linux-kernel - Re: RCU used on incoming CPU before rcu_cpu

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Date:   Thu, 9 Mar 2017 16:12:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com, fweisbec@...il.com
Subject: Re: RCU used on incoming CPU before rcu_cpu_starting() called

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.

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 :/

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);
 }