[PATCH] kexec: disable cpu hotplug until the rebooting cpu is stable

[Pingfan Liu]

The following identical code piece appears in both
migrate_to_reboot_cpu() and smp_shutdown_nonboot_cpus():

	if (!cpu_online(primary_cpu))
		primary_cpu = cpumask_first(cpu_online_mask);

Although the kexec-reboot task can get through a cpu_down() on its cpu,
this code looks a little confusing.

Make things straight forward by keep cpu hotplug disabled until
smp_shutdown_nonboot_cpus() holds cpu_add_remove_lock. By this way, the
rebooting cpu can keep unchanged.

Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
Cc: Vincent Donnefort <vincent.donnefort@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: YueHaibing <yuehaibing@xxxxxxxxxx>
Cc: Baokun Li <libaokun1@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: kexec@xxxxxxxxxxxxxxxxxxx
To: linux-kernel@xxxxxxxxxxxxxxx
---
 kernel/cpu.c        | 16 ++++++++++------
 kernel/kexec_core.c | 10 ++++------
 2 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 407a2568f35e..bc687d59ca90 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1227,20 +1227,24 @@ int remove_cpu(unsigned int cpu)
 }
 EXPORT_SYMBOL_GPL(remove_cpu);
 
+/* primary_cpu keeps unchanged after migrate_to_reboot_cpu() */
 void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
 {
 	unsigned int cpu;
 	int error;
 
+	/*
+	 * Block other cpu hotplug event, so primary_cpu is always online if
+	 * it is not touched by us
+	 */
 	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.
+	 * migrate_to_reboot_cpu() disables CPU hotplug assuming that
+	 * no further code needs to use CPU hotplug (which is true in
+	 * the reboot case). However, the kexec path depends on using
+	 * CPU hotplug again; so re-enable it here.
 	 */
-	if (!cpu_online(primary_cpu))
-		primary_cpu = cpumask_first(cpu_online_mask);
+	__cpu_hotplug_enable();
 
 	for_each_online_cpu(cpu) {
 		if (cpu == primary_cpu)
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 68480f731192..db4fa6b174e3 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1168,14 +1168,12 @@ int kernel_kexec(void)
 		kexec_in_progress = true;
 		kernel_restart_prepare("kexec reboot");
 		migrate_to_reboot_cpu();
-
 		/*
-		 * migrate_to_reboot_cpu() disables CPU hotplug assuming that
-		 * no further code needs to use CPU hotplug (which is true in
-		 * the reboot case). However, the kexec path depends on using
-		 * CPU hotplug again; so re-enable it here.
+		 * migrate_to_reboot_cpu() disables CPU hotplug. If an arch
+		 * relies on the cpu teardown to achieve reboot, it needs to
+		 * re-enable CPU hotplug there.
 		 */
-		cpu_hotplug_enable();
+
 		pr_notice("Starting new kernel\n");
 		machine_shutdown();
 	}
-- 
2.31.1