[PATCHv3 1/2] cpu/hotplug: Keep cpu hotplug disabled until the rebooting cpu is stable

[Pingfan Liu <kernelfans@gmail.com>]

The following code chunk repeats in both
migrate_to_reboot_cpu() and smp_shutdown_nonboot_cpus():

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

This is due to a breakage like the following:
kernel_kexec()
   migrate_to_reboot_cpu();
   cpu_hotplug_enable();
                        -----------> comes a cpu_down(this_cpu) on other cpu
   machine_shutdown();
     smp_shutdown_nonboot_cpus(); // re-check "if (!cpu_online(primary_cpu))" to protect against the former breakin

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 keeping cpu hotplug disabled until
smp_shutdown_nonboot_cpus() holds cpu_add_remove_lock. By this way, the
breakage is squashed out and the rebooting cpu can keep unchanged.

Note: this patch only affects the kexec-reboot on arches(arm/arm64/ia64/riscv) ,
which rely on the cpu hot-removing mechanism.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Vincent Donnefort <vincent.donnefort@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: YueHaibing <yuehaibing@huawei.com>
Cc: Baokun Li <libaokun1@huawei.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: kexec@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
 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 d0a9aa0b42e8..bf901fc90329 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1228,20 +1228,24 @@ 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