Re: [PATCH v6 2/2] PCI/ERR: Split the fatal and non-fatal error recovery handling

["Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 10/14/20 8:07 AM, Ethan Zhao wrote:
> On Wed, Oct 14, 2020 at 5:00 PM Kuppuswamy Sathyanarayanan
> <sathyanarayanan.nkuppuswamy@gmail.com> wrote:
>>
>> Commit bdb5ac85777d ("PCI/ERR: Handle fatal error recovery")
>> merged fatal and non-fatal error recovery paths, and also made
>> recovery code depend on hotplug handler for "remove affected
>> device + rescan" support. But this change also complicated the
>> error recovery path and which in turn led to the following
>> issues.
>>
>> 1. We depend on hotplug handler for removing the affected
>> devices/drivers on DLLSC LINK down event (on DPC event
>> trigger) and DPC handler for handling the error recovery. Since
>> both handlers operate on same set of affected devices, it leads
>> to race condition, which in turn leads to  NULL pointer
>> exceptions or error recovery failures.You can find more details
>> about this issue in following link.
>>
>> https://lore.kernel.org/linux-pci/20201007113158.48933-1-haifeng.zhao@intel.com/T/#t
>>
>> 2. For non-hotplug capable devices fatal (DPC) error recovery
>> is currently broken. Current fatal error recovery implementation
>> relies on PCIe hotplug (pciehp) handler for detaching and
>> re-enumerating the affected devices/drivers. So when dealing with
>> non-hotplug capable devices, recovery code does not restore the state
>> of the affected devices correctly. You can find more details about
>> this issue in the following links.
>>
>> https://lore.kernel.org/linux-pci/20200527083130.4137-1-Zhiqiang.Hou@nxp.com/
>> https://lore.kernel.org/linux-pci/12115.1588207324@famine/
>> https://lore.kernel.org/linux-pci/0e6f89cd6b9e4a72293cc90fafe93487d7c2d295.1585000084.git.sathyanarayanan.kuppuswamy@linux.intel.com/
>>
>> In order to fix the above two issues, we should stop relying on hotplug
>    Yes, it doesn't rely on hotplug handler to remove and rescan the device,
> but it couldn't prevent hotplug drivers from doing another replicated
> removal/rescanning.
> it doesn't make sense to leave another useless removal/rescanning there.
> Maybe that's why these two paths were merged to one and made it rely on
> hotplug.
No, as per PCIe spec, hotplug and DPC has no functional dependency. Hence
depending on it to handle some of its recovery function is in-correct and
would lead to issues in non-hotplug capable platforms (which is true
currently).
> 

>> +       else
>> +               udev = dev->bus->self;
>> +
>> +       parent = udev->subordinate;
>> +       pci_walk_bus(parent, pci_dev_set_disconnected, NULL);
>> +
>> +        pci_lock_rescan_remove();
>     Though here you have lock, but hotplug will do another
> 'pci_stop_and_remove_bus_device()'
>     without merging it with the hotplug driver, you have no way to
> remove the replicated actions in
>    hotplug handler.
No, the core operation (remove/add device) is syncronzied and done in
only one thread. Please check the following flow. Even in hotplug
handler, before removing the device, it attempts to hold pci_lock_rescan_remove()
lock. So holding the same lock in DPC handler will syncronize the DPC/hotplug
handlers. Also if one of the thread (DPC or hotplug) removes/adds the affected devices,
other thread will not repeat the same action (since the device is already removed/added).

->pciehp_ist()
   ->pciehp_handle_presence_or_link_change()
     ->pciehp_disable_slot()
       ->__pciehp_disable_slot()
         ->remove_board()
           ->pciehp_unconfigure_device()
             ->pci_lock_rescan_remove()
> 
> 
>    Thanks,
>    Ethan
>> +        pci_dev_get(dev);
>> +        list_for_each_entry_safe_reverse(pdev, temp, &parent->devices,
>> +                                        bus_list) {
>> +               pci_stop_and_remove_bus_device(pdev);
>> +       }
>> +
>> +       result = reset_link(udev);
>> +
>> +       if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE) {
>> +               /*
>> +                * If the error is reported by a bridge, we think this error
>> +                * is related to the downstream link of the bridge, so we
>> +                * do error recovery on all subordinates of the bridge instead
>> +                * of the bridge and clear the error status of the bridge.
>> +                */
>> +               pci_aer_clear_fatal_status(dev);
>> +               if (pcie_aer_is_native(dev))
>> +                       pcie_clear_device_status(dev);
>> +       }
>> +
>> +       if (result == PCI_ERS_RESULT_RECOVERED) {
>> +               if (pcie_wait_for_link(udev, true))
>     And another  pci_rescan_bus() like in the hotplug handler.
As I have mentioned before, holding the same lock should make them synchronized
and not repeat the underlying functionality of pci_rescan_bus() in both threads
at the same time.
>> +                       pci_rescan_bus(udev->bus);
>> +               pci_info(dev, "Device recovery from fatal error successful\n");
>> +        } else {
>> +               pci_uevent_ers(dev, PCI_ERS_RESULT_DISCONNECT);
>> +               pci_info(dev, "Device recovery from fatal error failed\n");

>> --
>> 2.17.1
>>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer