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

Josef Bacik
2011-06-10T18:29:40Z

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,

Josef