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Date: Fri, 10 Jun 2011 11:35:53 -0700 (PDT)
From: Sage Weil <sage@newdream.net>
To: Josef Bacik <josef@redhat.com>
cc: Chris Mason <chris.mason@oracle.com>, Jim Schutt <jaschut@sandia.gov>,
        dave <dave@jikos.cz>, miaox <miaox@cn.fujitsu.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>, ceph-devel@vger.kernel.org
Subject: Re: 3.0-rcX BUG at fs/btrfs/ioctl.c:432 - bisected
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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
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