Messages in this thread | | | Date | Fri, 27 Jun 2008 07:58:45 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Fri, Jun 27, 2008 at 11:19:59AM +0530, Dhaval Giani wrote: > 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?
This is my concern as well. Gautham, at which point in the above timeline is the offlining CPU marked DYING? Before stop_machine(), right?
If so, can't we just disable irqs, check for DYING or DEAD, and invoke smp_send_reschedule() only if not DYING or DEAD?
Thanx, Paul
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