Attachement 

http://bugzilla.kernel.org/attachment.cgi?id=19998&action=view

Causes the following OOPS as soon as stress-testing starts. Is it possible that bdi->unplug_io_data can be NULL in blk_backing_dev_wop ? Should we simply discard those ?

[  138.345195] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[  138.346301] IP: [<ffffffff803f997d>] elv_wait_on_page+0xd/0x20
[  138.346301] PGD 434c05067 PUD 434c06067 PMD 0
[  138.346301] Oops: 0000 [#1] PREEMPT SMP
[  138.346301] LTT NESTING LEVEL : 0
[  138.346301] last sysfs file: /sys/block/md1/md/raid_disks
[  138.346301] Dumping ftrace buffer:
[  138.346301]    (ftrace buffer empty)
[  138.346301] CPU 3
[  138.346301] Modules linked in: e1000e loop ltt_tracer ltt_trace_control ltt_type_serializer ltte
[  138.346301] Pid: 1272, comm: kjournald Not tainted 2.6.28.1 #69
[  138.346301] RIP: 0010:[<ffffffff803f997d>]  [<ffffffff803f997d>] elv_wait_on_page+0xd/0x20
[  138.346301] RSP: 0018:ffff88043cc19cd0  EFLAGS: 00010286
[  138.346301] RAX: 0000000000000000 RBX: ffff88043f460938 RCX: 0000000000000000
[  138.346301] RDX: ffff880438490000 RSI: ffffe200193f0bc0 RDI: ffff88043e580a40
[  138.346301] RBP: ffff88043cc19cd0 R08: ffff88043d09de78 R09: 0000000000000001
[  138.346301] R10: 0000000000000001 R11: 0000000000000001 R12: ffff88043cc19d50
[  138.346301] R13: ffff88043cc19d60 R14: 0000000000000002 R15: ffff8800280590c8
[  138.346301] FS:  0000000000000000(0000) GS:ffff88043f804d00(0000) knlGS:0000000000000000
[  138.346301] CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
[  138.346301] CR2: 0000000000000000 CR3: 0000000434817000 CR4: 00000000000006e0
[  138.346301] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  138.346301] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  138.346301] Process kjournald (pid: 1272, threadinfo ffff88043cc18000, task ffff88043d09d8c0)
[  138.346301] Stack:
[  138.346301]  ffff88043cc19ce0 ffffffff803fd2a2 ffff88043cc19d00 ffffffff802f6762
[  138.346301]  ffff88043cc19d60 0000000000000000 ffff88043cc19d40 ffffffff8067ace2
[  138.346301]  ffffffff802f6710 ffff880438490000 0000000000000002 0000000000000002
[  138.346301] Call Trace:
[  138.346301]  [<ffffffff803fd2a2>] blk_backing_dev_wop+0x12/0x20
[  138.346301]  [<ffffffff802f6762>] sync_buffer+0x52/0x80
[  138.346301]  [<ffffffff8067ace2>] __wait_on_bit+0x62/0x90
[  138.346301]  [<ffffffff802f6710>] ? sync_buffer+0x0/0x80
[  138.346301]  [<ffffffff802f6710>] ? sync_buffer+0x0/0x80
[  138.346301]  [<ffffffff8067ad89>] out_of_line_wait_on_bit+0x79/0x90
[  138.346301]  [<ffffffff802566f0>] ? wake_bit_function+0x0/0x50
[  138.346301]  [<ffffffff802f6649>] __wait_on_buffer+0xf9/0x130
[  138.346301]  [<ffffffff8036c0c5>] journal_commit_transaction+0x7d5/0x1540
[  138.346301]  [<ffffffff80265991>] ? trace_hardirqs_on_caller+0x1b1/0x210
[  138.346301]  [<ffffffff8067d457>] ? _spin_unlock_irqrestore+0x47/0x80
[  138.346301]  [<ffffffff80249cef>] ? try_to_del_timer_sync+0x5f/0x70
[  138.346301]  [<ffffffff803708c8>] kjournald+0xe8/0x250
[  138.346301]  [<ffffffff802566b0>] ? autoremove_wake_function+0x0/0x40
[  138.346301]  [<ffffffff803707e0>] ? kjournald+0x0/0x250
[  138.346301]  [<ffffffff802561de>] kthread+0x4e/0x90
[  138.346301]  [<ffffffff80256190>] ? kthread+0x0/0x90
[  138.346301]  [<ffffffff8020d8d9>] child_rip+0xa/0x11
[  138.346301]  [<ffffffff8020cd58>] ? restore_args+0x0/0x30
[  138.346301]  [<ffffffff80256190>] ? kthread+0x0/0x90
[  138.346301]  [<ffffffff8020d8cf>] ? child_rip+0x0/0x11