Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

[Jan Kara <jack@suse.cz>]

On Mon 02-05-11 10:41:55, Christoph Hellwig wrote:
> On Mon, May 02, 2011 at 04:20:55PM +0200, Jan Kara wrote:
> >   Hmm, but what prevents the following race?
> > 
> >   Thread 1					Thread 2
> > ..
> > xfs_trans_alloc()
> >   xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
> > 						freeze_super()
> 
>                                                   sb->s_frozen = SB_FREEZE_TRANS;
> > 						  ...
> > 						  xfs_fs_freeze()
> > 						    ...
> > 						    xfs_quiesce_attr()
> 
> 						      waits for all active
> 						      transactions
> 
> > 						    ...
> 
>    xfs_trans_alloc
>      -> blocks in xfs_wait_for_freeze
  But why should it block when xfs_wait_for_freeze() gets called before
freeze_super() gets called? The other thread calls freeze_super() just
after xfs_wait_for_freeze() in thread 1 and before _xfs_trans_alloc() gets
called.  Or am I missing some serialization somewhere?

>      (thus doesn't get to _xfs_trans_alloc)
> 
> >   _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...
> 
> I can't see the problem in this scheme.  Note that we want
> _xfs_trans_alloc to be able to create a transaction for
> xfs_fs_log_dummy, so that we can write the dummy log record after
> freezing out all other transactions, so that one is special cased
> and doesn't do the xfs_wait_for_freeze.
  OK.

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