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

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Date:	Wed, 12 Feb 2014 11:48:44 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	ego@...ux.vnet.ibm.com, "paulus@...ba.org" <paulus@...ba.org>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"walken@...gle.com" <walken@...gle.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [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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