Re: [PATCH] rcu: Avoid invalid wakeup for rcuc kthreads in RCU_KTHREAD_OFFCPU status

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Nov 16, 2022 at 02:07:28PM +0000, Zhang, Qiang1 wrote:
> On Tue, Nov 15, 2022 at 09:19:26PM +0800, Zqiang wrote:
> >And more important! On unpark time RCU_KTHREAD_OFFCPU isn't cleared. Only the
> >rcuc kthread does it, and after your patch it couldn't be awaken to perform
> >that, unless rcuc is lucky enough to have rcu_data.rcu_cpu_has_work = 1
> >by the time it unparks and that isn't guaranteed. So rcuc may sleep forever.
> 
> Thanks for review, yes I should register an unpark function to clear RCU_KTHREAD_OFFCPU.
> Is the following modification more appropriate?
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3ccad468887e..a2248af0ccda 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2375,7 +2375,8 @@ static void rcu_wake_cond(struct task_struct *t, int status)
>          * If the thread is yielding, only wake it when this
>          * is invoked from idle
>          */
> -       if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)))
> +       if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)) &&
> +                               status != RCU_KTHREAD_OFFCPU)
>                 wake_up_process(t);
>  }
> 
> @@ -2407,7 +2408,14 @@ static void invoke_rcu_core(void)
> 
>  static void rcu_cpu_kthread_park(unsigned int cpu)
>  {
> -       per_cpu(rcu_data.rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> +       WARN_ON_ONCE(cpu != smp_processor_id());
> +       __this_cpu_write(rcu_data.rcu_cpu_kthread_status, RCU_KTHREAD_OFFCPU);
> +}
> +
> +static void rcu_cpu_kthread_unpark(unsigned int cpu)
> +{
> +       WARN_ON_ONCE(cpu != smp_processor_id());
> +       __this_cpu_write(rcu_data.rcu_cpu_kthread_status, RCU_KTHREAD_ONCPU);
>  }
> 
>  static int rcu_cpu_kthread_should_run(unsigned int cpu)
> @@ -2460,6 +2468,7 @@ static struct smp_hotplug_thread rcu_cpu_thread_spec = {
>         .thread_comm            = "rcuc/%u",
>         .setup                  = rcu_cpu_kthread_setup,
>         .park                   = rcu_cpu_kthread_park,
> +       .unpark                 = rcu_cpu_kthread_unpark,

Well, personally I don't think it's worth the burden because wake_up_process()
already does an early exit if it's not dealing with a TASK_[UN]INTERRUPTIBLE task and
the window is so short and rare that it doesn't look like a good candidate for
extra optimization;

Thanks.