Re: [PATCH] smpboot: add missing get_online_cpus() when register

[Lai Jiangshan]

On 08/02/2014 05:54 AM, David Rientjes wrote:
> On Thu, 31 Jul 2014, Lai Jiangshan wrote:
> 
>> If the smpboot_register_percpu_thread() is called after smpboot_create_threads()
>> but before __cpu_up(), the smpboot thread of the online-ing CPU is not created,
>> and it results a bug.  So we use get_online_cpus() to prevent it.
>>
> 
> Do you have an example of the bug to include?  

Sorry, no, I don't have.

> Maintainers are going to 
> need to understand the implications of the problem before the 
> stable@xxxxxxxxxx annotation is warranted.

It is possible that smpboot_register_percpu_thread() can be called
any time in current kernel.  Repeating the module ehca and check while
repeating online/offline the CPUs, the bug is possible to hit.  I have not such
devices to test.

Let Thomas make the choice.

> 
>> smpboot_unregister_percpu_thread() travels all possible CPU, it doesn't need
>> get_online_cpus() which is removed in the patch.
>>
>> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>> CC: stable@xxxxxxxxxx
>> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> ---
>>  kernel/smpboot.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
>> index eb89e18..8adab87 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;
>>  }
> 
> I think the {get,put}_online_cpus() pair should be nested inside the 
> smpboot_threads_lock for better lock ordering since not all cases 
> smpboot_threads_lock will require it.
> 
> That way, you can also do put_online_cpus() before 
> smpboot_destroy_threads(), which you have already proven doesn't need it:
> 
> @@ -280,14 +280,17 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
>  	int ret = 0;
>  
>  	mutex_lock(&smpboot_threads_lock);
> +	get_online_cpus();

get_online_cpus() can't be nested in smpboot_threads_lock.


>  	for_each_online_cpu(cpu) {
>  		ret = __smpboot_create_thread(plug_thread, cpu);
>  		if (ret) {
> +			put_online_cpus();
>  			smpboot_destroy_threads(plug_thread);
>  			goto out;
>  		}
>  		smpboot_unpark_thread(plug_thread, cpu);
>  	}
> +	put_online_cpus();
>  	list_add(&plug_thread->list, &hotplug_threads);
>  out:
>  	mutex_unlock(&smpboot_threads_lock);
> 
>>  EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
>> @@ -303,11 +305,9 @@ EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
>>   */
>>  void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread)
>>  {
>> -	get_online_cpus();
>>  	mutex_lock(&smpboot_threads_lock);
>>  	list_del(&plug_thread->list);
>>  	smpboot_destroy_threads(plug_thread);
>>  	mutex_unlock(&smpboot_threads_lock);
>> -	put_online_cpus();
>>  }
>>  EXPORT_SYMBOL_GPL(smpboot_unregister_percpu_thread);
> 
> This makes sense.
> .
> 

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