Re: [RFC][PATCH] ext2: Resolve i_nlink race in ext2_rename

From: Josh Hunt
Date: Thu Feb 24 2011 - 15:18:40 EST

This is a multi-part message in MIME format.On 02/24/2011 03:20 AM, Jan Kara wrote:
> On Thu 24-02-11 06:37:49, Al Viro wrote:
>> On Wed, Feb 23, 2011 at 10:21:41PM -0800, Josh Hunt wrote:
>>> [resending: left Jan off the original mail by accident]
>>> We have a multi-threaded workload which is currently "losing" files in the form
>>> of unattached inodes. The workload is link, rename, unlink intensive. This is
>>> happening on an ext2 filesystem and have reproduced the issue in kernel
>>> 2.6.37. Here's a sample strace:
>>> open("/a/tmp/tmpfile.1296184058", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 9
>>> link("/a/tmp/tmpfile.1296184058", "/a/tmp/tmpfile.28117.1296184059") = 0
>>> rename("/a/tmp/tmpfile.28117.1296184059", "/a/tmp/tmpfile") = 0
>>> stat64("/a/tmp/tmpfile", {st_mode=S_IFREG|0644, st_size=24248267, ...}) = 0
>>> link("/a/tmp/tmpfile", "/a/tmp/submit/tmpfile") = 0
>>> open("/a/tmp/tmpfile.1296184058", O_RDONLY) = 13
>>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDWR|O_CREAT|O_EXCL, 0600) = 824
>>> rename("/a/tmp/submit/tmpfile", "/a/tmp/submit/tmpfile.send.q9SNoL") = 0
>>> unlink("/a/tmp/tmpfile.1296184058") = 0
>>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 827
>>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 828
>>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 829
>>> unlink("/a/tmp/submit/tmpfile.send.q9SNoL") = 0
>>> The application behavior shown above repeats indefinitely with most filenames
>>> changing during each iteration except for 'tmpfile'. Looking into this issue I
>>> see that vfs_rename_other() only takes i_mutex for the new inode and the new
>>> inode's directory as well as the old directory's mutex. This works for
>>> modifying the dir entry and appears to be fine for most filesystems, but
>>> ext2 and a few others (exofs, minix, nilfs2, omfs, sysv, ufs) modify i_nlink
>>> inside of their respective rename functions without grabbing the i_mutex. The
>>> modifications are done through calls to inode_inc_link_count(old_inode) and
>>> inode_dec_link_count(old_inode), etc.
>>> Taking the mutex for the old inode appears to resolve the issue of the
>>> lost files/unattached inodes that I am seeing with this workload. I've attached
>>> a patch below doing what I've described above. If this is an accepted solution
>>> I believe other filesystems may also be affected by this and I could provide
>>> a patch for those as well.
>> I don't know... The thing is, we mostly do that to make life easier for
>> fsck in case of crash. Other than that, there's no reason to play with
>> link count of that sucker at all. The question is, do we really want
>> such rename() interrupted by dirty shutdown to result in what looks like two
>> legitimate links to that inode without any indications of what had happened?
>> Note that fsck (at least on ext2) will correct link counts anyway and if
>> nothing else, we probably want some noise pointing to the inode in question...
> Yeah, I agree that playing with the link count is not worth it. It is
> even more disputable because it would have some reasonable effect only if
> we happened to write out the moved inode after it is linked to the new
> directory and before it is unlinked from the old one. Moreover we'd need
> to writeout the new directory and not the old directory before crash
> happens. All this is highly unlikely and even if that happens, it is
> questionable whether the result is worth it. So I'll just do away with
> those games with link count...
> The patch is attached. Josh, can you test it as well? Thanks.
> Honza

I'm not seeing the problem with your patch as was expected since we're
not messing with i_nlink anymore. Al suggested marking the inode as
dirty where we were previously doing the old_inode dec. I believe this
is needed as well since we are updating it's ctime. I've attached a
version marking the inode dirty and it also fixes the comment making
reference to calling inode_dec_link_count().

I'm not completely clear on the historical reasons for messing with the
link count of old_inode in the first place. It was just to simulate the
linking and unlinking of the old_inode?