Re: RCU used on incoming CPU before rcu_cpu_starting() called

Paul E. McKenney
2017-03-09T15:30:14Z

On Thu, Mar 09, 2017 at 04:12:55PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 09, 2017 at 02:08:23PM +0100, Thomas Gleixner wrote:
> > On Wed, 8 Mar 2017, Paul E. McKenney wrote:
> > > [ 30.694013] lockdep_rcu_suspicious+0xe7/0x120
> > > [ 30.694013] get_work_pool+0x82/0x90
> > > [ 30.694013] __queue_work+0x70/0x5f0
> > > [ 30.694013] queue_work_on+0x33/0x70
> > > [ 30.694013] clear_sched_clock_stable+0x33/0x40
> > > [ 30.694013] early_init_intel+0xe7/0x2f0
> > > [ 30.694013] init_intel+0x11/0x350
> > > [ 30.694013] identify_cpu+0x344/0x5a0
> > > [ 30.694013] identify_secondary_cpu+0x18/0x80
> > > [ 30.694013] smp_store_cpu_info+0x39/0x40
> > > [ 30.694013] start_secondary+0x4e/0x100
> > > [ 30.694013] start_cpu+0x14/0x14
> > >
> > > Here is the relevant code from x86's smp_callin():
> > >
> > > /*
> > > * Save our processor parameters. Note: this information
> > > * is needed for clock calibration.
> > > */
> > > smp_store_cpu_info(cpuid);
> > >
> > > The problem is that smp_store_cpu_info() indirectly invokes
> > > schedule_work(), which wants to use RCU. But RCU isn't informed
> > > of the incoming CPU until the call to notify_cpu_starting(), which
> > > causes lockdep to complain bitterly about the use of RCU by the
> > > premature call to schedule_work().
> >
> > Right. And that want's to be fixed, not hacked around by silencing RCU.
> >
> > Peter????
>
> I'm thinking this is hotplug? 30 seconds after boot is far too late for
> SMP bringup, or you have a stupid slow machine.

And this certainly does qualify as "shortly", thank you!

Yes, this only happens on hotplug with lockdep enabled, specifically
on rcutorture scenarios TASKS01 and TREE05.

> Because it only calls schedule_work() after SMP-init. In which case
> there's then two cases, either:
>
> - TSC was stable, hotplug wrecked it, TSC is now unstable, and we're
> screwed.
>
> - TSC was unstable, hotplug triggers and we want to mark it unstable
> _again_.
>
> If this is the second, the below should fix it, if its the first, I've
> no idea yet on how to fix that properly :/

I have applied this patch and started tests on TREE05 and TASKS01, should
get results shortly.

Thanx, Paul

> Bloody hotplug..
>
> ---
> kernel/sched/clock.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index a08795e..eecf388 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -172,7 +172,7 @@ void clear_sched_clock_stable(void)
>
> smp_mb(); /* matches sched_clock_init_late() */
>
> - if (sched_clock_running == 2)
> + if (sched_clock_running == 2 && sched_clock_stable())
> schedule_work(&sched_clock_work);
> }
>
>