Re: acpi_bus_trim does not detach devices in post order

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

(2013/08/07 9:57), Rafael J. Wysocki wrote:
> On Wednesday, August 07, 2013 09:35:51 AM Yasuaki Ishimatsu wrote:
>> (2013/08/06 23:26), Rafael J. Wysocki wrote:
>>> On Tuesday, August 06, 2013 07:06:37 PM Yasuaki Ishimatsu wrote:
>>>> (2013/08/06 12:07), Yasuaki Ishimatsu wrote:
>>>>>
>>>>> I acked the following commit but I hit a problem by the commit.
>>>>> So I report it.
>>>>>
>>>>> commit cecdb193c8d91a42d9489d00618cc3dfff92e55a
>>>>> Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>>> Date:   Tue Jan 15 13:24:02 2013 +0100
>>>>>
>>>>>        ACPI / scan: Change the implementation of acpi_bus_trim()
>>>>>
>>>>> Before applying the commit, acpi_bus_trim() detachs devices in post order.
>>>>>
>>>>> When I hot add memory devices and processor devices by container device
>>>>> in my x86 box, memory devices are added first and processor devices are added
>>>>> second. So I expect that processor devices are removed first and memory
>>>>> devices are removed second when I remove them. And before applying the
>>>>> commit, acpi_bus_trim() did so.
>>>>>
>>>>> But after appling the commit, acpi_bus_trim() does not detach devices in
>>>>> post order. So when I remove them, memory devices are removed first and
>>>>> processor devices are removed second.
>>>>>
>>>>> By this, I hit a problem.
>>>>>
>>>>> In Linux on x86 arch, NUMA node is depend on memory devices. So new NUMA
>>>>> node is created at memory hot adding. Thus when I hot add memory devices and
>>>>> processor devices, we must hot add memory device first. Otherwise, processor
>>>>> devices are not set to correct NUMA node number.
>>>>>
>>>>> And Linux expects that when removing them, processor devices are removed
>>>>> first before removing memory devices. But acpi_bus_trim() does not do so.
>>>>> By this, NUMA node is not cleared in my x86 box when hot removing memory device
>>>>> and processor devices. When removing memory devices, NUMA node is cleared.
>>>>> But if there are processor devices related with the NUMA node, NUMA node is
>>>>> not be cleared at memory hot removing.
>>>>>
>>>>> So when I remove them, NUMA node's sysfs file remained as follows:
>>>>
>>>> I had little mistake. CPU also tries to clear NUMA node.
>>>> But current implementation has bug. So I'll fix it.
>>>
>>
>>> Do I understand correctly that with your fix at
>>>
>>> https://patchwork.kernel.org/patch/2839298/
>>>
>>> the current acpi_bus_trim() implementation will be sufficient?
>>
>> No. The patch just fixed implementation of CPU hotplug.
>
> My question was not sufficiently precise. :-)
>

> I wanted to ask if your patch was sufficient to address the specific breakage
> you were seeing without modifying acpi_bus_trim().

Ah. Yes. By my patch, node sysfs is deleted correctly.

>
>> A problem I think is that acpi_bus_trim() does not detach devices in
>> post-order.
>
> That is not exactly post-order, but children are guaranteed to be processed
> before their parents.  If that guarantee is sufficient, there's no problem.
> Otherwise, acpi_bus_trim() may need to be modified, but first I'd like to
> see a real life example where that really matters.

Currently I have no idea.

Thanks,
Yasuaki Ishimatsu

>> And my patch does not fix it. So if some device has dependency
>> of other device, similar problem will occur.
>
> If there is a dependency that is not a parent-child one, we'll have a problem,
> but in that case relying on ordering will not be robust enough anyway in my
> opinion.
>
> Thanks,
> Rafael
>
>