looks reasonable to me from a correctness perspective, anyway, as long
as holding the j_list_lock over that callback is ok.

It does fix the oops-on-reboot that I could reproduce.

-Eric


> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 8b119e1..ebc667b 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -974,6 +974,9 @@ restart_loop:
>  	journal->j_committing_transaction = NULL;
>  	spin_unlock(&journal->j_state_lock);
>  
> +	if (journal->j_commit_callback)
> +		journal->j_commit_callback(journal, commit_transaction);
> +
>  	if (commit_transaction->t_checkpoint_list == NULL &&
>  	    commit_transaction->t_checkpoint_io_list == NULL) {
>  		__jbd2_journal_drop_transaction(journal, commit_transaction);
> @@ -995,11 +998,8 @@ restart_loop:
>  	}
>  	spin_unlock(&journal->j_list_lock);
>  
> -	if (journal->j_commit_callback)
> -		journal->j_commit_callback(journal, commit_transaction);
> -
>  	trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
> -		   journal->j_devname, commit_transaction->t_tid,
> +		   journal->j_devname, journal->j_commit_sequence,
>  		   journal->j_tail_sequence);
>  	jbd_debug(1, "JBD: commit %d complete, head %d\n",
>  		  journal->j_commit_sequence, journal->j_tail_sequence);

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