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

From: Joel Fernandes
Date: Fri Nov 25 2022 - 21:28:07 EST


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
>