On Mon, Jan 13, 2020 at 02:03:31PM -0500, 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, so it by scheduling mmdrop() on another online CPU.
>
> According to the commit a79e53d85683 ("x86/mm: Fix pgd_lock deadlock"),
> mmdrop() is not interrupt-safe, and called from
> smp_call_function_single() could end up running mmdrop() from the IPI
> interrupt handler.
>
<deletes ~100 lines of gunk>
Surely the critical information contained in these nearly 100 lines of
splat can be more consicely represented?
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 90e4b00ace89..1863a6fc4d82 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_async, mm, 0);
> }
Bah.. that's horrible. Surely we can find a better place to do this in
the whole hotplug machinery.
Perhaps you can have takedown_cpu() do the mmdrop()?
