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

[Jinjie Ruan]

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 at arm.com>
Cc: Jonathan Cameron <jic23 at kernel.org>
Cc: James Morse <james.morse at arm.com>
Cc: stable at 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 at arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
---
 arch/arm64/kernel/smp.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 1aa324104afb..543d3459f6e3 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -535,23 +535,13 @@ void arch_unregister_cpu(int cpu)
 {
 	acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
 	struct cpu *c = &per_cpu(cpu_devices, cpu);
-	acpi_status status;
 	unsigned long long sta;
-
-	if (!acpi_handle) {
-		pr_err_once("Removing a CPU without associated ACPI handle\n");
-		return;
-	}
+	acpi_status status;
 
 	status = acpi_evaluate_integer(acpi_handle, "_STA", NULL, &sta);
-	if (ACPI_FAILURE(status))
-		return;
-
-	/* For now do not allow anything that looks like physical CPU HP */
-	if (cpu_present(cpu) && !(sta & ACPI_STA_DEVICE_PRESENT)) {
+	if (!ACPI_FAILURE(status) &&
+	    cpu_present(cpu) && !(sta & ACPI_STA_DEVICE_PRESENT))
 		pr_err_once("Changing CPU present bit is not supported\n");
-		return;
-	}
 
 	unregister_cpu(c);
 }
-- 
2.34.1