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SubjectRe: [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:





What do you think?

Srivatsa S. Bhat

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