Subject: Re: [PATCH] kthread: kthread_bind fails to enforce CPU affinity (fixes kernel BUG at kernel/smpboot.c:134!)

From: Lai Jiangshan <[email-masked]>
Date: Tue, 9 Dec 2014 10:24:37 +0800

On 12/08/2014 09:54 PM, Steven Rostedt wrote:
> On Mon,  8 Dec 2014 14:27:01 +1100
> Anton Blanchard <[email-masked]> wrote:
> 
>> I have a busy ppc64le KVM box where guests sometimes hit the infamous
>> "kernel BUG at kernel/smpboot.c:134!" issue during boot:
>>
>> BUG_ON(td->cpu != smp_processor_id());
>>
>> Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops
>> output confirms it:
>>
>> CPU: 0
>> Comm: watchdog/130
>>
>> The issue is in kthread_bind where we set the cpus_allowed mask, but do
>> not touch task_thread_info(p)->cpu. The scheduler assumes the previously
>> scheduled CPU is in the cpus_allowed mask, but in this case we are
>> moving a thread to another CPU so it is not.
>>
> 
> Does this happen always on boot up, and always with the watchdog thread?
> 
> I followed the logic that starts the watchdog threads.
> 
> watchdog_enable_all_cpus()
>   smpboot_register_percpu-thread() {
> 
>     for_each_online_cpu(cpu) { ... }
> 
> Where watchdog_enable_all_cpus() can be called by
> lockup_detector_init() before SMP is started, but also by
> proc_dowatchdog() which is called by the sysctl commands (after SMP is
> up and running).
> 
> I noticed there's no "get_online_cpus()" anywhere, although the
> unregister_percpu_thread() has it. Is it possible that we created a
> thread on a CPU that wasn't fully online yet?
> 
> Perhaps the following patch is needed? Even if this isn't the solution
> to this bug, it is probably needed as watchdog_enable_all_cpus() can be
> called after boot up too.
> 
> -- Steve

Hi, Steven, tglx

See this https://lkml.org/lkml/2014/7/30/804
"[PATCH] smpboot: add missing get_online_cpus() when register"

Thanks,
Lai

> 
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index eb89e1807408..60d35ac5d3f1 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -279,6 +279,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
>  	unsigned int cpu;
>  	int ret = 0;
>  
> +	get_online_cpus();
>  	mutex_lock(&smpboot_threads_lock);
>  	for_each_online_cpu(cpu) {
>  		ret = __smpboot_create_thread(plug_thread, cpu);
> @@ -291,6 +292,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
>  	list_add(&plug_thread->list, &hotplug_threads);
>  out:
>  	mutex_unlock(&smpboot_threads_lock);
> +	put_online_cpus();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
> --
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