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

[Joel Fernandes]

On Fri, Nov 25, 2022 at 11:54:27PM +0800, 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.
> 
> Therefore, this commit add cpus_read_lock/unlock() before executing
> num_online_cpus().

I am not sure if this is needed. The only way what you suggest can happen is
if the tasks-RCU protected data is accessed after the num_online_cpus() value is
decremented on the CPU going offline.

However, the number of online CPUs value is changed on a CPU other than the
CPU going offline.

So there's no way the CPU going offline can run any code (it is already
dead courtesy of CPUHP_AP_IDLE_DEAD). So a corruption is impossible.

Or, did I miss something?

thanks,

 - Joel



> 
> 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);
> -- 
> 2.25.1
>