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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
