Re: [PATCH] Btrfs: fix infinite loop during fsync after rename operations

[Josef Bacik <josef@toxicpanda.com>]

On 1/15/20 8:21 AM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Recently fsstress (from fstests) sporadically started to trigger an
> infinite loop during fsync operations. This turned out to be because
> support for the rename exchange and whiteout operations was added to
> fsstress in fstests. These operations, unlike any others in fsstress,
> cause file names to be reused, whence triggering this issue. However
> it's not necessary to use rename exchange and rename whiteout operations
> trigger this issue, simple rename operations and file creations are
> enough to trigger the issue.
> 
> The issue boils down to when we are logging inodes that conflict (that
> had the name of any inode we need to log during the fsync operation),
> we keep logging them even if they were already logged before, and after
> that we check if there's any other inode that conflicts with them and
> then add it again to the list of inodes to log. Skipping already logged
> inodes fixes the issue.
> 
> Consider the following example:
> 
>    $ mkfs.btrfs -f /dev/sdb
>    $ mount /dev/sdb /mnt
> 
>    $ mkdir /mnt/testdir                           # inode 257
> 
>    $ touch /mnt/testdir/zz                        # inode 258
>    $ ln /mnt/testdir/zz /mnt/testdir/zz_link
> 
>    $ touch /mnt/testdir/a                         # inode 259
> 
>    $ sync
> 
>    # The following 3 renames achieve the same result as a rename exchange
>    # operation (<rename_exchange> /mnt/testdir/zz_link to /mnt/testdir/a).
> 
>    $ mv /mnt/testdir/a /mnt/testdir/a/tmp
>    $ mv /mnt/testdir/zz_link /mnt/testdir/a
>    $ mv /mnt/testdir/a/tmp /mnt/testdir/zz_link
> 
>    # The following rename and file creation give the same result as a
>    # rename whiteout operation (<rename_whiteout> zz to a2).
> 
>    $ mv /mnt/testdir/zz /mnt/testdir/a2
>    $ touch /mnt/testdir/zz                        # inode 260
> 
>    $ xfs_io -c fsync /mnt/testdir/zz
>      --> results in the infinite loop
> 
> The following steps happen:
> 
> 1) When logging inode 260, we find that its reference named "zz" was
>     used by inode 258 in the previous transaction (through the commit
>     root), so inode 258 is added to the list of conflicting indoes that
>     need to be logged;
> 
> 2) After logging inode 258, we find that its reference named "a" was
>     used by inode 259 in the previous transaction, and therefore we add
>     inode 259 to the list of conflicting inodes to be logged;
> 
> 3) After logging inode 259, we find that its reference named "zz_link"
>     was used by inode 258 in the previous transaction - we add inode 258
>     to the list of conflicting inodes to log, again - we had already
>     logged it before at step 3. After logging it again, we find again
>     that inode 259 conflicts with him, and we add again 259 to the list,
>     etc - we end up repeating all the previous steps.
> 
> So fix this by skipping logging of conflicting inodes that were already
> logged.
> 
> Fixes: 6b5fc433a7ad67 ("Btrfs: fix fsync after succession of renames of different files")
> CC: stable@vger.kernel.org
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef