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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH v2] sched/core: fix illegal RCU from offline CPUs
Date:   Mon, 20 Jan 2020 15:35:09 -0500
Message-Id: <F96BF662-BB11-4AFB-BF05-CB1720441A92@lca.pw>
References: <20200120101652.GM14879@hirez.programming.kicks-ass.net>
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
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 20, 2020, at 5:17 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> Bah.. that's horrible. Surely we can find a better place to do this in
> the whole hotplug machinery.
>=20
> 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 e=
ven more horrible?

If you really prefer it anyway, maybe something like touching every arch=E2=80=
=99s __cpu_die() to also call mmdrop() depends on arches?

BTW, how to obtain the other CPU=E2=80=99s current task mm? Is that cpu_rq(c=
pu)->curr->active_mm?=