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SubjectRe: [PATCH] [RFC] remove ext3 inode from orphan list when link and unlink race
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On Sat, 2007-01-13 at 00:02 +0300, Alex Tomas wrote:
> interesting ..
>
> I thought VFS doesn't allow concurrent operations.
> if unlink goes first, then link should wait on the
> parent's i_mutex and then found no source name.

I don't think the VFS ever takes the source's parent's i_mutex. Unless
the source and destination's parent is the same, in which case the
i_mutex is taken after the source has already been looked up.

> thanks, Alex
>
> >>>>> Eric Sandeen (ES) writes:
>
> ES> )
> ES> I've been looking at a case where many threads are opening, unlinking, and
> ES> hardlinking files on ext3 . At unmount time I see an oops, because the superblock's
> ES> orphan list points to a freed inode.
>
> ES> I did some tracing of the inodes, and it looks like this:
>
> ES> ext3_unlink():[/src/linux-2.6.18/fs/ext3/namei.c:2123] adding orphan
> ES> i_state:0x7 cpu:1 i_count:2 i_nlink:0
>
> ES> ext3_orphan_add():[/src/linux-2.6.18/fs/ext3/namei.c:1890] ext3_orphan_add
> ES> i_state:0x7 cpu:1 i_count:2 i_nlink:0
>
> ES> iput():[/src/linux-2.6.18/fs/inode.c:1139] iput enter
> ES> i_state:0x7 cpu:1 i_count:2 i_nlink:0
>
> ES> ext3_link():[/src/linux-2.6.18/fs/ext3/namei.c:2202] ext3_link enter
> ES> i_state:0x7 cpu:3 i_count:1 i_nlink:0
>
> ES> ext3_inc_count():[/src/linux-2.6.18/fs/ext3/namei.c:1627] done
> ES> i_state:0x7 cpu:3 i_count:1 i_nlink:1
>
> ES> The unlink gets there first, finds i_count > 0 (in use) but nlink goes to 0, so
> ES> it puts it on the orphan inode list. Then link comes along, and bumps the link
> ES> back up to 1. So now we are on the orphan inode list, but we are not unlinked.
>
> ES> Eventually when count goes to 0, and we still have 1 link, again no action is
> ES> taken to remove the inode from the orphan list, because it is still linked (i.e.
> ES> we don't go through ext3_delete())
>
> ES> When this inode is eventually freed, the sb orphan list gets corrupted, because
> ES> we have freed it without first removing it from the orphan list.
>
> ES> I think the simple solution is to remove the inode from the orphan list
> ES> when we bump the link back up from 0 to 1. I put that test in there because
> ES> there are other potential reasons that we might be on the list (truncates,
> ES> direct IO).
>
> ES> Comments?
>
> ES> Thanks,
> ES> -Eric
>
> ES> p.s. ext3_inc_count and ext3_dec_count seem misnamed, have an unused
> ES> arg, and are very infrequently called. I'll probably submit a patch
> ES> to just put the single line of code into the caller, too.
>
> ES> ---
>
> ES> Remove inode from the orphan list in ext3_link() if we might have
> ES> raced with ext3_unlink(), which potentially put it on the list.
> ES> If we're on the list with nlink > 0, we'll never get cleaned up
> ES> properly and eventually may corrupt the list.
>
> ES> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>
> ES> Index: linux-2.6.19/fs/ext3/namei.c
> ES> ===================================================================
> ES> --- linux-2.6.19.orig/fs/ext3/namei.c
> ES> +++ linux-2.6.19/fs/ext3/namei.c
> ES> @@ -2204,6 +2204,9 @@ retry:
> inode-> i_ctime = CURRENT_TIME_SEC;
> ES> ext3_inc_count(handle, inode);
> ES> atomic_inc(&inode->i_count);
> ES> + /* did we race w/ unlink? */
> ES> + if (inode->i_nlink == 1)
> ES> + ext3_orphan_del(handle, inode);
>
> ES> err = ext3_add_nondir(handle, dentry, inode);
> ES> ext3_journal_stop(handle);

--
David Kleikamp
IBM Linux Technology Center

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