On Mon, May 09 2022 at 12:13, Pingfan Liu wrote:
> 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:
I don't see what's broken here.
> 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.
Confusing != broken.
> +/* primary_cpu keeps unchanged after migrate_to_reboot_cpu() */
This comment makes no sense.
> 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.
You want to reduce confusion, but in reality this is even more confusing
than before.
> */
> - if (!cpu_online(primary_cpu))
> - primary_cpu = cpumask_first(cpu_online_mask);
> + __cpu_hotplug_enable();
How is this decrement solving anything? At the end of this function, the
counter is incremented again. So what's the point of this exercise?
> 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.
What does that for arch/powerpc/kernel/kexec_machine64.c now?
Nothing, as far as I can tell. Which means you basically reverted
011e4b02f1da ("powerpc, kexec: Fix "Processor X is stuck" issue during
kexec from ST mode") unless I'm completely confused.
> */
> - cpu_hotplug_enable();
This is tinkering at best. Can we please sit down and rethink this whole
machinery instead of applying random duct tape to it?
Thanks,
tglx
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