Re: acpi_bus_trim does not detach devices in post order

[Yasuaki Ishimatsu]

(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
> > >
> > > 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


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