Re: [PATCH 09/19] arm64: smp: Defer RCU registration during secondary CPU bringup

[Jinjie Ruan <ruanjinjie@huawei.com>]

在 2026/9/8 0:40, Will Deacon 写道:
> Calling rcutree_report_cpu_starting() early during boot can lead to
> livelocks with the generic CPU hotplug mechanism if the boot CPU blocks
> on an RCU grace period while the CPU being onlined is spinning in
> cpuhp_ap_sync_alive().
> 
> In preparation for enabling the generic CPU hotplug code on arm64, split
> up the trace_hardirqs_off() call during secondary CPU bringup so that we
> update lockdep early but defer the tracing updates until after
> notify_cpu_starting() has registered the new CPU with RCU, allowing us
> to drop the explicit call to rcutree_report_cpu_starting() entirely.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/smp.c | 5 ++---
>  include/linux/rcutree.h | 2 +-
>  2 files changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index f4cabf9e19e6..ff68640d0c0b 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -217,8 +217,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>  	if (system_uses_irq_prio_masking())
>  		init_gic_priority_masking();
>  
> -	rcutree_report_cpu_starting(cpu);
> -	trace_hardirqs_off();

Hi Will,

I think we need to handle the printk problem before this patch as we
discussed earlier.

Otherwise defer the rcutree_report_cpu_starting() will trigger a
false-positive lockdep"suspicious RCU usage" splat during early lock
acquisitions as commit ce3d31ad3cac ("arm64/smp: Move
rcu_cpu_starting() earlier") pointed out.

Link:
https://lore.kernel.org/all/483e471d-d51b-4991-98d8-c0b4a2cdf8d8@huawei.com/

Best regards,
Jinjie

> +	lockdep_hardirqs_off(CALLER_ADDR0);
>  
>  	/*
>  	 * If the system has established the capabilities, make sure
> @@ -242,6 +241,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>  	 * Enable GIC and timers.
>  	 */
>  	notify_cpu_starting(cpu);
> +	trace_hardirqs_off_finish();
>  
>  	ipi_setup(cpu);
>  
> @@ -411,7 +411,6 @@ void __noreturn cpu_die_early(void)
>  
>  	/* Mark this CPU absent */
>  	set_cpu_present(cpu, 0);
> -	rcutree_report_cpu_dead();
>  
>  	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
>  		update_cpu_boot_status(CPU_KILL_ME);
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index 16a04202888b..e6ad4f2a475c 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -116,7 +116,7 @@ int rcutree_offline_cpu(unsigned int cpu);
>  
>  void rcutree_migrate_callbacks(int cpu);
>  
> -/* Called from hotplug and also arm64 early secondary boot failure */
> +/* Called from hotplug */
>  void rcutree_report_cpu_dead(void);
>  
>  #endif /* __LINUX_RCUTREE_H */