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Date:   Thu, 27 Jan 2022 17:41:44 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Donnefort <vincent.donnefort@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        YueHaibing <yuehaibing@huawei.com>,
        Baokun Li <libaokun1@huawei.com>,
        Randy Dunlap <rdunlap@infradead.org>, kexec@lists.infradead.org
Subject: Re: [PATCHv2] kexec: disable cpu hotplug until the rebooting cpu is
 stable
Message-ID: <20220127094144.GC13508@MiWiFi-R3L-srv>
References: <20220127090215.32000-1-kernelfans@gmail.com>
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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
>