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

Baoquan He
2022-01-27T09:42:00Z

Hi Pingfan,

On 01/27/22 at 05:02pm, Pingfan Liu wrote:
> 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.

If I didn't go through code wrongly, you may miss the x86 case.
Several ARCHes do call smp_shutdown_nonboot_cpus() in machine_shutdown()
in kexec reboot code path, while x86 doesn't. If I am right, you may
need reconsider if this patch is needed or need be adjustd.

Are you optimizing code path, or you meet a real problem? I haven't
checked v1, but I also didn't see it's told in patch log which case it
is.

>
> 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
>