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 12/19/2012 01:13 AM, Oleg Nesterov wrote:
> On 12/18, Srivatsa S. Bhat wrote:
>>
>> So now that we can't avoid disabling and enabling interrupts,
> 
> Still I think it would be better to not use local_irq_save/restore
> directly.

Sure, we can use this_cpu_add() itself. I explicitly used
local_irq_save/restore here just to explain my question.

> And,
> 
>> I was
>> wondering if we could exploit this to avoid the smp_mb()..
>>
>> Maybe this is a stupid question, but I'll shoot it anyway...
>> Does local_irq_disable()/enable provide any ordering guarantees by any chance?
> 
> Oh, I do not know.
> 
> But please look at the comment above prepare_to_wait(). It assumes
> that even spin_unlock_irqrestore() is not the full barrier.
> 

Semi-permeable barrier.. Hmm.. 

> In any case. get_online_cpus_atomic() has to use irq_restore, not
> irq_enable. And _restore does nothing "special" if irqs were already
> disabled, so I think we can't rely on sti.
> 

Right, I forgot about the _restore part.

>> I tried thinking about other ways to avoid that smp_mb() in the reader,
> 
> Just in case, I think there is no way to avoid mb() in _get (although
> perhaps it can be "implicit").
> 

Actually, I was trying to avoid mb() in the _fastpath_, when there is no
active writer. I missed stating that clearly, sorry.

> The writer changes cpu_online_mask and drops the lock. We need to ensure
> that the reader sees the change in cpu_online_mask after _get returns.
> 

The write_unlock() will ensure the completion of the update to cpu_online_mask,
using the same semi-permeable logic that you pointed above. So readers will
see the update as soon as the writer releases the lock, right?

>> but was unsuccessful. So if the above assumption is wrong, I guess we'll
>> just have to go with the version that uses synchronize_sched() at the
>> writer-side.
> 
> In this case we can also convert get_online_cpus() to use percpu_rwsem
> and avoid mutex_lock(&cpu_hotplug.lock), but this is minor I guess.
> I do not think get_online_cpus() is called too often.
> 

Yes, we could do that as well. I remember you saying that you had some
patches for percpu_rwsem to help use it in cpu hotplug code (to make it
recursive, IIRC).

So, I guess we'll go with the synchronize_sched() approach for percpu rwlocks
then. Tejun, it is still worthwhile to expose this as a generic percpu rwlock
and then use it inside cpu hotplug code, right?


Regards,
Srivatsa S. Bhat