Comment 2 for bug 543617

The reproducer works here. I use ext4 on MD (raid1) for '/' with /tmp as part of '/'. Additionally, I get the following trace in dmesg when trying to umount:
[214920.810112] INFO: task umount:16606 blocked for more than 120 seconds.
[214920.810118] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[214920.810121] umount D 00000000ffffffff 0 16606 16551 0x00000000
[214920.810128] ffff8800b27cbd18 0000000000000082 0000000000015b00 0000000000015b00
[214920.810135] ffff8800bcf15f80 ffff8800b27cbfd8 0000000000015b00 ffff8800bcf15bc0
[214920.810140] 0000000000015b00 ffff8800b27cbfd8 0000000000015b00 ffff8800bcf15f80
[214920.810146] Call Trace:
[214920.810158] [<ffffffff81162770>] ? bdi_sched_wait+0x0/0x20
[214920.810163] [<ffffffff8116277e>] bdi_sched_wait+0xe/0x20
[214920.810170] [<ffffffff815553ff>] __wait_on_bit+0x5f/0x90
[214920.810174] [<ffffffff81162770>] ? bdi_sched_wait+0x0/0x20
[214920.810181] [<ffffffff815554a8>] out_of_line_wait_on_bit+0x78/0x90
[214920.810187] [<ffffffff810835c0>] ? wake_bit_function+0x0/0x40
[214920.810192] [<ffffffff81162734>] ? bdi_queue_work+0xa4/0xe0
[214920.810196] [<ffffffff81163aaf>] bdi_sync_writeback+0x6f/0x80
[214920.810201] [<ffffffff81163ae0>] sync_inodes_sb+0x20/0x30
[214920.810205] [<ffffffff811675c2>] __sync_filesystem+0x82/0x90
[214920.810209] [<ffffffff811677cb>] sync_filesystem+0x4b/0x70
[214920.810214] [<ffffffff811422c7>] generic_shutdown_super+0x27/0x100
[214920.810218] [<ffffffff811423d1>] kill_block_super+0x31/0x50
[214920.810222] [<ffffffff811434a1>] deactivate_super+0x71/0x90
[214920.810227] [<ffffffff8115bc1f>] mntput_no_expire+0xbf/0x110
[214920.810231] [<ffffffff8115c357>] sys_umount+0x57/0xc0
[214920.810236] [<ffffffff810131f2>] system_call_fastpath+0x16/0x1b