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Re: [Xen-devel] [PATCH 3/3] Xen physical cpus interface

Konrad Rzeszutek Wilk wrote:
> On Fri, May 11, 2012 at 01:12:13PM +0000, Liu, Jinsong wrote:
>> Liu, Jinsong wrote:
>>> Just notice your reply (so quick :)
>>> Agree and will update later, except 1 concern below.
>>> Konrad Rzeszutek Wilk wrote:
>>>>> Hmm, it's good if it's convenient to do it automatically via
>>>>> dev->release. However, dev container (pcpu) would be free at some
>>>>> other error cases, so I prefer do it 'manually'.
>>>> You could also call pcpu_release(..) to do it manually.
>>> that means kfree(pcpu) would be done twice at some error cases, do
>>> you think it really good? 
>> Ping.
>> I think error recovery should be kept inside error logic level
>> itself, if try to recover upper level error would bring trouble. 
>> In our example, there are 2 logic levels:
>> pcpu level (as container), and dev level (subfield used for sys)
> So you need to untangle free_pcpu from doing both. Meaning one does
> the SysFS and the other deals with free-ing the structure and
> removing itself from the list.

free_cpu is very samll, just consist of the 2 parts your said:
* pcpu_sys_remove() deal with sysfs
* list_del/kfree(pcpu) deal with pcpu

>> dev->release should only recover error occurred at dev/sys level,
>> and the pcpu error should be recovered at pcpu level. 
>> If dev->release try to recover its container pcpu level error, like
>> list_del/kfree(pcpu), it would make confusing. i.e., considering
>> pcpu_sys_create(), 2 error cases: device_register fail, and
>> device_create_file fail --> how can the caller decide kfree(pcpu) or
>> not?   
> Then you should free it manually. But you can do this by a wrapper
> function:
> __pcpu_release(..) {
>       ..
>       /* Does the removing itself from the list and kfree the pcpu */
> }
> pcpu_release(..) {
>       struct pcpcu *p= container_of(..)
>       __pcpu_release(p);
> }
> dev->release = &pcpu_release;

Too weird way. If we want to release dev itself it's good to use dev->release, 
but for pcpu I doubt it.
(consider the example I gave --> why we create issue (it maybe solved in weird 
method I agree), just for using dev->release?)

In kernel many dev->release keep NULL.
An example of using dev->release is cpu/mcheck/mce.c --> mce_device_release(), 
it *just* deal dev itself.


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