When hot removing a pci express module that has a pcie switch and supports
SRIOV, we got:

[ 5918.610127] pciehp 0000:80:02.2:pcie04: pcie_isr: intr_loc 1
[ 5918.615779] pciehp 0000:80:02.2:pcie04: Attention button interrupt received
[ 5918.622730] pciehp 0000:80:02.2:pcie04: Button pressed on Slot(3)
[ 5918.629002] pciehp 0000:80:02.2:pcie04: pciehp_get_power_status: SLOTCTRL a8 value read 1f9
[ 5918.637416] pciehp 0000:80:02.2:pcie04: PCI slot #3 - powering off due to button press.
[ 5918.647125] pciehp 0000:80:02.2:pcie04: pcie_isr: intr_loc 10
[ 5918.653039] pciehp 0000:80:02.2:pcie04: pciehp_green_led_blink: SLOTCTRL a8 write cmd 200
[ 5918.661229] pciehp 0000:80:02.2:pcie04: pciehp_set_attention_status: SLOTCTRL a8 write cmd c0
[ 5924.667627] pciehp 0000:80:02.2:pcie04: Disabling domain:bus:device=0000:b0:00
[ 5924.674909] pciehp 0000:80:02.2:pcie04: pciehp_get_power_status: SLOTCTRL a8 value read 2f9
[ 5924.683262] pciehp 0000:80:02.2:pcie04: pciehp_unconfigure_device: domain:bus:dev = 0000:b0:00
[ 5924.693976] libfcoe_device_notification: NETDEV_UNREGISTER eth6
[ 5924.764979] libfcoe_device_notification: NETDEV_UNREGISTER eth14
[ 5924.873539] libfcoe_device_notification: NETDEV_UNREGISTER eth15
[ 5924.995209] libfcoe_device_notification: NETDEV_UNREGISTER eth16
[ 5926.114407] sxge 0000:b2:00.0: PCI INT A disabled
[ 5926.119342] BUG: unable to handle kernel NULL pointer dereference at (null)
[ 5926.127189] IP: [<ffffffff81353a3b>] pci_stop_bus_device+0x33/0x83
[ 5926.133377] PGD 0
[ 5926.135402] Oops: 0000 [#1] SMP
[ 5926.138659] CPU 2
[ 5926.140499] Modules linked in:
...
[ 5926.143754]
[ 5926.275823] Call Trace:
[ 5926.278267]  [<ffffffff81353a38>] pci_stop_bus_device+0x30/0x83
[ 5926.284180]  [<ffffffff81353af4>] pci_remove_bus_device+0x1a/0xba
[ 5926.290264]  [<ffffffff81366311>] pciehp_unconfigure_device+0x110/0x17b
[ 5926.296866]  [<ffffffff81365dd9>] ? pciehp_disable_slot+0x188/0x188
[ 5926.303123]  [<ffffffff81365d6f>] pciehp_disable_slot+0x11e/0x188
[ 5926.309206]  [<ffffffff81365e68>] pciehp_power_thread+0x8f/0xe0
...

 +-[0000:80]-+-00.0-[81-8f]--
 |           +-01.0-[90-9f]--
 |           +-02.0-[a0-af]--
 |           +-02.2-[b0-bf]----00.0-[b1-b3]--+-02.0-[b2]--+-00.0 Device
 |           |                               |            +-00.1 Device
 |           |                               |            +-00.2 Device
 |           |                               |            \-00.3 Device
 |           |                               \-03.0-[b3]--+-00.0 Device
 |           |                                            +-00.1 Device
 |           |                                            +-00.2 Device
 |           |                                            \-00.3 Device

root complex: 80:02.2
pci express modules: have pcie switch and are listed as b0:00.0, b1:02.0 and b1:03.0.
end devices  are b2:00.0 and b3.00.0.
VFs are: b2:00.1,... b2:00.3, and b3:00.1,...,b3:00.3

Root cause: when doing pci_stop_bus_device() with phys fn, it will stop
virt fn and remove the fn, so
	list_for_each_safe(l, n, &bus->devices)
will have problem to refer freed n that is pointed to vf entry.

Solution is just replacing list_for_each_safe() with
list_for_each_prev_safe().  This will make sure we can get valid n pointer
to PF instead of the freed VF pointer (because newly added devices are
inserted to the bus->devices list tail).

During reviewing the patch, Bjorn said:
|   The PCI hot-remove path calls pci_stop_bus_devices() via
|   pci_remove_bus_device().
|
|   pci_stop_bus_devices() traverses the bus->devices list (point A below),
|   stopping each device in turn, which calls the driver remove() method.  When
|   the device is an SR-IOV PF, the driver calls pci_disable_sriov(), which
|   also uses pci_remove_bus_device() to remove the VF devices from the
|   bus->devices list (point B).
|
|       pci_remove_bus_device
|         pci_stop_bus_device
|           pci_stop_bus_devices(subordinate)
|             list_for_each(bus->devices)             <-- A
|               pci_stop_bus_device(PF)
|                 ...
|                   driver->remove
|                     pci_disable_sriov
|                       ...
|                         pci_remove_bus_device(VF)
|                             <remove from bus_list>  <-- B
|
|   At B, we're changing the same list we're iterating through at A, so when
|   the driver remove() method returns, the pci_stop_bus_devices() iterator has
|   a pointer to a list entry that has already been freed.

Discussion thread can be found : https://lkml.org/lkml/2011/10/15/141
				 https://lkml.org/lkml/2012/1/23/360

-v5: According to Linus to make remove more robust, Change to
     list_for_each_prev_safe instead. That is more reasonable, because
     those devices are added to tail of the list before.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>