Re: [PATCH v4 1/2] arm64: smp: Fix hot-unplug tearing by forcing unregistration

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jun 10, 2026 at 03:52:01PM +0800, Jinjie Ruan wrote:
> Sashiko review pointed out the following issue[1].
> 
> Commit eba4675008a6 ("arm64: arch_register_cpu() variant to check if
> an ACPI handle is now available.") introduced architectural safety
> blocks inside arch_unregister_cpu(). If a hot-unplug operation is
> determined to be a physical hardware removal (where _STA evaluates to
> !ACPI_STA_DEVICE_PRESENT), or if firmware evaluation fails, it aborts
> the unregistration transaction early to protect unreadied arm64
> infrastructure.
> 
> However, returning early from arch_unregister_cpu() causes a catastrophic
> state tearing because the generic ACPI layer (acpi_processor_post_eject())
> unconditionally continues its cleanup flow. This leaves the stale sysfs
> device leaked in the memory, deadlocking any subsequent hot-add attempts
> on the same CPU.
> 
> Fix it by simplifying arch_unregister_cpu() to always proceed with
> the unregistration, as a pr_err_once() warning is sufficient to make
> it more visible for currently not supported physical CPU removal.
> Also remove the redundant NULL check on acpi_handle as it cannot be
> NULL when calling arch_unregister_cpu().
> 
> [1]: https://sashiko.dev/#/patchset/20260520022023.126670-1-ruanjinjie@huawei.com
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Jonathan Cameron <jic23@kernel.org>
> Cc: James Morse <james.morse@arm.com>
> Cc: stable@vger.kernel.org
> Fixes: eba4675008a6e ("arm64: arch_register_cpu() variant to check if an ACPI handle is now available.")
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>