Messages in this thread | | | Date | Thu, 26 Jun 2008 08:27:28 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Tue, Jun 24, 2008 at 01:01:44PM +0200, Ingo Molnar wrote: > > * Gautham R Shenoy <ego@in.ibm.com> wrote: > > > > hm, not sure - we might just be fighting the symptom and we might > > > now create a silent resource leak instead. Isnt a full RCU quiescent > > > state forced (on all CPUs) before a CPU is cleared out of > > > cpu_online_map? That way the to-be-offlined CPU should never > > > actually show up in rcp->cpumask. > > > > No, this does not happen currently. The rcp->cpumask is always > > initialized to cpu_online_map&~nohz_cpu_mask when we start a new > > batch. Hence, before the batch ends, if a cpu goes offline we _can_ > > have a stale rcp->cpumask, till the RCU subsystem has handled it's > > CPU_DEAD notification. > > > > Thus for a tiny interval, the rcp->cpumask would contain the offlined > > CPU. One of the alternatives is probably to handle this using > > CPU_DYING notifier instead of CPU_DEAD where we can call > > __rcu_offline_cpu(). > > > > The warn_on that dhaval was hitting was because of some cpu-offline > > that was called just before we did a local_irq_save inside call_rcu(). > > But at that time, the rcp->cpumask was still stale, and hence we ended > > up sending a smp_reschedule() to an offlined cpu. So the check may not > > create any resource leak. > > the check may not - but the problem it highlights might and with the > patch we'd end up hiding potential problems in this area. > > Paul, what do you think about this mixed CPU hotplug plus RCU workload?
RCU most certainly needs to work correctly in face of arbitrary sequences of CPU-hotplug events, and should therefore be tested with arbitrary CPU-hotplug tests. And RCU also most certainly needs to refrain from issuing spurious warning messages that might over time be ignored, possibly causing someone to miss a real bug. My concern with this patch is in the second spurious-warning area.
Not sure I answered the actual question, though...
Thanx, Paul
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