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

[Jiang Liu]

On 11/30/2012 05:25 AM, Rafael J. Wysocki wrote:
> On Thursday, November 29, 2012 01:56:17 PM Toshi Kani wrote:
>> On Thu, 2012-11-29 at 13:39 -0700, Toshi Kani wrote:
>>> On Thu, 2012-11-29 at 21:30 +0100, 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:
>>>>>>> 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.
>>>
>>> Agreed.
>>
>> BTW, it is not an optimization I am after for this phase.  There are
>> many error cases during hot-plug operations.  It is difficult to assure
>> that rollback is successful for every error condition in terms of
>> testing and maintaining the code.  So, it is easier to fail beforehand
>> when possible.
> 
> OK, but as I said it is necessary to ensure that the conditions will be met
> in the next phases as well if we don't fail.
Yes, that's absolutely an requirement. Otherwise QA people will call you
when doing stress tests.

> 
> Thanks,
> Rafael
> 
> 

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