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

[Paul E. McKenney]

On Wed, Oct 28, 2020 at 02:23:34PM -0400, Qian Cai wrote:
> 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:
> 
>  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>

Acked-by: Paul E. McKenney <paulmck@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
>