Xen guest (DomU) kernel panicked after a failed attempt to detach a disk and a `fdisk` was performed on guest.

Issue

• Xen guest (DomU) kernel panicked after a failed attempt to detach a disk and a fdisk was performed on guest with the following RIP:

Unable to handle kernel NULL pointer dereference at 0000000000000010 RIP: 
 [<ffffffff88076016>] :xenblk:blkif_release+0x2d/0x53
PGD cadcb067 PUD 69749067 PMD 0 
Oops: 0000 [1] SMP 
last sysfs file: /block/ram0/dev
CPU 0 
Modules linked in: oracleasm(U) lockd sunrpc ib_iser libiscsi2 scsi_transport_iscsi2 scsi_transport_iscsi ib_srp rds ib_sdp ib_ipoib ipoib_helper ipv6 xfrm_nalgo crypto_api rdma_ucm rdma_cm ib_ucm ib_uverbs ib_umad ib_cm iw_cm ib_addr ib_sa ib_mad ib_core dm_multipath scsi_dh scsi_mod parport_pc lp parport pcspkr xennet dm_raid45 dm_message dm_region_hash dm_mem_cache dm_snapshot dm_zero dm_mirror dm_log dm_mod xenblk ext3 jbd uhci_hcd ohci_hcd ehci_hcd
Pid: 9612, comm: fdisk Tainted: G     ---- 2.6.18-308.el5xen #1
RIP: e030:[<ffffffff88076016>]  [<ffffffff88076016>] :xenblk:blkif_release+0x2d/0x53
..
Process fdisk (pid: 9612, threadinfo ffff8800b70d0000, task ffff8800fd7fd860)
..
Call Trace:
 [<ffffffff802da05c>] __blkdev_put+0x6d/0x169
 [<ffffffff80213580>] __fput+0xd3/0x1bd
 [<ffffffff80224425>] filp_close+0x5c/0x64
 [<ffffffff8021e577>] sys_close+0x88/0xbd
 [<ffffffff8025f2f9>] tracesys+0xab/0xb6
...
 <0>Kernel panic - not syncing: Fatal exception