Subject: Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Wed, 26 Jun 2013 19:39:40 +0530

On 06/26/2013 03:30 AM, Paul E. McKenney wrote:
> On Wed, Jun 26, 2013 at 01:57:55AM +0530, Srivatsa S. Bhat wrote:
>> Once stop_machine() is gone from the CPU offline path, we won't be able
>> to depend on disabling preemption to prevent CPUs from going offline
>> from under us.
>>
>> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
>> while being protected by a spinlock. Use the get/put_online_cpus_atomic()
>> APIs to prevent CPUs from going offline, while invoking from atomic context.
> 
> I am not completely sure that this is needed.  Here is a (quite possibly
> flawed) argument for its not being needed:
> 
> o	rcu_gp_init() holds off CPU-hotplug operations during
> 	grace-period initialization.  Therefore, RCU will avoid
> 	looking for quiescent states from CPUs that were offline
> 	(and thus in an extended quiescent state) at the beginning
> 	of the grace period.
> 
> o	If force_qs_rnp() is looking for a quiescent state from
> 	a given CPU, and if it senses that CPU as being offline,
> 	then even without synchronization we know that the CPU
> 	was offline some time during the current grace period.
> 
> 	After all, it was online at the beginning of the grace
> 	period (otherwise, we would not be looking at it at all),
> 	and our later sampling of its state must have therefore
> 	happened after the start of the grace period.  Given that
> 	the grace period has not yet ended, it also has to happened
> 	before the end of the grace period.
> 
> o	Therefore, we should be able to sample the offline state
> 	without synchronization.
>

Thanks a lot for explaining the synchronization design in detail, Paul!
I agree that get/put_online_cpus_atomic() is not necessary here.

Regarding the debug checks under CONFIG_DEBUG_HOTPLUG_CPU, to avoid
false-positives, I'm thinking of introducing a few _nocheck() variants,
on a case-by-case basis, like cpu_is_offline_nocheck() (useful here in RCU)
and for_each_online_cpu_nocheck() (useful in percpu-counter code, as
pointed out by Tejun Heo). These fine synchronization details are kinda
hard to encapsulate in that debug logic, so we can use the _nocheck()
variants here to avoid getting splats when running with DEBUG_HOTPLUG_CPU
enabled.

Regards,
Srivatsa S. Bhat