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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