Comment 2 for bug 620242

Also on observed on one of my machines.

  - Linux RAID10 (f2)
  - SATA disks
  - Ext4
  - KVM
  - Multi gigabyte files (qcow2 images)
  - Multiple frozen, unkillable 'lvcreate --snapshot' commands

[1360167.174975] INFO: task flush-251:4:479 blocked for more than 120 seconds.
[1360167.187216] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[1360167.211359] flush-251:4 D ffff88031d2e33d8 0 479 2 0x00000000
[1360167.211363] ffff88031f7c9bb0 0000000000000046 0000000000015bc0 0000000000015bc0
[1360167.211366] ffff880322589ab0 ffff88031f7c9fd8 0000000000015bc0 ffff8803225896f0
[1360167.211369] 0000000000015bc0 ffff88031f7c9fd8 0000000000015bc0 ffff880322589ab0
[1360167.211371] Call Trace:
[1360167.211377] [<ffffffff811f632d>] ext4_force_commit+0x8d/0xe0
[1360167.211381] [<ffffffff81085090>] ? autoremove_wake_function+0x0/0x40
[1360167.211385] [<ffffffff811de405>] ext4_write_inode+0x75/0x110
[1360167.211388] [<ffffffff81166a64>] writeback_single_inode+0x2b4/0x3d0
[1360167.211390] [<ffffffff8116750e>] writeback_inodes_wb+0x40e/0x5e0
[1360167.211393] [<ffffffff810586b0>] ? finish_task_switch+0x50/0xe0
[1360167.211395] [<ffffffff811677ea>] wb_writeback+0x10a/0x1d0
[1360167.211398] [<ffffffff81077719>] ? try_to_del_timer_sync+0x79/0xd0
[1360167.211402] [<ffffffff81558d9d>] ? schedule_timeout+0x19d/0x300
[1360167.211404] [<ffffffff81167abd>] wb_do_writeback+0x12d/0x1a0
[1360167.211406] [<ffffffff81167b83>] bdi_writeback_task+0x53/0xe0
[1360167.211409] [<ffffffff8110f546>] bdi_start_fn+0x86/0x100
[1360167.211411] [<ffffffff8110f4c0>] ? bdi_start_fn+0x0/0x100
[1360167.211413] [<ffffffff81084d16>] kthread+0x96/0xa0
[1360167.211416] [<ffffffff810141ea>] child_rip+0xa/0x20
[1360167.211418] [<ffffffff81084c80>] ? kthread+0x0/0xa0
[1360167.211419] [<ffffffff810141e0>] ? child_rip+0x0/0x20