Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 02/12/2014 02:21 AM, Toshi Kani wrote:
> On Wed, 2014-02-12 at 00:50 +0530, Srivatsa S. Bhat wrote:
>> On 02/11/2014 11:05 PM, Toshi Kani wrote:
>  :
>>> How about this?  foo_cpu_notifier returns NOP when foo_notifier_ready is
>>> false.
>>>
>>>         register_cpu_notifier(&foobar_cpu_notifier);
>>>
>>>         get_online_cpus();
>>>
>>>         for_each_online_cpu(cpu)
>>>                 init_cpu(cpu);
>>>
>>> 	foo_notifier_ready = true;
>>>
>>>         put_online_cpus();
>>>
>>
>> Nah, that looks a lot like some quick-n-dirty hack ;-(
>> It would also amount to burdening the various subsystems to add weird-looking
>> pieces of code such as this in their callbacks:
>>
>> 	if (!foo_notifier_ready)
>> 		return NOTIFY_OK;
>>
>> This only makes it all the more evident that the callback registration APIs
>> exposed by the CPU hotplug core is poorly designed.
>>
>> What we need instead, is an elegant, well-defined and easy-to-use set of
>> interfaces/APIs exposed by the core CPU hotplug code to the various
>> subsystems. I don't think we should worry so much about the fact that
>> we can't use the familiar get/put_online_cpus() in this type of callback
>> registration scenario. We can introduce a sane set of APIs that work
>> well in such situations and use them consistently.
> 
>> For example, something like the code snippet shown below looks pretty
>> neat to me:
>>
>> 	cpu_notifier_register_begin();
>>
>> 	for_each_online_cpu(cpu)
>> 		init_cpu(cpu);
>>
>> 	register_cpu_notifier(&foobar_cpu_notifier);
>>
>> 	cpu_notifier_register_done();
>>
>> What do you think?
> 
> I agree that it is cleaner for the callers as long as people understand
> how to use them.  Can you document them properly so that they know when
> they need to use them instead of the familiar get/put_online_cpus()?
> 

Sure.. I had updated the documentation with the semantics introduced in
this patchset, in patch 2:

http://thread.gmane.org/gmane.linux.kernel/1641638/focus=1641695

Similarly I'll keep the docs updated with these new APIs in v2 as well.

Thank you!
 
Regards,
Srivatsa S. Bhat