Re: [3.11.4] Thunderbolt/PCI unplug oops in pci_pme_list_scan

[Yinghai Lu <yinghai@kernel.org>]

On Thu, Oct 24, 2013 at 8:33 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
>
>>>> Bisection points to 928bea964827d7824b548c1f8e06eccbbc4d0d7d .
>>>
>>> This is "PCI: Delay enabling bridges until they're needed" by Yinghai.
>>
>> that double disabling should be addressed by:
>>
>> https://lkml.org/lkml/2013/4/25/608
>>
>> [PATCH] PCI: Remove duplicate pci_disable_device for pcie port
>
> I'll look at that patch again.  I had some questions about it the
> first time, but perhaps it makes more sense after 928bea9648 has been
> applied.
>
> Andreas originally reported a GPF oops in pci_pme_list_scan().  I
> posted a refcounting patch, which made the problem go away, but I
> can't explain why, and I don't want to apply it without understanding
> that.  Decoding his oops shows this:
>
>   24: 0f 1f 00             nopl   (%rax)
>   27: 48 8b 50 10           mov    0x10(%rax),%rdx
>   2b:* 48 8b 52 38           mov    0x38(%rdx),%rdx <-- trapping instruction
>   2f: 48 85 d2             test   %rdx,%rdx
>
> %rax is the pci_dev pointer, so 0x10(%rax) is the dev->bus pointer,
> which we put in %rdx.  The oops says %rdx = 6b6b6b6b6b6b6b6b, which is
> POISON_FREE, so I think we loaded dev->bus out of a struct pci_dev
> that has already been freed.
>
> pci_pme_list_scan() holds pci_pme_list_mutex while it traverses
> pci_pme_list, and the pci_stop_and_remove_bus_device() path removes
> the pci_dev by calling pci_pme_active(), which also holds
> pci_pme_list_mutex, so I don't understand how pci_pme_list_scan() can
> see a pci_dev that has already been freed.
>
> If I understand Andreas correctly, 928bea9648 also fixes the crash,
> even without my refcounting change.  Can you explain why?

928bea will make the dev->enable_cnt increase wrongly, as we have
pci_enable_device for child
   pci_enable_bridge for parent
     pci_enable_bridge for grandparent
       pci_enable_device for grandparent
   pci_enable_device for parent
       pci_enable_brdige for grandparent
         pci_enable_device for grandparent.
...

in that case grandprent will be enabled two times, and will enable_cnt will have
extra increase.

so later pci_disable_device will not really call do_pci_disable_device
do the really work, as enable_cnt still big.

solution could be:
let pci_enable_bridge call __pci_enable_device.
and __pci_enable_device will not call pci_enable_bridge.

Thanks

Yinghai