Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/13/2012 01:06 AM, Oleg Nesterov wrote:
> On 12/12, Oleg Nesterov wrote:
>>
>> On 12/12, Srivatsa S. Bhat wrote:
>>>
>>> On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
>>>>
>>>> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
>>>> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
>>>> into a single cacheline...
>>>
>>> Even I realized this (that we could use a global) after posting out the
>>> series.. But do you think that it would be better to retain the per-cpu
>>> variant itself, due to the cache effects?
>>
>> I don't really know, up to you. This was the question ;)
> 
> But perhaps there is another reason to make it per-cpu...
> 
> It seems we can avoid cpu_hotplug.active_writer == current check in
> get/put.
> 
> take_cpu_down() can clear this_cpu(writer_signal) right after it takes
> hotplug_rwlock for writing. It runs with irqs and preemption disabled,
> nobody else will ever look at writer_signal on its CPU.
> 

Hmm.. And then the get/put_ on that CPU will increment/decrement the per-cpu
refcount, but we don't care.. because we only need to ensure that they don't
deadlock by taking the rwlock for read.

This sounds great!

Regards,
Srivatsa S. Bhat