Re: [PATCH v2] acpi : acpi_bus_trim() stops removing devices when failing to remove the device

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

Hi Greg,

Sorry for late reply.

2012/10/20 2:59, Greg Kroah-Hartman wrote:
> On Fri, Oct 19, 2012 at 06:29:52AM +0200, Rafael J. Wysocki wrote:
>> On Thursday 11 of October 2012 19:12:28 Yasuaki Ishimatsu wrote:
>>> acpi_bus_trim() stops removing devices, when acpi_bus_remove() return error
>>> number. But acpi_bus_remove() cannot return error number correctly.
>>> acpi_bus_remove() only return -EINVAL, when dev argument is NULL. Thus even if
>>> device cannot be removed correctly, acpi_bus_trim() ignores and continues to
>>> remove devices. acpi_bus_hot_remove_device() uses acpi_bus_trim() for removing
>>> devices. Therefore acpi_bus_hot_remove_device() can send "_EJ0" to firmware,
>>> even if the device is running on the system. In this case, the system cannot
>>> work well.
>>>
>>> Vasilis hit the bug at memory hotplug and reported it as follow:
>>> https://lkml.org/lkml/2012/9/26/318
>>>
>>> So acpi_bus_trim() should check whether device was removed or not correctly.
>>> The patch adds error check into some functions to remove the device.
>>>
>>> Applying the patch, acpi_bus_trim() stops removing devices when failing
>>> to remove the device. But I think there is no impact with the
>>> exceptionof CPU and Memory hotplug path. Because other device also fails
>>> but the fail is an irregular case like device is NULL.
>>>
>>> v1->v2
>>> - add a rollback for reinstalling a notify handler.
>>>
>>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>>
>> Greg, do you think there may be any problems with the changes in dd.c?
>
> Yes, I don't like it.
>
> remove should always work, just like the exit call in a module.  It
> means that the core wants to remove the driver, so it is going to
> happen, a driver can't refuse it.
>
> Which brings me to the larger question, why would this solve anything?

Now we are developing physical memory hot plug.

https://lkml.org/lkml/2012/10/23/213

So if we aplly the patch-set, we can hot remove a physical memory
by the following way.

"echo 1 > /sys/bus/acpi/devices/PNP/eject"

In this case, acpi_bus_hot_remove_device() tries to remove memory
device by acpi_bus_trim(). But if the memory has irremovable memory,
memory hot remove fails. And the memory remains in kernel.
However acpi_bus_trim() cannot notice that memory hot remove fails and
retruns 0. So acpi_bus_hot_remove_device() continues to remove memory
devices and sends _EJ0 method to firmware. Thus the memory device cannot
be used. But the memory remains in kernel yet. So if someone access the
memory, kernel panic occurs.

Thanks,
Yasuaki Ishimatsu

> If the kernel wants to unbind a device, why would we ever not want that
> to happen?
>
> So, NAK on this patch, sorry.  Fix up the ACPI core to handle this
> properly, don't mess with the driver core here.
>
> greg k-h
>