On Tue, Jun 26, 2018 at 10:10:39AM -0700, Paul E. McKenney wrote: > Without special fail-safe quiescent-state-propagation checks, grace-period > hangs can result from the following scenario: > > 1. CPU 1 goes offline. > > 2. Because CPU 1 is the only CPU in the system blocking the current > grace period, as soon as rcu_cleanup_dying_idle_cpu()'s call to > rcu_report_qs_rnp() returns. > > 3. At this point, the leaf rcu_node structure's ->lock is no longer > held: rcu_report_qs_rnp() has released it, as it must in order > to awaken the RCU grace-period kthread. > > 4. At this point, that same leaf rcu_node structure's ->qsmaskinitnext > field still records CPU 1 as being online. This is absolutely > necessary because the scheduler uses RCU, and ->qsmaskinitnext
Can you expand a bit on this, where does the scheduler care about the online state of the CPU that's about to call into arch_cpu_idle_dead()? > contains RCU's idea as to which CPUs are online. Therefore, > invoking rcu_report_qs_rnp() after clearing CPU 1's bit from > ->qsmaskinitnext would result in a lockdep-RCU splat due to > RCU being used from an offline CPU. > > 5. RCU's grace-period kthread awakens, sees that the old grace period > has completed and that a new one is needed. It therefore starts > a new grace period, but because CPU 1's leaf rcu_node structure's > ->qsmaskinitnext field still shows CPU 1 as being online, this new > grace period is initialized to wait for a quiescent state from the > now-offline CPU 1. If we're past cpuhp_report_idle_cpu() -> rcu_report_dead(), then cpu_offline() is true. Is that not sufficient state to avoid this? > 6. Without the fail-safe force-quiescent-state checks, there would > be no quiescent state from the now-offline CPU 1, which would > eventually result in RCU CPU stall warnings and memory exhaustion.
