Messages in this thread Patch in this message | | | Date | Fri, 27 Jun 2008 10:17:38 +0530 | From | Gautham R Shenoy <> | Subject | Re: [PATCH] fix rcu vs hotplug race |
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On Thu, Jun 26, 2008 at 08:27:28AM -0700, Paul E. McKenney wrote: > 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.
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! . . . rcu_cpu_notify: . -------- WINDOW ENDS ------------------------------------------ rcu_offline_cpu() /* Which calls cpu_quiet() * which removes * cpu from rcp->cpumask. */
If a new batch was started just before calling stop_machine_run(), the "tobe-offlined" cpu is still present in rcp-cpumask.
During a cpu-offline, from take_cpu_down(), we queue an rt-prio idle task as the next task to be picked by the scheduler. We also call cpu_disable() which will disable any further interrupts and remove the cpu's bit from the cpu_online_map.
Once the stop_machine_run() successfully calls take_cpu_down(), it calls schedule(). That's the last time a schedule is called on the offlined cpu, and hence the last time when rdp->passed_quiesc will be set to 1 through rcu_qsctr_inc().
But the cpu_quiet() will be on this cpu will be called only when the next RCU_SOFTIRQ occurs on this CPU. So at this time, the offlined CPU is still set in rcp->cpumask.
Now coming back to the idle_task which truely offlines the CPU, it does check for a pending RCU and raises the softirq, since it will find rdp->passed_quiesc to be 0 in this case. However, since the cpu is offline I am not sure if the softirq will trigger on the CPU.
Even if it doesn't the rcu_offline_cpu() will find that rcp->completed is not the same as rcp->cur, which means that our cpu could be holding up the grace period progression. Hence we call cpu_quiet() and move ahead.
But because of the window explained in the timeline, we could still have a call_rcu() before the RCU subsystem executes it's CPU_DEAD notification, and we send smp_send_reschedule() to offlined cpu while trying to force the quiescent states. The appended patch adds comments and prevents checking for offlined cpu everytime.
Author: Gautham R Shenoy <ego@in.ibm.com> Date: Fri Jun 27 09:33:55 2008 +0530
cpu_online_map is updated by the _cpu_down() using stop_machine_run(). Since force_quiescent_state is invoked from irqs disabled section, stop_machine_run() won't be executing while a cpu is executing force_quiescent_state(). Hence the cpu_online_map is stable while we're in the irq disabled section.
However, a cpu might have been offlined _just_ before we disabled irqs while entering force_quiescent_state(). And rcu subsystem might not yet have handled the CPU_DEAD notification, leading to the offlined cpu's bit being set in the rcp->cpumask.
Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent sending smp_reschedule() to an offlined CPU.
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com> Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c index f4ffbd0..a38895a 100644 --- a/kernel/rcuclassic.c +++ b/kernel/rcuclassic.c @@ -89,8 +89,22 @@ static void force_quiescent_state(struct rcu_data *rdp, /* * Don't send IPI to itself. With irqs disabled, * rdp->cpu is the current cpu. + * + * cpu_online_map is updated by the _cpu_down() + * using stop_machine_run(). Since we're in irqs disabled + * section, stop_machine_run() is not exectuting, hence + * the cpu_online_map is stable. + * + * However, a cpu might have been offlined _just_ before + * we disabled irqs while entering here. + * And rcu subsystem might not yet have handled the CPU_DEAD + * notification, leading to the offlined cpu's bit + * being set in the rcp->cpumask. + * + * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent + * sending smp_reschedule() to an offlined CPU. */ - cpumask = rcp->cpumask; + cpus_and(cpumask, rcp->cpumask, cpu_online_map); cpu_clear(rdp->cpu, cpumask); for_each_cpu_mask(cpu, cpumask) smp_send_reschedule(cpu);
> > Not sure I answered the actual question, though... > > Thanx, Paul
-- Thanks and Regards gautham
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