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Date:   Mon, 20 Jan 2020 11:16:52 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qian Cai <cai@lca.pw>
Cc:     mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        paulmck@kernel.org, tglx@linutronix.de, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs
Message-ID: <20200120101652.GM14879@hirez.programming.kicks-ass.net>
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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()?