Messages in this thread | | | Date | Fri, 27 Jun 2008 10:48:55 +0530 | From | Dipankar Sarma <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Fri, Jun 27, 2008 at 10:17:38AM +0530, Gautham R Shenoy wrote: > IMHO the warning is a spurious one. > Here's the timeline. > CPU_A CPU_B > -------------------------------------------------------------- > cpu_down(): . > . . > . . > stop_machine(): /* disables preemption, . > * and irqs */ . > . . > . . > take_cpu_down(); . > . . > . . > . . > cpu_disable(); /*this removes cpu . > *from cpu_online_map . > */ . > . . > . . > restart_machine(); /* enables irqs */ . > ------WINDOW DURING WHICH rcp->cpumask is stale --------------- > . call_rcu(); > . /* disables irqs here */ > . .force_quiescent_state(); > .CPU_DEAD: .for_each_cpu(rcp->cpumask) > . . smp_send_reschedule(); > . . > . . WARN_ON() for offlined CPU! > .
Exactly. The call_rcu()s are coming from a different subsystem and can happen anytime during the CPU hotplug path. So, RCU subsystem doesn't have anything to do to keep rcu->cpumask consistent. It is *safe* even if we miss poking a cpu or two while forcing quiescent state in all CPUs. The worst that can happen is a delay in grace period. No correctness problem here.
Thanks Dipankar
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