On Wed, Jan 26, 2022 at 12:29 AM Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
> On 25/01/22 11:39, 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);
> >
> > 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.
> >
>
> So is this supposed to be a refactor with no change in behaviour? AFAICT it
> actually does change things (and isn't necessarily clearer).
>
Yes, as you have seen, it does change behavior. Before this patch,
there is a breakage:
migrate_to_reboot_cpu();
cpu_hotplug_enable();
----------> technical, here can
comes a cpu_down(this_cpu)
machine_shutdown();
And this patch squeezes out this breakage.
> > 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();
> > +
>
> Not all archs map machine_shutdown() to smp_shutdown_nonboot_cpus(), other
> archs will now be missing a cpu_hotplug_enable() prior to a kexec
> machine_shutdown(). That said, AFAICT none of those archs rely on the
> hotplug machinery in machine_shutdown(), so it might be OK, but that's not
> obvious at all.
>
At the first glance, it may be not obvious, but tracing down
cpu_hotplug_enable() to the variable cpu_hotplug_disabled, you can
find out the limited involved functions are all related to
cpu_up/cpu_down.
IOW, if no code path connects with the interface of cpu_up/cpu_down,
then kexec-reboot will not be affected.
And after this patch, it is more clear how to handle the following cases:
arch/arm/kernel/reboot.c:94: smp_shutdown_nonboot_cpus(reboot_cpu);
arch/arm64/kernel/process.c:88: smp_shutdown_nonboot_cpus(reboot_cpu);
arch/ia64/kernel/process.c:578: smp_shutdown_nonboot_cpus(reboot_cpu);
Thanks,
Pingfan
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