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

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Date:	Wed, 12 Feb 2014 00:50:10 +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 02/11/2014 11:05 PM, Toshi Kani wrote:
> On Tue, 2014-02-11 at 22:48 +0530, Gautham R Shenoy wrote:
>> On Tue, Feb 11, 2014 at 09:33:56AM -0700, Toshi Kani wrote:
>>>
>>> I agree that introducing a reader-writer semaphore allows concurrent
>>> executions.  Adding yet another hotplug lock is a bit unfortunate,
>>> though.
>>>
>>
>> I agree with this last part. We already have enough locks for
>> cpu-hotplug. Another one sounds one too many!!
>>
>>
>>> This may be a dumb question, but can't we simply do this way?
>>>
>>>         get_online_cpus();
>>>
>>>         for_each_online_cpu(cpu)
>>>                 init_cpu(cpu);
>>>
>>>         put_online_cpus();
>>>
>>       -------- Someone chooses to hotplug a cpu here ------
>>       -------- But this subsystem might miss out on knowing
>>                about it since it hasn't registered its
>>                notifier yet! 
>>
>>>         register_cpu_notifier(&foobar_cpu_notifier);
> 
> 
> 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?

Regards,
Srivatsa S. Bhat

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