Re: [RFC PATCH v3 0/3] acpi: Introduce prepare_remove device operation

[Jiang Liu]

On 11/30/2012 04:30 AM, Rafael J. Wysocki wrote:
> On Thursday, November 29, 2012 10:03:12 AM Toshi Kani wrote:
>> On Thu, 2012-11-29 at 11:15 +0100, Rafael J. Wysocki wrote:
>>> On Wednesday, November 28, 2012 11:41:36 AM Toshi Kani wrote:
>>>> On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:
>>>>> On 2012/11/24 1:50, Vasilis Liaskovitis wrote:
>>>>>> As discussed in https://patchwork.kernel.org/patch/1581581/
>>>>>> the driver core remove function needs to always succeed. This means we need
>>>>>> to know that the device can be successfully removed before acpi_bus_trim / 
>>>>>> acpi_bus_hot_remove_device are called. This can cause panics when OSPM-initiated
>>>>>> or SCI-initiated eject of memory devices fail e.g with:
>>>>>> echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
>>>>>>
>>>>>> since the ACPI core goes ahead and ejects the device regardless of whether the
>>>>>> the memory is still in use or not.
>>>>>>
>>>>>> For this reason a new acpi_device operation called prepare_remove is introduced.
>>>>>> This operation should be registered for acpi devices whose removal (from kernel
>>>>>> perspective) can fail.  Memory devices fall in this category.
>>>>>>
>>>>>> acpi_bus_remove() is changed to handle removal in 2 steps:
>>>>>> - preparation for removal i.e. perform part of removal that can fail. Should
>>>>>>   succeed for device and all its children.
>>>>>> - if above step was successfull, proceed to actual device removal
>>>>>
>>>>> Hi Vasilis,
>>>>> We met the same problem when we doing computer node hotplug, It is a good idea
>>>>> to introduce prepare_remove before actual device removal.
>>>>>
>>>>> I think we could do more in prepare_remove, such as rollback. In most cases, we can
>>>>> offline most of memory sections except kernel used pages now, should we rollback
>>>>> and online the memory sections when prepare_remove failed ?
>>>>
>>>> I think hot-plug operation should have all-or-nothing semantics.  That
>>>> is, an operation should either complete successfully, or rollback to the
>>>> original state.
>>>
>>> That's correct.
>>>
>>>>> As you may know, the ACPI based hotplug framework we are working on already addressed
>>>>> this problem, and the way we slove this problem is a bit like yours.
>>>>>
>>>>> We introduce hp_ops in struct acpi_device_ops:
>>>>> struct acpi_device_ops {
>>>>> 	acpi_op_add add;
>>>>> 	acpi_op_remove remove;
>>>>> 	acpi_op_start start;
>>>>> 	acpi_op_bind bind;
>>>>> 	acpi_op_unbind unbind;
>>>>> 	acpi_op_notify notify;
>>>>> #ifdef	CONFIG_ACPI_HOTPLUG
>>>>> 	struct acpihp_dev_ops *hp_ops;
>>>>> #endif	/* CONFIG_ACPI_HOTPLUG */
>>>>> };
>>>>>
>>>>> in hp_ops, we divide the prepare_remove into six small steps, that is:
>>>>> 1) pre_release(): optional step to mark device going to be removed/busy
>>>>> 2) release(): reclaim device from running system
>>>>> 3) post_release(): rollback if cancelled by user or error happened
>>>>> 4) pre_unconfigure(): optional step to solve possible dependency issue
>>>>> 5) unconfigure(): remove devices from running system
>>>>> 6) post_unconfigure(): free resources used by devices
>>>>>
>>>>> In this way, we can easily rollback if error happens.
>>>>> How do you think of this solution, any suggestion ? I think we can achieve
>>>>> a better way for sharing ideas. :)
>>>>
>>>> Yes, sharing idea is good. :)  I do not know if we need all 6 steps (I
>>>> have not looked at all your changes yet..), but in my mind, a hot-plug
>>>> operation should be composed with the following 3 phases.
>>>>
>>>> 1. Validate phase - Verify if the request is a supported operation.  All
>>>> known restrictions are verified at this phase.  For instance, if a
>>>> hot-remove request involves kernel memory, it is failed in this phase.
>>>> Since this phase makes no change, no rollback is necessary to fail.  
>>>
>>> Actually, we can't do it this way, because the conditions may change between
>>> the check and the execution.  So the first phase needs to involve execution
>>> to some extent, although only as far as it remains reversible.
>>
>> For memory hot-remove, we can check if the target memory ranges are
>> within ZONE_MOVABLE.  We should not allow user to change this setup
>> during hot-remove operation.  Other things may be to check if a target
>> node contains cpu0 (until it is supported), the console UART (assuming
>> we cannot delete it), etc.  We should avoid doing rollback as much as we
>> can.
> 
> Yes, we can make some checks upfront as an optimization and fail early if
> the conditions are not met, but for correctness we need to repeat those
> checks later anyway.  Once we've decided to go for the eject, the conditions
> must hold whatever happens.
Hi Rafael,
	Another reason for us to split hotplug operations into minor/tiny
steps is to support cancellation other than error handling. Theoretical
it may take infinite time to hot-remove a memory device, so we should provide
an interface for user to cancel ongoing hot-removal operations. Currently that's
done by timeout in the memory hot-remove code path, but I think it not the 
best solutions. We should provide choices to users:
1) wait for ever to remove a hot-removal operation
2) cancel an ongoing hot-removal operation if it takes too long

Regards!
Gerry
> 
> Thanks,
> Rafael
> 
> 

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