[patch, -git] pcie hotplug bootup crash fix

[Ingo Molnar <mingo@elte.hu>]

-tip tree testing found that the the PCI hotplug ISR routine crashes 
with a NULL pointer dereference under certain circumstances.

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:

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 config with which it occurs is:

  http://redhat.com/~mingo/misc/config-Sat_May_24_18_17_56_CEST_2008.bad

the fix is to check for NULL slots.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 drivers/pci/hotplug/pciehp_ctrl.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/drivers/pci/hotplug/pciehp_ctrl.c
===================================================================
--- linux.orig/drivers/pci/hotplug/pciehp_ctrl.c
+++ linux/drivers/pci/hotplug/pciehp_ctrl.c
@@ -118,6 +118,9 @@ u8 pciehp_handle_presence_change(u8 hp_s
 
 	p_slot = pciehp_find_slot(ctrl, hp_slot + ctrl->slot_device_offset);
 
+	if (!p_slot || !p_slot->hpc_ops)
+		return 0;
+
 	/* Switch is open, assume a presence change
 	 * Save the presence state
 	 */