Messages in this thread |  | | | Date | Fri, 27 Jun 2008 11:19:59 +0530 | | From | Dhaval Giani <> | | Subject | Re: [PATCH] fix rcu vs hotplug race |
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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