RE: [PATCH] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug

[Zhang, Qiang1]

>Hi Zqiang,
>
>On 11/25/2022 9:24 PM, Zqiang wrote:
> Currently, for the case of num_online_cpus() <= 1, return directly,
> indicates the end of current grace period and then release old data.
> it's not accurate, for SMP system, when num_online_cpus() is equal
> one, maybe another cpu that in offline process(after invoke
> __cpu_disable()) is still in the rude RCU-Tasks critical section
> holding the old data, this lead to memory corruption.
> 
>
>
>Was this race seen in your testing? For the outgoing CPU, once that
>CPU marks itself offline (and decrements __num_online_cpus), do we
>have tracing active on that CPU, and synchronize_rcu_tasks_rude()
>not waiting for it could potentially lead to memory corruption?

Hi Neeraj

Indeed, I didn't see race in the actual production environment,
Maybe my commit information description is not accurate enough,
like the scene I described with joel.

If in cpuhp_invoke_callback, some callback is in rude rcu-tasks read ctrical section,
and still holding old data, but in this time, synchronize_rcu_tasks_rude() not waiting,
and release old data.

Suppose the system has two cpus

	CPU0                                                                     CPU1
					     cpu_stopper_thread
                                                                                  take_cpu_down
						    __cpu_disable
							dec __num_online_cpus 
 rcu_tasks_rude_wait_gp                                      cpuhp_invoke_callback  
	num_online_cpus() == 1
		return;
        
when __num_online_cpus == 1, the CPU1 not completely offline.


>
>As per my understanding, given that outgoing/incoming CPU 
>decrements/increments the __num_online_cpus value, and num_online_cpus()
>is a plain read, problem could happen when the incoming CPU updates the
>__num_online_cpus  value, however, rcu_tasks_rude_wait_gp()'s 
>num_online_cpus() call didn't observe the increment. So, 
>cpus_read_lock/unlock() seems to be required to handle this case.

Yes, the same problem will be encountered when going online, due to
access  __num_online_cpus  that is not protected by cpus_read_lock/unlock() 
in rcu_tasks_rude_wait_gp().

Do I need to change the commit information to send v2?

Thanks
Zqiang

>
>
>Thanks
>Neeraj
>
> Therefore, this commit add cpus_read_lock/unlock() before executing
> num_online_cpus().
> 
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
>   kernel/rcu/tasks.h | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 4a991311be9b..08e72c6462d8 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
>   {
>   }
>   
> +static DEFINE_PER_CPU(struct work_struct, rude_work);
> +
>   // Wait for one rude RCU-tasks grace period.
>   static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>   {
> +	int cpu;
> +	struct work_struct *work;
> +
> +	cpus_read_lock();
>   	if (num_online_cpus() <= 1)
> -		return;	// Fastpath for only one CPU.
> +		goto end;// Fastpath for only one CPU.
>   
>   	rtp->n_ipis += cpumask_weight(cpu_online_mask) > -	schedule_on_each_cpu(rcu_tasks_be_rude);
> +	for_each_online_cpu(cpu) {
> +		work = per_cpu_ptr(&rude_work, cpu);
> +		INIT_WORK(work, rcu_tasks_be_rude);
> +		schedule_work_on(cpu, work);
> +	}
> +
> +	for_each_online_cpu(cpu)
> +		flush_work(per_cpu_ptr(&rude_work, cpu));
> +
> +end:
> +	cpus_read_unlock();
>   }
>   
>   void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);