Date:	Fri, 10 Jun 2011 14:34:21 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Sage Weil <sage@...dream.net>
CC:	Chris Mason <chris.mason@...cle.com>,
	Jim Schutt <jaschut@...dia.gov>, dave <dave@...os.cz>,
	miaox <miaox@...fujitsu.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	ceph-devel@...r.kernel.org
Subject: Re: 3.0-rcX BUG at fs/btrfs/ioctl.c:432 - bisected

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
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