pciehp: fix NULL dereference in interrupt handler

Fix the following NULL dereference problem reported from Pierre Ossman
and Ingo Molnar.

pciehp: HPC vendor_id 8086 device_id 27d0 ss_vid 0 ss_did 0
pciehp: pciehp_find_slot: slot (device=0x0) not found
BUG: unable to handle kernel NULL pointer dereference at 0000000000000070
IP: [<ffffffff80494a8b>] pciehp_handle_presence_change+0x7e/0x113
PGD 0
Oops: 0000 [1]
CPU 0
Modules linked in:
Pid: 1, comm: swapper Tainted: G        W 2.6.26-rc3-sched-devel.git-00001-g2b99b26-dirty #170
RIP: 0010:[<ffffffff80494a8b>]  [<ffffffff80494a8b>] pciehp_handle_presence_change+0x7e/0x113
RSP: 0000:ffff81003f83fbb0  EFLAGS: 00010046
RAX: 0000000000000039 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000046
RBP: ffff81003f83fbd0 R08: 0000000000000001 R09: ffffffff80245103
R10: 0000000000000020 R11: 0000000000000000 R12: ffff81003ea53a30
R13: 0000000000000000 R14: 0000000000000011 R15: ffffffff80495926
FS:  0000000000000000(0000) GS:ffffffff80be7400(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000000070 CR3: 0000000000201000 CR4: 00000000000006a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 1, threadinfo ffff81003f83e000, task ffff81003f840000)
Stack:  0000000000000008 ffff81003f83fbf6 ffff81003ea53a30 0000000000000008
 ffff81003f83fc10 ffffffff80495ab4 0000000000000011 0000000000000002
 0000000000000202 0000000000000202 00000000fffffff4 ffff81003ea53a30
Call Trace:
 [<ffffffff80495ab4>] pcie_isr+0x18e/0x1bc
 [<ffffffff80260831>] request_irq+0x106/0x12f
 [<ffffffff80495fb6>] pcie_init+0x15e/0x6cc
 [<ffffffff804933a3>] pciehp_probe+0x64/0x541
 [<ffffffff8048f4e7>] pcie_port_probe_service+0x4c/0x76
 [<ffffffff8054af70>] driver_probe_device+0xd4/0x1f0
 [<ffffffff8054b108>] __driver_attach+0x7c/0x7e
 [<ffffffff8054b08c>] ? __driver_attach+0x0/0x7e
 [<ffffffff8054a4b6>] bus_for_each_dev+0x53/0x7d
 [<ffffffff8054ad3c>] driver_attach+0x1c/0x1e
 [<ffffffff8054a9c2>] bus_add_driver+0xdd/0x25b
 [<ffffffff80c09d3d>] ? pcied_init+0x0/0x8b
 [<ffffffff8054b288>] driver_register+0x5f/0x13e
 [<ffffffff80c09d3d>] ? pcied_init+0x0/0x8b
 [<ffffffff8048f441>] pcie_port_service_register+0x47/0x49
 [<ffffffff80c09d52>] pcied_init+0x15/0x8b
 [<ffffffff80bf3938>] kernel_init+0x75/0x243
 [<ffffffff808639d2>] ? _spin_unlock_irq+0x2b/0x3a
 [<ffffffff80228d1f>] ? finish_task_switch+0x57/0x9a
 [<ffffffff8020c258>] child_rip+0xa/0x12
 [<ffffffff8020bcec>] ? restore_args+0x0/0x30
 [<ffffffff80bf38c3>] ? kernel_init+0x0/0x243
 [<ffffffff8020c24e>] ? child_rip+0x0/0x12

Code: 83 80 00 00 00 48 39 f0 75 e1 0f b6 c9 48 c7 c2 00 0e 8d 80 48 c7 c6 8a 60 a6 80 48 c7 c7 10 db a8 80 31 c0 e8 3f 8d d9 ff 31 db <48> 8b 43 70 48 8d 75 ef 48 89 df ff 50 30 80 7d ef 00 74 37 48
RIP  [<ffffffff80494a8b>] pciehp_handle_presence_change+0x7e/0x113
 RSP <ffff81003f83fbb0>
CR2: 0000000000000070
Kernel panic - not syncing: Fatal exception

The situation under which it occurs is hw and timing related: it appears
to happen on a system that has PCI hotplug hardware but with no active
hotplug cards, and another interrupt in the same (shared) IRQ line
arrives too early, before the hotplug-slot entry has been set up - as
triggered by CONFIG_DEBUG_SHIRQ=y:

This patch contains the following two fixes.

(1) Clear all events bits in Slot Status register to prevent the pciehp
    driver from detecting the spurious events that would have been occur
    before pciehp loading.

(2) Add check whether slot initialization had been already done.

This is short term fix. We need more structural fixes to install
interrupt handler after slot initialization is done.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>