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

Josef Bacik
2011-06-10T18:34:36Z

On 06/10/2011 02:35 PM, Sage Weil wrote:
> 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.
>

Oh yeah your patch is good :),

Josef