through my testing i've noticed that umount will hang when i'm rebooting the 
box.  Usually i've severely screwed the box (like making gfs2 oops) before 
such reboots so I've ignored it, but this time I'm getting it and I didn't do 
anything wrong.  I've captured sysrq from the box when it happens

SysRq : Show Blocked State

                         free                        sibling
  task             PC    stack   pid father child younger older
umount.gfs2   D C797DE8C  1980  4092   4091                     (NOTLB)
       c797dea0 00000082 00000002 c797de8c c797de88 00000000 c043c154 00000046 
       00000046 c0427f2c 00000007 c7eff560 c06b1440 635085a4 00005b39 000bb253 
       c7eff684 c1204460 00000000 ceb2ec80 c07a9c00 00000286 ffffffff 00000000 
Call Trace:
 [<c043c154>] tick_handle_periodic+0x17/0x67
 [<c0427f2c>] profile_tick+0x43/0x5e
 [<c060f67e>] schedule_timeout+0x70/0x8d
 [<c042e5b6>] process_timeout+0x0/0x5
 [<c042edf2>] msleep+0xd/0x12
 [<d0a9b29c>] gfs2_gl_hash_clear+0x110/0x11d [gfs2]
 [<d0aa88da>] gfs2_put_super+0x1b7/0x1d7 [gfs2]
 [<c047008f>] generic_shutdown_super+0x52/0xb7
 [<c0470111>] kill_block_super+0x1d/0x2d
 [<c04701b9>] deactivate_super+0x52/0x65
 [<c0480ed8>] sys_umount+0x1f2/0x21a
 [<c044e5cb>] audit_syscall_entry+0x111/0x143
 [<c0480f17>] sys_oldumount+0x17/0x1a
 [<c0404e4c>] syscall_call+0x7/0xb
 =======================

and it stays like this.  Once i've tracked down the delete problem I'm working 
on I'll go back to this one.