Messages in this thread | | | Date | Fri, 27 Jun 2008 11:19:59 +0530 | From | Dhaval Giani <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Fri, Jun 27, 2008 at 10:48:55AM +0530, Dipankar Sarma wrote: > 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. >
One question. What is preventing a CPU from clearing its mask after we have checked whether it is online but before we have called into smp_send_reschedule?
-- regards, Dhaval
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