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

[Jiang Liu]

On 11/29/2012 02:41 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.
> 
>> 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.  
> 
> 2. Execute phase - Perform hot-add / hot-remove operation that can be
> rolled-back in case of error or cancel.
> 
> 3. Commit phase - Perform the final hot-add / hot-remove operation that
> cannot be rolled-back.  No error / cancel is allowed in this phase.  For
> instance, eject operation is performed at this phase.  
Hi Toshi,
	There are one more step needed. Linux provides sysfs interfaces to
online/offline CPU/memory sections, so we need to protect from concurrent
operations from those interfaces when doing physical hotplug. Think about
following sequence:
Thread 1
1. validate conditions for hot-removal
2. offline memory section A
3.						online memory section A			
4. offline memory section B
5 hot-remove memory device hosting A and B.
Regards!
Gerry
> 
> 
> Thanks,
> -Toshi
> 
> 
> 
> 
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