Re: acpi_bus_trim does not detach devices in post order

From: Yasuaki Ishimatsu
Date: Tue Aug 06 2013 - 23:21:56 EST

(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@xxxxxxxxx>
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

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

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.

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


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