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 12/12/2012 11:53 PM, Oleg Nesterov wrote:
> On 12/12, Srivatsa S. Bhat wrote:
>>
>> On 12/12/2012 10:54 PM, Oleg Nesterov wrote:
>>
>>> And when I look at get_online_cpus_atomic() again it uses rmb(). This
>>> doesn't look correct, we need the full barrier between this_cpu_inc()
>>> and writer_active().
>>
>> Hmm..
>>
>>> At the same time reader_nested_percpu() can be checked before mb().
>>
>> I thought that since the increment and the check (reader_nested_percpu)
>> act on the same memory location, they will naturally be run in the given
>> order, without any need for barriers. Am I wrong?
> 
> And this is what I meant, you do not need a barrier before
> reader_nested_percpu().
> 

Ah, ok!

> But you need to ensure that WRITE(reader_percpu_refcnt) and READ(writer_signal)
> can't be reordered, so you need mb() in between. rmb() can serialize LOADs and
> STOREs.
> 

OK, got it. (I know you meant s/can/can't).

I'm trying to see if we can somehow exploit the fact that the writer can
potentially tolerate if a reader ignores his signal (to switch to rwlocks)
for a while... and use this to get rid of barriers in the reader path (without
using synchronize_sched() at the writer, of course). And perhaps also take advantage
of the fact that the read_lock() acts as a one-way barrier..

I don't know, maybe its not possible after all.. :-/
 
Regards,
Srivatsa S. Bhat