Re: [PATCH] [RFC] remove ext3 inode from orphan list when link and unlink race

[Alex Tomas <alex@clusterfs.com>]

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.

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);


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