Re: [PATCH v3 05/12] smp: Free call_function_data via RCU in smpcfd_dead_cpu

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-18 12:56:31 [+0800], Chuyi Zhou wrote:
> Use rcu_read_lock() to protect csd in smp_call_function_many_cond() and
> wait for all read critical sections to exit before releasing percpu csd
> data. This is preparation for enabling preemption during csd_lock_wait()
> and can prevent accessing cfd->csd data that has already been freed in
> smpcfd_dead_cpu().
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
>  kernel/smp.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 9728ba55944d..32c293d8be0e 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -77,6 +77,7 @@ int smpcfd_dead_cpu(unsigned int cpu)
>  {
>  	struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
>  
> +	synchronize_rcu();
>  	free_cpumask_var(cfd->cpumask);
>  	free_cpumask_var(cfd->cpumask_ipi);
>  	free_percpu(cfd->csd);

This seems to make sense. But it could delay CPU shutdown and then the
stress-cpu-hotplug.sh. And this one helped to find bugs.

What is expectation of shutting down a CPU? Will it remain off for
_long_ at which point we care about this memory or is temporary and we
could skip to free the memory here because we allocate it only once on
the UP side?

Sebastian