[PATCHv2] kexec: disable cpu hotplug until the rebooting cpu is stable

Pingfan Liu
2022-01-27T09:02:33Z

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);

This is due to a breakage like the following:
migrate_to_reboot_cpu();
cpu_hotplug_enable();
--> comes a cpu_down(this_cpu) on other cpu
machine_shutdown();

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, which rely on
cpu hotplug 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: kexec@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
v1 -> v2:
improve commit log

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 9c92147f0812..87bdf21de950 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1240,20 +1240,24 @@ 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