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



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