[PATCH RFC] reboot: hotplug cpus in migrate_to_reboot_cpu()

[Hsin-Yi Wang <hsinyi@chromium.org>]

Currently system reboots use arch specific codes (eg. smp_send_stop) to
offline non reboot cpus. Some arch like arm64, arm, and x86... set offline
masks to cpu without really offlining them. Thus it causes some race
condition and kernel warning comes out sometimes when system reboots. We
can do cpu hotplug in migrate_to_reboot_cpu() to avoid this issue.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
---
kernel warnings at reboot:
[1] https://lore.kernel.org/lkml/20190820100843.3028-1-hsinyi@chromium.org/
[2] https://lore.kernel.org/lkml/20190727164450.GA11726@roeck-us.net/
---
 kernel/cpu.c    | 35 +++++++++++++++++++++++++++++++++++
 kernel/reboot.c | 18 ------------------
 2 files changed, 35 insertions(+), 18 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index fc28e17940e0..2f4d51fe91e3 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -31,6 +31,7 @@
 #include <linux/relay.h>
 #include <linux/slab.h>
 #include <linux/percpu-rwsem.h>
+#include <linux/reboot.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -1366,6 +1367,40 @@ int __boot_cpu_id;
 
 #endif /* CONFIG_SMP */
 
+void migrate_to_reboot_cpu(void)
+{
+	/* The boot cpu is always logical cpu 0 */
+	int cpu = reboot_cpu;
+
+	/* Make certain the cpu I'm about to reboot on is online */
+	if (!cpu_online(cpu))
+		cpu = cpumask_first(cpu_online_mask);
+
+	/* Prevent races with other tasks migrating this task */
+	current->flags |= PF_NO_SETAFFINITY;
+
+	/* Make certain I only run on the appropriate processor */
+	set_cpus_allowed_ptr(current, cpumask_of(cpu));
+
+	/* Hotplug other cpus if possible */
+	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
+		int i, err;
+
+		cpu_maps_update_begin();
+
+		for_each_online_cpu(i) {
+			if (i == cpu)
+				continue;
+			err = _cpu_down(i, 0, CPUHP_OFFLINE);
+			if (err)
+				pr_info("Failed to offline cpu %d\n", i);
+		}
+		cpu_hotplug_disabled++;
+
+		cpu_maps_update_done();
+	}
+}
+
 /* Boot processor state steps */
 static struct cpuhp_step cpuhp_hp_states[] = {
 	[CPUHP_OFFLINE] = {
diff --git a/kernel/reboot.c b/kernel/reboot.c
index c4d472b7f1b4..f0046be34a60 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -215,24 +215,6 @@ void do_kernel_restart(char *cmd)
 	atomic_notifier_call_chain(&restart_handler_list, reboot_mode, cmd);
 }
 
-void migrate_to_reboot_cpu(void)
-{
-	/* The boot cpu is always logical cpu 0 */
-	int cpu = reboot_cpu;
-
-	cpu_hotplug_disable();
-
-	/* Make certain the cpu I'm about to reboot on is online */
-	if (!cpu_online(cpu))
-		cpu = cpumask_first(cpu_online_mask);
-
-	/* Prevent races with other tasks migrating this task */
-	current->flags |= PF_NO_SETAFFINITY;
-
-	/* Make certain I only run on the appropriate processor */
-	set_cpus_allowed_ptr(current, cpumask_of(cpu));
-}
-
 /**
  *	kernel_restart - reboot the system
  *	@cmd: pointer to buffer containing command to execute for restart
-- 
2.23.0.444.g18eeb5a265-goog