On Thu, Mar 22, 2012 at 11:13:17AM +0800, Miao Xie wrote:

The reason the deadlock is that:
  Task					Btrfs-cleaner
  umount()
    down_write(&s->s_umount)
    close_ctree()
      wait for the end of
      btrfs-cleaner
					start_transaction
					  reserve space
					    shrink_delalloc()
					      writeback_inodes_sb_nr_if_idle()
						down_read(&sb->s_umount)
So, the deadlock has happened.

Every time a deadlock involving writeback_inodes_sb...if_idle()
comes up, I give the same response. If the s_umount is write locked,
then the sb is not idle. IOWs, writeback_inodes_sb...if_idle()
should be doing down_read_trylock(), not down_read().

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com