linux-kernel - Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs

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Date:   Mon, 20 Jan 2020 15:35:09 -0500
From:   Qian Cai <cai@....pw>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        paulmck@...nel.org, tglx@...utronix.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs

> On Jan 20, 2020, at 5:17 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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()?

The problem is that no all arch_cpu_idle_dead() will call idle_task_exit(). For example, alpha and parisc are not, so it needs to deal with some kind of ifdef dance in takedown_cpu() to conditionally call mmdrop() which sounds even more horrible?

If you really prefer it anyway, maybe something like touching every arch’s __cpu_die() to also call mmdrop() depends on arches?

BTW, how to obtain the other CPU’s current task mm? Is that cpu_rq(cpu)->curr->active_mm?