Re: 3.0-rcX BUG at fs/btrfs/ioctl.c:432 - bisected

[Sage Weil]

On Fri, 10 Jun 2011, Sage Weil wrote:
> On Fri, 10 Jun 2011, Chris Mason wrote:
> > Excerpts from Jim Schutt's message of 2011-06-10 13:06:22 -0400:
> > 
> > [ two different btrfs crashes ]
> > 
> > I think your two crashes in btrfs were from the uninit variables and
> > those should be fixed in rc2.
> > 
> > > When I did my bisection, my criteria for success/failure was
> > > "did mkcephfs succeed?".  When I apply this criteria to a recent
> > > linus kernel (e.g. 06e86849cf4019), which includes the fix you
> > > mentioned (aa0467d8d2a00e), I get still a different failure mode,
> > > which doesn't actually reference btrfs:
> > > 
> > > [  276.364178] BUG: unable to handle kernel NULL pointer dereference at 000000000000000a
> > > [  276.365127] IP: [<ffffffffa05434b1>] journal_start+0x3e/0x9c [jbd]
> > 
> > Looking at the resulting code in the oops, we're here in journal_start:
> > 
> >         if (handle) {
> >                 J_ASSERT(handle->h_transaction->t_journal == journal);
> > 
> > handle comes from current->journal_info, and we're doing a deref on
> > handle->h_transaction, which is probably 0xa.
> > 
> > So, we're leaving crud in current->journal_info and ext3 is finding it.
> > 
> > Perhaps its from ceph starting a transaction but leaving it running?
> > The bug came with Josef's transaction performance fixes, but it is
> > probably a mixture of his code with the ioctls ceph is using.
> 
> Ah, yeah, that's the problem.  We saw a similar problem a while back with 
> the start/stop transaction ioctls.  In this case, create_snapshot is doing
> 
> 	trans = btrfs_start_transaction(root->fs_info->extent_root, 5);
> 	if (IS_ERR(trans)) {
> 		ret = PTR_ERR(trans);
> 		goto fail;
> 	}
> 
> which sets current->journal_info.  Then
> 
> 	ret = btrfs_snap_reserve_metadata(trans, pending_snapshot);
> 	BUG_ON(ret);
> 
> 	list_add(&pending_snapshot->list,
> 		 &trans->transaction->pending_snapshots);
> 	if (async_transid) {
> 		*async_transid = trans->transid;
> 		ret = btrfs_commit_transaction_async(trans,
> 				     root->fs_info->extent_root, 1);
> 	} else {
> 		ret = btrfs_commit_transaction(trans,
> 				       root->fs_info->extent_root);
> 	}
> 
> but the async snap creation ioctl takes the async path, which runs 
> btrfs_commit_transaction in a worker thread.
> 
> I'm not sure what the right thing to do is here is... can whatever is in 
> journal_info be attached to trans instead in 
> btrfs_commit_transaction_async()?

It looks like it's not used for anything in btrfs, actually; it's just set 
and cleared.  What's the point of that?

Anyway, assuming it's useful, I think the below would fix the problem.  
Want to give it a shot, Jim?

Thanks!
sage


diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c571734..fd04ad7 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1196,6 +1196,9 @@ int btrfs_commit_transaction_async(struct btrfs_trans_handle *trans,
        put_transaction(cur_trans);
        mutex_unlock(&root->fs_info->trans_mutex);
 
+       if (current->journal_info == trans)
+               current->journal_info = NULL;
+
        return 0;
 }
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