On Mon 02-05-11 09:22:04, Christoph Hellwig wrote:

On Mon, May 02, 2011 at 03:16:19PM +0200, Jan Kara wrote:

quoted

Dave, Christoph, any opinions on this?

The busyloop in xfs_quiesce_attr which waits for all active transactions
to finish is supposed to fix this issue.

  Hmm, but what prevents the following race?

  Thread 1					Thread 2
..
xfs_trans_alloc()
  xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
						freeze_super()
						  ...
						  xfs_fs_freeze()
						    ...
						    xfs_quiesce_attr()
						    ...
  _xfs_trans_alloc()
    atomic_inc(&mp->m_active_trans);
    ... goes on modifying the filesystem

  It seems to be a similar problem as in ext4 - the atomic_inc() and
vfs_check_frozen() are in the wrong order...

Note that XFS traditionally expects a two stage freeze process where
we first freeze new VFS-level writes, then flush the caches and then
stop transactions, wait for them to finish and do the remainder of
the freeze process, but I really messed that process up when moving
the sequence to generic code.  Funnily enough it seems to work
neverless.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR