cpu/hotplug: Create a new function to shutdown nonboot cpus

This function will be used later in machine_shutdown() for some
architectures.

disable_nonboot_cpus() is not safe to use when doing machine_down(),
because it relies on freeze_secondary_cpus() which in turn is a
suspend/resume related freeze and could abort if the logic detects any
pending activities that can prevent finishing the offlining process.
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200323135110.30522-3-qais.yousef@arm.com

include/linux/cpu.h

@@ -120,6 +120,7 @@ extern void cpu_hotplug_enable(void);
 void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
 int remove_cpu(unsigned int cpu);
+extern void smp_shutdown_nonboot_cpus(unsigned int primary_cpu);
 
 #else /* CONFIG_HOTPLUG_CPU */
 
@@ -131,6 +132,7 @@ static inline int  cpus_read_trylock(void) { return true; }
 static inline void lockdep_assert_cpus_held(void) { }
 static inline void cpu_hotplug_disable(void) { }
 static inline void cpu_hotplug_enable(void) { }
+static inline void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) { }
 #endif	/* !CONFIG_HOTPLUG_CPU */
 
 /* Wrappers which go away once all code is converted */

kernel/cpu.c

@@ -1069,6 +1069,48 @@ int remove_cpu(unsigned int cpu)
 }
 EXPORT_SYMBOL_GPL(remove_cpu);
 
+void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
+{
+	unsigned int cpu;
+	int error;
+
+	cpu_maps_update_begin();
+
+	/*
+	 * Make certain the cpu I'm about to reboot on is online.
+	 *
+	 * This is inline to what migrate_to_reboot_cpu() already do.
+	 */
+	if (!cpu_online(primary_cpu))
+		primary_cpu = cpumask_first(cpu_online_mask);
+
+	for_each_online_cpu(cpu) {
+		if (cpu == primary_cpu)
+			continue;
+
+		error = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
+		if (error) {
+			pr_err("Failed to offline CPU%d - error=%d",
+				cpu, error);
+			break;
+		}
+	}
+
+	/*
+	 * Ensure all but the reboot CPU are offline.
+	 */
+	BUG_ON(num_online_cpus() > 1);
+
+	/*
+	 * Make sure the CPUs won't be enabled by someone else after this
+	 * point. Kexec will reboot to a new kernel shortly resetting
+	 * everything along the way.
+	 */
+	cpu_hotplug_disabled++;
+
+	cpu_maps_update_done();
+}
+
 #else
 #define takedown_cpu		NULL
 #endif /*CONFIG_HOTPLUG_CPU*/