[PATCH v1] cpu: Add missing check to cpuhp_smt_enable()

["Rafael J. Wysocki" <rafael@kernel.org>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Christian has reported that commit a430c11f4015 ("intel_idle: Rescan
"dead" SMT siblings during initialization") broke the use case in
which both nosmt and maxcpus were added to the kernel command line
because it caused CPUs that were not SMT siblings to be brought
online during the intel_idle driver initialization in violation of the
maxcpus limit.

The underlying reason for this is a missing topology_is_primary_thread()
check in cpuhp_smt_enable() which causes that function to put online
more CPUs than just the SMT siblings that it is supposed to handle.

Add the missing check to address the issue.

Fixes: a430c11f4015 ("intel_idle: Rescan "dead" SMT siblings during initialization")
Fixes: f694481b1d31 ("ACPI: processor: Rescan "dead" SMT siblings during initialization")
Closes: https://lore.kernel.org/linux-pm/724616a2-6374-4ba3-8ce3-ea9c45e2ae3b@arm.com/
Reported-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Cc: 6.16+ <stable@vger.kernel.org> # 6.16+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 kernel/cpu.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2710,6 +2710,12 @@
 	cpu_maps_update_begin();
 	cpu_smt_control = CPU_SMT_ENABLED;
 	for_each_present_cpu(cpu) {
+		/*
+		 * Avoid accidentally onlining primary thread CPUs that have
+		 * been taken offline.
+		 */
+		if (topology_is_primary_thread(cpu))
+			continue;
 		/* Skip online CPUs and CPUs on offline nodes */
 		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
 			continue;