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