On 12/08/2014 09:54 PM, Steven Rostedt wrote:
> On Mon, 8 Dec 2014 14:27:01 +1100
> Anton Blanchard <an...@samba.org> 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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