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SubjectRe: [PATCH] fix rcu vs hotplug race
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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