When a wide port is being utilized to a target, if one disables only one of the
phys, we get an OS crash:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000238
IP: [<ffffffff814ca9b1>] mutex_lock+0x21/0x50
PGD 4103f5067 PUD 41dba9067 PMD 0
Oops: 0002 [#1] SMP
last sysfs file: /sys/bus/pci/slots/5/address
CPU 0
Modules linked in: pm8001(U) ses enclosure fuse nfsd exportfs autofs4 ipmi_devintf ipmi_si ipmi_msghandler nfs lockd fscache nfs_acl auth_rpcgss 8021q fcoe libfcoe garp libfc scsi_transport_fc stp scsi_tgt llc sunrpc cpufreq_ondemand acpi_cpufreq freq_table ipv6 sr_mod cdrom dm_mirror dm_region_hash dm_log uinput sg i2c_i801 i2c_core iTCO_wdt iTCO_vendor_support e1000e mlx4_ib ib_mad ib_core mlx4_en mlx4_core ext3 jbd mbcache sd_mod crc_t10dif usb_storage ata_generic pata_acpi ata_piix libsas(U) scsi_transport_sas dm_mod [last unloaded: pm8001]

Modules linked in: pm8001(U) ses enclosure fuse nfsd exportfs autofs4 ipmi_devintf ipmi_si ipmi_msghandler nfs lockd fscache nfs_acl auth_rpcgss 8021q fcoe libfcoe garp libfc scsi_transport_fc stp scsi_tgt llc sunrpc cpufreq_ondemand acpi_cpufreq freq_table ipv6 sr_mod cdrom dm_mirror dm_region_hash dm_log uinput sg i2c_i801 i2c_core iTCO_wdt iTCO_vendor_support e1000e mlx4_ib ib_mad ib_core mlx4_en mlx4_core ext3 jbd mbcache sd_mod crc_t10dif usb_storage ata_generic pata_acpi ata_piix libsas(U) scsi_transport_sas dm_mod [last unloaded: pm8001]
Pid: 5146, comm: scsi_wq_5 Not tainted 2.6.32-71.29.1.el6.lustre.7.x86_64 #1 Storage Server
RIP: 0010:[<ffffffff814ca9b1>]  [<ffffffff814ca9b1>] mutex_lock+0x21/0x50
RSP: 0018:ffff8803e4e33d30  EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000238 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff8803e664c800 RDI: 0000000000000238
RBP: ffff8803e4e33d40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000238 R14: ffff88041acb7200 R15: ffff88041c51ada0
FS:  0000000000000000(0000) GS:ffff880028200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000000238 CR3: 0000000410143000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process scsi_wq_5 (pid: 5146, threadinfo ffff8803e4e32000, task ffff8803e4e294a0)
Stack:
 ffff8803e664c800 0000000000000000 ffff8803e4e33d70 ffffffffa001f06e
<0> ffff8803e4e33d60 ffff88041c51ada0 ffff88041acb7200 ffff88041bc0aa00
<0> ffff8803e4e33d90 ffffffffa0032b6c 0000000000000014 ffff88041acb7200
Call Trace:
 [<ffffffffa001f06e>] sas_port_delete_phy+0x2e/0xa0 [scsi_transport_sas]
 [<ffffffffa0032b6c>] sas_unregister_devs_sas_addr+0xac/0xe0 [libsas]
 [<ffffffffa0034914>] sas_ex_revalidate_domain+0x204/0x330 [libsas]
 [<ffffffffa00307f0>] ? sas_revalidate_domain+0x0/0x90 [libsas]
 [<ffffffffa0030855>] sas_revalidate_domain+0x65/0x90 [libsas]
 [<ffffffff8108c7d0>] worker_thread+0x170/0x2a0
 [<ffffffff81091ea0>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff8108c660>] ? worker_thread+0x0/0x2a0
 [<ffffffff81091b36>] kthread+0x96/0xa0
 [<ffffffff810141ca>] child_rip+0xa/0x20
 [<ffffffff81091aa0>] ? kthread+0x0/0xa0
 [<ffffffff810141c0>] ? child_rip+0x0/0x20
Code: ff ff 85 c0 75 ed eb d6 66 90 55 48 89 e5 48 83 ec 10 48 89 1c 24 4c 89 64 24 08 0f 1f 44 00 00 48 89 fb e8 92 f4 ff ff 48 89 df <f0> ff 0f 79 05 e8 25 00 00 00 65 48 8b 04 25 08 cc 00 00 48 2d
RIP  [<ffffffff814ca9b1>] mutex_lock+0x21/0x50
 RSP <ffff8803e4e33d30>
CR2: 0000000000000238

The following patch is admittedly a band-aid, and does not solve the root cause, but it still is a good candidate for hardening as a pointer check before reference.

Signed-off-by: Mark Salyzyn <mark_salyzyn@xyus.xyratex.com>
Cc: Luben Tuikov <ltuikov@yahoo.com>
Cc: Darrick J Wong <djwong@us.ibm.com>
Cc: James Bottomley <jbottomley@parallels.com>
Cc: Xiangliang Yu <yuxiangl@marvell.com>
Cc: Jack Wang <jack_wang@usish.com>

 sas_expander.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff -rup scsi-misc-2.6/drivers/scsi/libsas/sas_expander.c scsi-misc-2.6.new/drivers/scsi/libsas/sas_expander.c
--- scsi-misc-2.6/drivers/scsi/libsas/sas_expander.c	2011-08-31 08:32:21.000000000 -0400
+++ scsi-misc-2.6.new/drivers/scsi/libsas/sas_expander.c	2011-09-22 11:20:06.000000000 -0400
@@ -1769,10 +1769,12 @@ static void sas_unregister_devs_sas_addr
 		sas_disable_routing(parent, phy->attached_sas_addr);
 	}
 	memset(phy->attached_sas_addr, 0, SAS_ADDR_SIZE);
-	sas_port_delete_phy(phy->port, phy->phy);
-	if (phy->port->num_phys == 0)
-		sas_port_delete(phy->port);
-	phy->port = NULL;
+	if (phy->port) {
+		sas_port_delete_phy(phy->port, phy->phy);
+		if (phy->port->num_phys == 0)
+			sas_port_delete(phy->port);
+		phy->port = NULL;
+	}
 }
 
 static int sas_discover_bfs_by_root_level(struct domain_device *root,
