Re: [PATCH 0/4] Fix filesystem freezing

Jan Kara
2012-01-12T11:30:36Z

On Thu 12-01-12 13:48:41, Dave Chinner wrote:
> On Thu, Jan 12, 2012 at 02:20:49AM +0100, Jan Kara wrote:
> >
> > Hello,
> >
> > filesystem freezing is currently racy and thus we can end up with dirty data
> > on frozen filesystem (see changelog of the first patch for detailed race
> > description and proposed fix). This patch series aims at fixing this.
>
> It only fixes the dirty data race (i.e. SB_FREEZE_WRITE). The same
> race conditions exist for SB_FREEZE_TRANS on XFS, and so need the
> same fix. That race has had one previous attempt at fixing it in
> XFS but that's not possible:
>
> b2ce397 Revert "xfs: fix filesystsem freeze race in xfs_trans_alloc"
> 7a249cf xfs: fix filesystsem freeze race in xfs_trans_alloc
>
> It was looking at that problem earlier today that lead to the
> solution Eric proposed. Essentially the method in these patches
> needs to replace the xfs specifc m_active_trans counter and delay
> during ->fs_freeze to prevent that race condition....
OK, I see. I just checked ext4 to make sure and ext4 seems to get this
right. Looking into Christoph's original patch it shouldn't be hard to fix
it. Instead of:
atomic_inc(&mp->m_active_trans);

if (wait_for_freeze)
xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);

we just need to do a bit more elaborate

retry:
if (wait_for_freeze)
xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
atomic_inc(&mp->m_active_trans);
if (wait_for_freeze && mp->m_super->s_frozen >= SB_FREEZE_TRANS) {
atomic_dec(&mp->m_active_trans);
goto retry;
}

Or does XFS support nested transactions (i.e. a thread already holding a
running transaction can call into xfs_trans_alloc() again)?
That would make things more complicated...

Using sb_start_write() instead of m_active_trans won't be that easy because
it can create A-A deadlocks (e.g. we do sb_start_write in
block_page_mkwrite() and then xfs_get_blocks() decides to start a
transaction and calls sb_start_write() again which might block if
filesystem freezing started in the mean time).

So it's up to XFS maintainers to decide what's best but I'd take
Christoph's patch with above fixup. I guess I'll put it in this series and
see what people say.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR