Re: [PATCH v3] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

[Will Deacon]

On Tue, Jun 02, 2026 at 08:14:26PM +0800, Jinjie Ruan wrote:
> On 6/2/2026 7:09 PM, Will Deacon wrote:
> > Please can you check the Sashiko review comment?
> > 
> > https://sashiko.dev/#/patchset/20260520022023.126670-1-ruanjinjie@xxxxxxxxxx
> 
> On arm64, arch_unregister_cpu() enforces a safety block against physical
> CPU hot-removal by aborting early if cpu_present() is true but the
> device is no longer physically present, thereby skipping
> unregister_cpu() and leaking the sysfs device node.
> If acpi_unmap_cpu() blindly clears the present bit while the sysfs
> device remains registered, a subsequent hot-add attempt will see the
> valid leaked device pointer, skip acpi_processor_hotadd_init() (and thus
> skip acpi_map_cpu()), leaving the hot-added CPU permanently absent and
> deadlocked.
> 
> Hi, Will,
> 
> what do you think about fix it like this?
> 
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -455,7 +455,6 @@ EXPORT_SYMBOL(acpi_map_cpu);
> 
>  int acpi_unmap_cpu(int cpu)
>  {
> -       set_cpu_present(cpu, false);
>         return 0;
>  }
>  EXPORT_SYMBOL(acpi_unmap_cpu);
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 5932e5b30b71..507c6d761434 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -554,6 +554,7 @@ void arch_unregister_cpu(int cpu)
>         }
> 
>         unregister_cpu(c);
> +       set_cpu_present(cpu, false);
>  }
>  #endif /* CONFIG_ACPI_HOTPLUG_CPU */

Hmm, not sure. Doesn't that break error handling cleanup paths that
expect acpi_unmap_cpu() to undo acpi_map_cpu()? See
acpi_processor_hotadd_init(), for example.

Will