Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier

[Paul E. McKenney]

On Wed, Oct 28, 2020 at 02:26:14PM -0400, Qian Cai wrote:
> The call to rcu_cpu_starting() in secondary_start_kernel() 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_backtrace+0x0/0x3c8
>   show_stack+0x14/0x60
>   dump_stack+0x14c/0x1c4
>   lockdep_rcu_suspicious+0x134/0x14c
>   __lock_acquire+0x1c30/0x2600
>   lock_acquire+0x274/0xc48
>   _raw_spin_lock+0xc8/0x140
>   vprintk_emit+0x90/0x3d0
>   vprintk_default+0x34/0x40
>   vprintk_func+0x378/0x590
>   printk+0xa8/0xd4
>   __cpuinfo_store_cpu+0x71c/0x868
>   cpuinfo_store_cpu+0x2c/0xc8
>   secondary_start_kernel+0x244/0x318
> 
> This is avoided by moving the call to rcu_cpu_starting up near the
> beginning of the secondary_start_kernel() function.
> 
> Link: https://lore.kernel.org/lkml/160223032121.7002.1269740091547117869.tip-bot2@tip-bot2/
> Signed-off-by: Qian Cai <cai@xxxxxxxxxx>

Interesting way to compute "cpu" earlier in the code, but nevertheless:

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

> ---
>  arch/arm64/kernel/smp.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 82e75fc2c903..09c96f57818c 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -222,6 +222,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>  	if (system_uses_irq_prio_masking())
>  		init_gic_priority_masking();
>  
> +	rcu_cpu_starting(cpu);
>  	preempt_disable();
>  	trace_hardirqs_off();
>  
> -- 
> 2.28.0
>