Re: [PATCH] powerpc/smp: Move rcu_cpu_starting() earlier

[Paul E. McKenney]

On Thu, Oct 29, 2020 at 11:09:07AM +1100, Michael Ellerman wrote:
> Qian Cai <cai@xxxxxxxxxx> writes:
> > The call to rcu_cpu_starting() in start_secondary() is not early enough
> > in the CPU-hotplug onlining process, which results in lockdep splats as
> > follows:
> 
> Since when?
> What kernel version?
> 
> I haven't seen this running CPU hotplug tests with PROVE_LOCKING=y on
> v5.10-rc1. Am I missing a CONFIG?

My guess would be that adding CONFIG_PROVE_RAW_LOCK_NESTING=y will
get you some splats.

							Thanx, Paul

> cheers
> 
> 
> >  WARNING: suspicious RCU usage
> >  -----------------------------
> >  kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
> >
> >  other info that might help us debug this:
> >
> >  RCU used illegally from offline CPU!
> >  rcu_scheduler_active = 1, debug_locks = 1
> >  no locks held by swapper/1/0.
> >
> >  Call Trace:
> >  dump_stack+0xec/0x144 (unreliable)
> >  lockdep_rcu_suspicious+0x128/0x14c
> >  __lock_acquire+0x1060/0x1c60
> >  lock_acquire+0x140/0x5f0
> >  _raw_spin_lock_irqsave+0x64/0xb0
> >  clockevents_register_device+0x74/0x270
> >  register_decrementer_clockevent+0x94/0x110
> >  start_secondary+0x134/0x800
> >  start_secondary_prolog+0x10/0x14
> >
> > This is avoided by moving the call to rcu_cpu_starting up near the
> > beginning of the start_secondary() function. Note that the
> > raw_smp_processor_id() is required in order to avoid calling into
> > lockdep before RCU has declared the CPU to be watched for readers.
> >
> > Link: https://lore.kernel.org/lkml/160223032121.7002.1269740091547117869.tip-bot2@tip-bot2/
> > Signed-off-by: Qian Cai <cai@xxxxxxxxxx>
> > ---
> >  arch/powerpc/kernel/smp.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 3c6b9822f978..8c2857cbd960 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -1393,13 +1393,14 @@ static void add_cpu_to_masks(int cpu)
> >  /* Activate a secondary processor. */
> >  void start_secondary(void *unused)
> >  {
> > -	unsigned int cpu = smp_processor_id();
> > +	unsigned int cpu = raw_smp_processor_id();
> >  
> >  	mmgrab(&init_mm);
> >  	current->active_mm = &init_mm;
> >  
> >  	smp_store_cpu_info(cpu);
> >  	set_dec(tb_ticks_per_jiffy);
> > +	rcu_cpu_starting(cpu);
> >  	preempt_disable();
> >  	cpu_callin_map[cpu] = 1;
> >  
> > -- 
> > 2.28.0