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SubjectRe: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline
On Wed, Jun 26, 2013 at 07:39:40PM +0530, Srivatsa S. Bhat wrote:
> 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.

Good point, and seems like a reasonable approach to me.

Thanx, Paul



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