Re: [PATCH] sched/core: fix illegal RCU from offline CPUs

[Tetsuo Handa]

On 2020/01/13 1:17, Qian Cai wrote:
> In the CPU-offline process, it calls mmdrop() after idle entry and the
> subsequent call to cpuhp_report_idle_dead(). Once execution passes the
> call to rcu_report_dead(), RCU is ignoring the CPU, which results in
> lockdep complaints when mmdrop() uses RCU from either memcg or
> debugobjects. Fix it by scheduling mmdrop() on another online CPU.
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 90e4b00ace89..41fb49f3dfce 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6194,7 +6194,8 @@ void idle_task_exit(void)
>  		current->active_mm = &init_mm;
>  		finish_arch_post_lock_switch();
>  	}
> -	mmdrop(mm);
> +	smp_call_function_single(cpumask_first(cpu_online_mask),
> +				(void (*)(void *))mmdrop, mm, 0);

mmdrop() might sleep, but

/*
 * smp_call_function_single - Run a function on a specific CPU
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait until function has completed on other CPUs.
 *
 * Returns 0 on success, else a negative status code.
 */

. Maybe mmdrop_async() instead?