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

[Neeraj Upadhyay <quic_neeraju@quicinc.com>]

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?

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.



Thanks
Neeraj

> Therefore, this commit add cpus_read_lock/unlock() before executing
> num_online_cpus().
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>   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);