Chris Webb reported:
  p0# uname -a
  Linux f7ea8425-d45b-490f-a738-d181d0df6963.host.elastichosts.com 2.6.30.4-elastic-lon-p #2 SMP PREEMPT Thu Aug 20 14:30:50 BST 2009 x86_64 Intel(R) Xeon(R) CPU E5420 @ 2.50GHz GenuineIntel GNU/Linux
  p0# zgrep SCAN_ASYNC /proc/config.gz
  # CONFIG_SCSI_SCAN_ASYNC is not set

  p0# cat /var/log/kern/2009-08-20
  [...]
  15:27:10.485 kernel: scsi9 : iSCSI Initiator over TCP/IP
  15:27:11.493 kernel: scsi 9:0:0:0: RAID              IET      Controller       0001 PQ: 0 ANSI: 5
  15:27:11.493 kernel: scsi 9:0:0:0: Attached scsi generic sg6 type 12
  15:27:11.495 kernel: scsi 9:0:0:1: Direct-Access     IET      VIRTUAL-DISK     0001 PQ: 0 ANSI: 5
  15:27:11.495 kernel: sd 9:0:0:1: Attached scsi generic sg7 type 0
  15:27:11.495 kernel: sd 9:0:0:1: [sdg] 4194304 512-byte hardware sectors: (2.14 GB/2.00 GiB)
  15:27:11.495 kernel: sd 9:0:0:1: [sdg] Write Protect is off
  15:27:11.495 kernel: sd 9:0:0:1: [sdg] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
  15:27:13.012 kernel: sdg:<6>scsi 9:0:0:1: [sdg] Unhandled error code
  15:27:13.012 kernel: scsi 9:0:0:1: [sdg] Result: hostbyte=0x07 driverbyte=0x00
  15:27:13.012 kernel: end_request: I/O error, dev sdg, sector 0
  15:27:13.012 kernel: Buffer I/O error on device sdg, logical block 0
  15:27:13.012 kernel: ldm_validate_partition_table(): Disk read failed.
  15:27:13.012 kernel: unable to read partition table
  15:27:13.014 kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
  15:27:13.014 kernel: IP: [<ffffffff803f0d77>] disk_part_iter_next+0x74/0xfd
  15:27:13.014 kernel: PGD 82ad0b067 PUD 82cd7e067 PMD 0 
  15:27:13.014 kernel: Oops: 0000 [#1] PREEMPT SMP 
  15:27:13.014 kernel: last sysfs file: /sys/devices/platform/host9/session4/iscsi_session/session4/ifacename
  15:27:13.014 kernel: CPU 5 
  15:27:13.014 kernel: Modules linked in:
  15:27:13.014 kernel: Pid: 13999, comm: async/0 Not tainted 2.6.30.4-elastic-lon-p #2 X7DBN
  15:27:13.014 kernel: RIP: 0010:[<ffffffff803f0d77>]  [<ffffffff803f0d77>] disk_part_iter_next+0x74/0xfd
  15:27:13.014 kernel: RSP: 0018:ffff88066afa3dd0  EFLAGS: 00010246
  15:27:13.014 kernel: RAX: ffff88082b58a000 RBX: ffff88066afa3e00 RCX: 0000000000000000
  15:27:13.014 kernel: RDX: 0000000000000000 RSI: ffff88082b58a000 RDI: 0000000000000000
  15:27:13.014 kernel: RBP: ffff88066afa3df0 R08: ffff88066afa2000 R09: ffff8806a204f000
  15:27:13.014 kernel: R10: 000000fb12c7d274 R11: ffff8806c2bf0628 R12: ffff88066afa3e00
  15:27:13.014 kernel: R13: ffff88082c829a00 R14: 0000000000000000 R15: ffff8806bc50c920
  15:27:13.014 kernel: FS:  0000000000000000(0000) GS:ffff88002818a000(0000) knlGS:0000000000000000
  15:27:13.014 kernel: CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
  15:27:13.014 kernel: CR2: 0000000000000010 CR3: 000000082ade3000 CR4: 00000000000426e0
  15:27:13.014 kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  15:27:13.014 kernel: DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
  15:27:13.014 kernel: Process async/0 (pid: 13999, threadinfo ffff88066afa2000, task ffff8806c2bf05e0)
  15:27:13.014 kernel: Stack:
  15:27:13.014 kernel: 0000000000000000 ffff88066afa3e00 ffff88066afa3e00 ffff88082c829a00
  15:27:13.014 kernel: ffff88066afa3e40 ffffffff80306feb ffff88082b58a000 0000000000000000
  15:27:13.014 kernel: 0000000000000001 ffff8806bc50c920 ffff88066afa3e40 ffff88082b58a000
  15:27:13.014 kernel: Call Trace:
  15:27:13.014 kernel: [<ffffffff80306feb>] register_disk+0x122/0x13a
  15:27:13.014 kernel: [<ffffffff803f0b0f>] add_disk+0xaa/0x106
  15:27:13.014 kernel: [<ffffffff80493609>] sd_probe_async+0x198/0x25b
  15:27:13.014 kernel: [<ffffffff80270482>] async_thread+0x10c/0x20d
  15:27:13.014 kernel: [<ffffffff802545ff>] ? default_wake_function+0x0/0xf
  15:27:13.014 kernel: [<ffffffff80270376>] ? async_thread+0x0/0x20d
  15:27:13.014 kernel: [<ffffffff8026ad89>] kthread+0x55/0x80
  15:27:13.014 kernel: [<ffffffff8022be6a>] child_rip+0xa/0x20
  15:27:13.014 kernel: [<ffffffff8026ad34>] ? kthread+0x0/0x80
  15:27:13.014 kernel: [<ffffffff8022be60>] ? child_rip+0x0/0x20
  15:27:13.014 kernel: Code: c8 ff 80 e1 0c b9 00 00 00 00 0f 44 c1 41 83 cd ff 48 8d 7a 20 48 be ff ff ff ff 08 00 00 00 48 b9 00 00 00 00 08 00 00 00 eb 50 <8b> 42 10 41 bd 01 00 00 00 eb db 4c 63 c2 4e 8d 04 c7 4d 8b 20 
  15:27:13.015 kernel: RIP  [<ffffffff803f0d77>] disk_part_iter_next+0x74/0xfd
  15:27:13.015 kernel: RSP <ffff88066afa3dd0>
  15:27:13.015 kernel: CR2: 0000000000000010
  15:27:13.015 kernel: ---[ end trace 6104b56ef5590e25 ]---

The problem is caused because the async scanning split in sd.c doesn't hold
any reference to the device when it kicks off the async piece.  What's
happening is that an iSCSI disconnect is destorying the device again *before*
the async sd scanning thread even starts.  Fix this by taking a reference
before starting the thread and dropping it again when the thread completes.
Reported-by: Chris Webb <chris@arachsys.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>