Re: [BUG] ext3: cannot unfreeze a filesystem due to a deadlock

Greg Freemyer
2011-09-09T03:06:25Z

On Wed, Sep 7, 2011 at 6:32 PM, Jan Kara <jack@suse.cz> wrote:
>
> On Wed 07-09-11 13:56:08, Greg Freemyer wrote:
> > On Wed, Sep 7, 2011 at 1:34 PM, Jan Kara <jack@suse.cz> wrote:
> > >  Hello,
> > >
> > >  Thanks for report!
> > >
> > > On Wed 07-09-11 12:29:30, Masayoshi MIZUMA wrote:
> > >> When I checked the freeze feature for ext3 filesystem using fsfreeze
> > >> command at 3.1.0-rc4, I think the following deadlock problem happened.
> > >>
> > >> How to reproduce:
> > >>  # mkfs -t ext3 /dev/sdd1
> > >>  # mount /dev/sdd1 /MNT
> > >>  # ./fsstress -d /MNT/tmp -n 10 -p 1000 > /dev/null 2>&1 &
> > >>  # fsfreeze -f /MNT
> > >>  # fsfreeze -u /MNT
> > >>
> > >>  If this deadlock is reproduced, "fsfreeze -u /MNT" does not return.
> > >>
> > >> The detail of deadlock:
> > >> o [flush-8:16:1523]
> > >>   wb_do_writeback
> > >>    wb_writeback
> > >>    ...
> > >>      ext3_journalled_writepage
> > >>       journal_start
> > >>        start_this_handle
> > >>        # waiting until journal->j_barrier_count turns 0...
> > >>        # j_barrier_count was incremented by journal_lock_updates()
> > >>        # via ext3_freeze().
> > >>
> > >> o [fsstress:2673]
> > >>   sys_sync
> > >>    sync_filesystems
> > >>     iterate_supers
> > >>      down_read(sb->s_umount)
> > >>      sync_one_sb
> > >>       __sync_filesystem
> > >>        writeback_inodes_sb
> > >>         writeback_inodes_sb_nr
> > >>          wait_for_completion
> > >>           wait_for_common
> > >>           # waiting for completion of [flush-8:16:1523]...
> > >>
> > >> o [fsfreeze:2749]
> > >>   sys_ioctl
> > >>    do_vfs_ioctl
> > >>     thaw_super
> > >>     # waiting for down_write(sb->s_umount)...
> > >>     # [fsfreeze:2673] did down_read(sb->s_umount).
> > >  Yes, this is a classical deadlock that can happen for any filesystem. The
> > > problem is flusher thread holds s_umount semaphore (either directly, or as
> > > in your case, indirectly via blocked sync) and tries to do some IO which
> > > blocks on frozen filesystem. It's particularly easy to hit for ext3 because
> > > it doesn't do vfs_check_frozen() checks but all other filesystems have the
> > > race window as well. Val Henson is working on fixing the problem - she even
> > > has some first version of patches I believe.
> > >
> > >                                                                Honza
> >
> > xfstests test 068 has been around since kernel 2.4 days and should
> > have caught it if xfs is impacted.
> >
> > I know I ran the 2002 version many times to prove to myself that
> > fsfreeze for xfs was stable when teamed with LVM.  (It wasn't when I
> > first wrote 068 way back then).
> >
> > 068 has been greatly simplified since 2002, but it still looks like it
> > should do a good job.
> >
> > Is there a problem with 068?  Does it need extra test coverage even for xfs?
>  I believe at least mmapped writes can trigger the deadlock even for xfs
> and fsstress (slightly surprisingly) does not test that. It's a narrow race
> window but it is there and it has been triggered in practice (for ext4 but
> it's a race in VFS code used by both XFS and ext4). So maybe extending
> fsstress would be a way to go?
>
>                                                                Honza

That's a surprisingly large hole in xfstests.

That sounds like a pretty core and significant change. I'll have to
leave that to one of the main developers.

Greg
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