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

Sage Weil
2011-06-10T18:33:54Z

On Fri, 10 Jun 2011, Josef Bacik wrote:
> On 06/10/2011 02:14 PM, Sage Weil wrote:
> > 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?
> >
>
> It is used now, check the beginning of start_transaction(). Thanks,

Oh I see, okay.

So clearing it in btrfs_commit_transaction_async should be fine then,
right? When btrfs_commit_transaction runs in the other thread it won't
care that current->journal_info is NULL.

sage