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 [off-list ref]
SUSE Labs, CR