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

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

[Pingfan Liu]

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.

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: 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 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();
+
 		pr_notice("Starting new kernel\n");
 		machine_shutdown();
 	}
-- 
2.31.1

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

[Valentin Schneider]

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).

> 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: 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 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.

>  		pr_notice("Starting new kernel\n");
>  		machine_shutdown();
>  	}
> -- 
> 2.31.1

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

[Pingfan Liu]

On Wed, Jan 26, 2022 at 12:29 AM Valentin Schneider
<valentin.schneider@arm.com> 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@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: 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 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

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

[Valentin Schneider]

On 26/01/22 10:45, Pingfan Liu wrote:
> On Wed, Jan 26, 2022 at 12:29 AM Valentin Schneider
> <valentin.schneider@arm.com> 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.
>

Ok, that's worth pointing out in the changelog.

>> > 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.
>

That's my point, this only works if the other archs truly do not rely on
hotplug for machine_shutdown(), which seems to be the case but it wouldn't
hurt for you to double-check that and explicitely call it out in the
changelog.

> 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);
>

FWIW riscv is also concerned.

> Thanks,
> Pingfan

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

[Pingfan Liu]

On Wed, Jan 26, 2022 at 11:06 PM Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 26/01/22 10:45, Pingfan Liu wrote:
> > On Wed, Jan 26, 2022 at 12:29 AM Valentin Schneider
> > <valentin.schneider@arm.com> 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.
> >
>
> Ok, that's worth pointing out in the changelog.
>
Sure, I will update it.

> >> > 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.
> >
>
> That's my point, this only works if the other archs truly do not rely on
> hotplug for machine_shutdown(), which seems to be the case but it wouldn't
> hurt for you to double-check that and explicitely call it out in the
> changelog.
>
OK, I will update the change log. BTW, besides x86, I have just
finished the test on powerpc, both of them works fine with this patch

> > 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);
> >
>
> FWIW riscv is also concerned.

I think its current statement is right.
arch/riscv/kernel/machine_kexec.c:135:
smp_shutdown_nonboot_cpus(smp_processor_id());

Thanks,

Pingfan
>
> > Thanks,
> > Pingfan