Hello!

I have exactly the same problem. 
The Server hangs on iscsiadm logouts.
My ISCSI Target ist a HP P4500

Here is my stack trace:

[ 2360.832232] sd 3:0:0:0: [sdb] 20971520 512-byte logical blocks: (10.7 GB/10.0 GiB)
[ 2360.833438] sd 3:0:0:0: [sdb] Write Protect is off
[ 2360.833445] sd 3:0:0:0: [sdb] Mode Sense: 77 00 00 08
[ 2360.834112] sd 3:0:0:0: [sdb] Write cache: disabled, read cache: disabled, doesn't support DPO or FUA
[ 2360.850500]  sdb: sdb1 sdb2 < sdb5 >
[ 2360.852989] sd 3:0:0:0: [sdb] Attached SCSI disk
[ 2403.635452] BUG: soft lockup - CPU#6 stuck for 22s! [kworker/u:0:5]
[ 2403.635805] Modules linked in: ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi usbhid hid veth ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack ipt_REJECT xt_CHECKSUM iptable_mangle xt_tcpudp iptable_filter ip_tables x_tables openvswitch_mod(O) bridge stp kvm_intel kvm nbd vesafb bonding dm_multipath psmouse serio_raw sb_edac edac_core hpilo ioatdma dca acpi_power_meter mac_hid lp parport hpsa tg3 [last unloaded: scsi_transport_iscsi]
[ 2403.635841] CPU 6 
[ 2403.635842] Modules linked in: ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi usbhid hid veth ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack ipt_REJECT xt_CHECKSUM iptable_mangle xt_tcpudp iptable_filter ip_tables x_tables openvswitch_mod(O) bridge stp kvm_intel kvm nbd vesafb bonding dm_multipath psmouse serio_raw sb_edac edac_core hpilo ioatdma dca acpi_power_meter mac_hid lp parport hpsa tg3 [last unloaded: scsi_transport_iscsi]
[ 2403.635864] 
[ 2403.635866] Pid: 5, comm: kworker/u:0 Tainted: G        W  O 3.2.0-31-generic #50-Ubuntu HP ProLiant DL360p Gen8
[ 2403.635869] RIP: 0010:[<ffffffff8165a70f>]  [<ffffffff8165a70f>] _raw_spin_unlock_irqrestore+0x1f/0x30
[ 2403.635878] RSP: 0018:ffff88080c0e5da0  EFLAGS: 00000286
[ 2403.635880] RAX: 0000000000000286 RBX: 0000000000000286 RCX: 0000000000000001
[ 2403.635881] RDX: ffff880fffbc3c08 RSI: 0000000000000286 RDI: 0000000000000286
[ 2403.635883] RBP: ffff88080c0e5da0 R08: ffffffff81cddaa0 R09: 0000000000000100
[ 2403.635884] R10: 0000000000000040 R11: 0000000000000000 R12: 0000000000000001
[ 2403.635885] R13: 0000000000000001 R14: ffffffff81cddaa0 R15: 0000000000000100
[ 2403.635887] FS:  0000000000000000(0000) GS:ffff88101f200000(0000) knlGS:0000000000000000
[ 2403.635889] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 2403.635890] CR2: 00007f43c5c87cd0 CR3: 0000000001c05000 CR4: 00000000000426e0
[ 2403.635892] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2403.635894] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 2403.635896] Process kworker/u:0 (pid: 5, threadinfo ffff88080c0e4000, task ffff88080c0c5c00)
[ 2403.635897] Stack:
[ 2403.635992]  ffff88080c0e5dd0 ffffffff8142f3b4 ffff881001bff130 ffff881001bff000
[ 2403.635996]  ffff881001bff018 0000000000000000 ffff88080c0e5e00 ffffffffa0106fec
[ 2403.635999]  ffff881001bff080 ffff88080c0a1100 ffff880fffd8ae00 ffffffff81e534c0
[ 2403.636003] Call Trace:
[ 2403.636128]  [<ffffffff8142f3b4>] scsi_remove_target+0xb4/0xe0
[ 2403.636136]  [<ffffffffa0106fec>] __iscsi_unbind_session+0xbc/0x190 [scsi_transport_iscsi]
[ 2403.636144]  [<ffffffff81084a6a>] process_one_work+0x11a/0x480
[ 2403.636148]  [<ffffffff81085814>] worker_thread+0x164/0x370
[ 2403.636151]  [<ffffffff810856b0>] ? manage_workers.isra.30+0x130/0x130
[ 2403.636154]  [<ffffffff8108a03c>] kthread+0x8c/0xa0
[ 2403.636159]  [<ffffffff81664c74>] kernel_thread_helper+0x4/0x10
[ 2403.636162]  [<ffffffff81089fb0>] ? flush_kthread_worker+0xa0/0xa0
[ 2403.636164]  [<ffffffff81664c70>] ? gs_change+0x13/0x13
[ 2403.636165] Code: 65 f8 c9 c3 0f 1f 84 00 00 00 00 00 55 48 89 e5 53 66 66 66 66 90 48 89 f3 e8 9e 35 9e ff 66 90 48 89 df 57 9d 66 66 90 66 90 5b <5d> c3 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 
[ 2403.637472] Call Trace:
[ 2403.637474]  [<ffffffff8142f3b4>] scsi_remove_target+0xb4/0xe0
[ 2403.637485]  [<ffffffffa0106fec>] __iscsi_unbind_session+0xbc/0x190 [scsi_transport_iscsi]
[ 2403.637491]  [<ffffffff81084a6a>] process_one_work+0x11a/0x480
[ 2403.637494]  [<ffffffff81085814>] worker_thread+0x164/0x370
[ 2403.637496]  [<ffffffff810856b0>] ? manage_workers.isra.30+0x130/0x130
[ 2403.637499]  [<ffffffff8108a03c>] kthread+0x8c/0xa0
[ 2403.637501]  [<ffffffff81664c74>] kernel_thread_helper+0x4/0x10
[ 2403.637504]  [<ffffffff81089fb0>] ? flush_kthread_worker+0xa0/0xa0
[ 2403.637506]  [<ffffffff81664c70>] ? gs_change+0x13/0x13

as reported the problem does not exist with kernel -29.

Thank you!