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

[Jan Kara <jack@suse.cz>]

  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

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