Why system hang up after running command, echo "- - -" > /sys/class/scsi_host/hostN/scan ?

Issue

• The system hang up after running below command.

 # echo "- - -" > /sys/class/scsi_host/hostN/scan

• The /var/log/messages contains following call trace:

May 20 00:09:42 hostname1 kernel: CPU 5:
May 20 00:09:42 hostname1 kernel: Modules linked in: dell_rbu iptable_nat ip_nat ip_conntrack nfnetlink iptable_mangle iptable_filter ip_tables x_tables mptctl mptbase ipmi_devintf ipmi_si ipmi_msghandler oracleasm(U) nfs fscache nfs_acl lockd sunrpc bonding ipv6 xfrm_nalgo crypto_api dm_multipath scsi_dh sddlmfdrv(PU) sddlmadrv(PU) video hwmon backlight sbs i2c_ec i2c_core button battery asus_acpi acpi_memhotplug ac parport_pc lp parport joydev sr_mod sg igb usb_storage 8021q bnx2 ide_cd pcspkr serio_raw cdrom dm_raid45 dm_message dm_region_hash dm_mem_cache dm_snapshot dm_zero dm_mirror dm_log dm_mod lpfc scsi_transport_fc ata_piix libata shpchp megaraid_sas sd_mod scsi_mod ext3 jbd uhci_hcd ohci_hcd ehci_hcd
May 20 00:09:42 hostname1 kernel: Pid: 14645, comm: bash Tainted: PF  M  2.6.18-164.el5 #1
May 20 00:09:42 hostname1 kernel: RIP: 0010:[<ffffffff88122016>]  [<ffffffff88122016>] :scsi_transport_fc:fc_user_scan+0x6a/0x8b
May 20 00:09:42 hostname1 kernel: RSP: 0000:ffff811b9730be38  EFLAGS: 00000282
May 20 00:09:42 hostname1 kernel: RAX: ffff81203c756850 RBX: 00000000ffffffff RCX: 00000000000000d0
May 20 00:09:42 hostname1 kernel: RDX: 0000000000000000 RSI: ffffffff8014e00f RDI: ffff81203d993060
May 20 00:09:42 hostname1 kernel: RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000028
May 20 00:09:42 hostname1 kernel: R10: ffffffff803e8260 R11: 0000000000000000 R12: ffff810100000010
May 20 00:09:42 hostname1 kernel: R13: 00000000000000a0 R14: ffff811b9730be68 R15: 0000003000000028
May 20 00:09:42 hostname1 kernel: FS:  00002b1514aa2dd0(0000) GS:ffff810170fa3540(0000) knlGS:0000000000000000
May 20 00:09:42 hostname1 kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
May 20 00:09:42 hostname1 kernel: CR2: 0000000000b76b70 CR3: 0000000afa771000 CR4: 00000000000006e0
May 20 00:09:42 hostname1 kernel:
May 20 00:09:42 hostname1 kernel: Call Trace:
May 20 00:09:42 hostname1 kernel:  [<ffffffff88122007>] :scsi_transport_fc:fc_user_scan+0x5b/0x8b
May 20 00:09:42 hostname1 kernel:  [<ffffffff8807ddb4>] :scsi_mod:store_scan+0x9b/0xc5
May 20 00:09:42 hostname1 kernel:  [<ffffffff8010a75d>] sysfs_write_file+0xb9/0xe8
May 20 00:09:42 hostname1 kernel:  [<ffffffff80016927>] vfs_write+0xce/0x174
May 20 00:09:42 hostname1 kernel:  [<ffffffff800171df>] sys_write+0x45/0x6e
May 20 00:09:42 hostname1 kernel:  [<ffffffff8005d28d>] tracesys+0xd5/0xe0
May 20 00:09:42 hostname1 kernel:
May 20 00:09:52 hostname1 kernel: BUG: soft lockup - CPU#5 stuck for 10s! [bash:14645]
May 20 00:09:52 hostname1 kernel: CPU 5:
May 20 00:09:52 hostname1 kernel: Modules linked in: dell_rbu iptable_nat ip_nat ip_conntrack nfnetlink iptable_mangle iptable_filter ip_tables x_tables mptctl mptbase ipmi_devintf ipmi_si ipmi_msghandler oracleasm(U) nfs fscache nfs_acl lockd sunrpc bonding ipv6 xfrm_nalgo crypto_api dm_multipath scsi_dh sddlmfdrv(PU) sddlmadrv(PU) video hwmon backlight sbs i2c_ec i2c_core button battery asus_acpi acpi_memhotplug ac parport_pc lp parport joydev sr_mod sg igb usb_storage 8021q bnx2 ide_cd pcspkr serio_raw cdrom dm_raid45 dm_message dm_region_hash dm_mem_cache dm_snapshot dm_zero dm_mirror dm_log dm_mod lpfc scsi_transport_fc ata_piix libata shpchp megaraid_sas sd_mod scsi_mod ext3 jbd uhci_hcd ohci_hcd ehci_hcd
May 20 00:09:52 hostname1 kernel: Pid: 14645, comm: bash Tainted: PF  M  2.6.18-164.el5 #1
May 20 00:09:52 hostname1 kernel: RIP: 0010:[<ffffffff8812202a>]  [<ffffffff8812202a>] :scsi_transport_fc:fc_user_scan+0x7e/0x8b
May 20 00:09:52 hostname1 kernel: RSP: 0000:ffff811b9730be38  EFLAGS: 00000297
May 20 00:09:52 hostname1 kernel: RAX: ffff81203db1aad8 RBX: 00000000ffffffff RCX: 00000000000000d0
May 20 00:09:52 hostname1 kernel: RDX: ffff81203c756850 RSI: ffffffff8014e00f RDI: ffff81203d993060
May 20 00:09:52 hostname1 kernel: RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000028
May 20 00:09:52 hostname1 kernel: R10: ffffffff803e8260 R11: 0000000000000000 R12: ffff810100000010
May 20 00:09:52 hostname1 kernel: R13: 00000000000000a0 R14: ffff811b9730be68 R15: 0000003000000028
May 20 00:09:52 hostname1 kernel: FS:  00002b1514aa2dd0(0000) GS:ffff810170fa3540(0000) knlGS:0000000000000000
May 20 00:09:52 hostname1 kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
May 20 00:09:52 hostname1 kernel: CR2: 0000000000b76b70 CR3: 0000000afa771000 CR4: 00000000000006e0
May 20 00:09:52 hostname1 kernel:
May 20 00:09:52 hostname1 kernel: Call Trace:
May 20 00:09:52 hostname1 kernel:  [<ffffffff88122007>] :scsi_transport_fc:fc_user_scan+0x5b/0x8b
May 20 00:09:52 hostname1 kernel:  [<ffffffff8807ddb4>] :scsi_mod:store_scan+0x9b/0xc5
May 20 00:09:52 hostname1 kernel:  [<ffffffff8010a75d>] sysfs_write_file+0xb9/0xe8
May 20 00:09:52 hostname1 kernel:  [<ffffffff80016927>] vfs_write+0xce/0x174
May 20 00:09:52 hostname1 kernel:  [<ffffffff800171df>] sys_write+0x45/0x6e
May 20 00:09:52 hostname1 kernel:  [<ffffffff8005d28d>] tracesys+0xd5/0xe0
May 20 00:09:52 hostname1 kernel:
May 20 00:10:02 hostname1 kernel: BUG: soft lockup - CPU#5 stuck for 10s! [bash:14645]